Tunneling in the Brillouin Zone: Theory of Backscattering in Valley Hall Edge Channels

Tirth Shah, Florian Marquardt, Vittorio Peano

A large set of recent experiments has been exploring topological transport in bosonic systems,e.g. of photons or phonons. In the vast majority, time-reversal symmetry is preserved, and bandstructures are engineered by a suitable choice of geometry, to produce topologically nontrivialbandgaps in the vicinity of high-symmetry points. However, this leaves open the possibility oflarge-quasimomentum backscattering, destroying the topological protection. Up to now, it has beenunclear what precisely are the conditions where this effect can be sufficiently suppressed. In thepresent work, we introduce a comprehensive semiclassical theory of tunneling transitions in momen-tum space, describing backscattering for one of the most important system classes, based on thevalley Hall effect. We predict that even for a smooth domain wall effective scattering centres developat locations determined by both the local slope of the wall and the energy. Moreover, our theoryprovides a quantitative analysis of the exponential suppression of the overall reflection amplitudewith increasing domain wall smoothness.

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Multiphoton non-local quantum interference controlled by an undetected photon

Kaiyi Qian, Kai Wang, Leizhen Chen, Hou Zhaohua, Mario Krenn, Shining Zhu, Xiao-Song Ma

The interference of quanta lies at the heart of quantum physics. The multipartite generalization<br>of single-quanta interference creates entanglement, the coherent superposition of states shared by several quanta. Entanglement allows non-local correlations between many quanta and hence is a key resource for quantum information technology. Entanglement is typically considered to be essential for creating non-local correlations, manifested by multipartite interference. Here, we show that this is not the case and demonstrate multiphoton non-local quantum interference without entanglement of any intrinsic properties of the photons. We harness the superposition of the physical origin of a four-photon product state, which leads to constructive and destructive interference of the photons’ mere existence. With the intrinsic indistinguishability in the generation process of photons, we realize four-photon frustrated quantum interference. We furthermore establish non-local control of multipartite quantum interference, in which we tune the phase of one undetected photon and observe the interference of the other three photons. Our work paves the way for fundamental studies of non-locality and potential applications in quantum technologies.

Synchronization of gigahertz core resonances in multiple photonic crystal fiber cores by timing-modulated harmonic mode locking

Dung-Han Yeh, Wenbin He, Meng Pang, Xin Jiang, Philip St.J. Russell

Synchronization of mechanical oscillators by optical forces is a topic that has been much explored in recent years, for example, in the context of SiN microdisk resonators. Here we report stable long-term synchronization of the core vibrations of three different photonic crystal fibers, driven intra-cavity by a 2 GHz train of timing-modulated pulses in a high harmonic opto-acoustically mode-locked fiber laser. The core resonances are equally spaced in frequency and are coupled purely by the optical field. Under the correct conditions, they become stably synchronized, being simultaneously driven by the timing-modulated pulse train. Floquet–Bloch theory, in which the pulses are treated as particles trapped in potential wells and coupled by optomechanical back-action, describes the complex temporal dynamics observed in the experiments. This unique system provides a novel means of modifying the temporal structure of pulse trains running at few-gigahertz repetition rates.

Real-Time Deformability Cytometry Detects Leukocyte Stiffening After Gadolinium-Based Contrast Agent Exposure

Angela Jacobi, Angela Ariza de Schellenberger, Yavuz Oguz Uca, Maik Herbig, Jochen Guck, Ingolf Sack

Objectives <br>Reports on gadolinium (Gd) retention in soft tissues after administration of Gd-based contrast agents (GBCAs) raise concerns about Gd-induced changes in the biophysical properties of cells and tissues. Here, we investigate if clinical GBCAs of both classes of linear and macrocyclic structure cause changes in the mechanical properties of leukocytes in human blood samples.<br><br>Material and Methods <br>Real-time deformability cytometry was applied to human blood samples from 6 donors. The samples were treated with 1 mM gadoteric acid (Dotarem), gadopentetic acid (Magnevist), gadobutrol (Gadovist), or Gd trichloride at 37°C for 1 hour to mimic clinical doses of GBCAs and exposure times. Leukocyte subtypes—lymphocytes, monocytes, and neutrophils—were identified based on their size and brightness and analyzed for deformability, which is inversely correlated with cellular stiffness.<br><br>Results <br>We observed significant stiffening (3%–13%, P < 0.01) of all investigated leukocyte subtypes, which was most pronounced for lymphocytes, followed by neutrophils and monocytes, and the effects were independent of the charge and steric structure of the GBCA applied. In contrast, no changes in cell size and brightness were observed, suggesting that deformability and cell stiffness measured by real-time deformability cytometry are sensitive to changes in the physical phenotypes of leukocytes after GBCA exposure.<br><br>Conclusions <br>Real-time deformability cytometry might provide a quantitative blood marker for critical changes in the physical properties of blood cells in patients undergoing GBCA-enhanced magnetic resonance imaging.

Phase Space Crystals: Condensed matter in dynamical systems

Lingzhen Guo

This book aims to develop a general framework of condensed matter theory in phase space, instead of configuration space, of a dynamical system. Different from Euclidean real space, phase space is embedded with symplectic geometry in classical mechanics or noncommutative geometry in quantum mechanics. Arbitrary lattice Hamiltonians and crystalline many-body states in phase space can be created with the Floquet approach. The book covers topics ranging from dynamical systems, Floquet theory, topological physics to quantum many-body physics and time crystals. The book fills in the blanks in the study of dynamical systems by considering many-body physics in the phase space.

Playing Ping Pong with Light: Directional Emission of White Light

Heribert Wankerl, Christopher Wiesmann, Laura Kreiner, Rainer Butendeich, Alexander Luce, Sandra Sobczyk, Maike Lorena Stern, Elmar Wolfgang Lang

Over the last decades, light-emitting diodes (LED) have replaced common light bulbs in almost every application, from flashlights in smartphones to automotive headlights. Illuminating nightly streets requires LEDs to emit a light spectrum that is perceived as pure white by the human eye. The power associated with such a white light spectrum is not only distributed over the contributing wavelengths but also over the angles of vision. For many applications, the usable light rays are required to exit the LED in forward direction, namely under small angles to the perpendicular. In this work, we demonstrate that a specifically designed multi-layer thin film on top of a white LED increases the power of pure white light emitted in forward direction. Therefore, the deduced multi-objective optimization problem is reformulated via a real-valued physics-guided objective function that represents the<br>hierarchical structure of our engineering problem. Variants of Bayesian optimization are employed to maximize this non-deterministic objective function based on ray tracing simulations. Eventually, the investigation of optical properties of suitable multi-layer thin films allowed to identify the mechanism behind the increased directionality of white light: angle and wavelength selective filtering causes the multi-layer thin film to play ping pong with<br>rays of light.

Accelerated Non-Reciprocal Transfer of Energy Around an Exceptional Point

Hugo Ribeiro, Florian Marquardt

We develop perturbative methods to study and control dynamical phenomena related to exceptional points in Non-Hermitian systems. In particular, we show how to find perturbative solutions based on the Magnus expansion that accurately describe the evolution of non-Hermitian systems when encircling an exceptional point. This allows us to use the recently proposed Magnus-based strategy for control to design fast non-reciprocal, topological operations whose fidelity error is orders of magnitude smaller than their much slower adiabatic counterparts.

Passive coupling of membrane tension and cell volume during active response of cells to osmosis

Chloé Roffay, Guillaume Molinard, Kyoohyun Kim, Marta Urbanska, Virginia Andrade, Victoria Barbarasa, Paulina Nowak, Vincent Mercier, José García-Calvo, et al.

During osmotic changes of their environment, cells actively regulate their volume and plasma membrane tension that can passively change through osmosis. How tension and volume are coupled during osmotic adaptation remains unknown, as their quantitative characterization is lacking. Here, we performed dynamic membrane tension and cell volume measurements during osmotic shocks. During the first few seconds following the shock, cell volume varied to equilibrate osmotic pressures inside and outside the cell, and membrane tension dynamically followed these changes. A theoretical model based on the passive, reversible unfolding of the membrane as it detaches from the actin cortex during volume increase quantitatively describes our data. After the initial response, tension and volume recovered from hypoosmotic shocks but not from hyperosmotic shocks. Using a fluorescent membrane tension probe (fluorescent lipid tension reporter [Flipper-TR]), we investigated the coupling between tension and volume during these asymmetric recoveries. Caveolae depletion and pharmacological inhibition of ion transporters and channels, mTORCs, and the cytoskeleton all affected tension and volume responses. Treatments targeting mTORC2 and specific downstream effectors caused identical changes to both tension and volume responses, their coupling remaining the same. This supports that the coupling of tension and volume responses to osmotic shocks is primarily regulated by mTORC2.

Mapping Tumor Spheroid Mechanics in Dependence of 3D Microenvironment Stiffness and Degradability by Brillouin Microscopy

Vaibhav Mahajan, Timon Beck, Paulina Gregorczyk, André Ruland, Simon Alberti, Jochen Guck, Carsten Werner, Raimund Schlüßler, Anna V. Taubenberger

Altered biophysical properties of cancer cells and of their microenvironment contribute to cancer progression. While the relationship between microenvironmental stiffness and cancer cell mechanical properties and responses has been previously studied using two-dimensional (2D) systems, much less is known about it in a physiologically more relevant 3D context and in particular for multicellular systems. To investigate the influence of microenvironment stiffness on tumor spheroid mechanics, we first generated MCF-7 tumor spheroids within matrix metalloproteinase (MMP)-degradable 3D polyethylene glycol (PEG)-heparin hydrogels, where spheroids showed reduced growth in stiffer hydrogels. We then quantitatively mapped the mechanical properties of tumor spheroids in situ using Brillouin microscopy. Maps acquired for tumor spheroids grown within stiff hydrogels showed elevated Brillouin frequency shifts (hence increased longitudinal elastic moduli) with increasing hydrogel stiffness. Maps furthermore revealed spatial variations of the mechanical properties across the spheroids’ cross-sections. When hydrogel degradability was blocked, comparable Brillouin frequency shifts of the MCF-7 spheroids were found in both compliant and stiff hydrogels, along with similar levels of growth-induced compressive stress. Under low compressive stress, single cells or free multicellular aggregates showed consistently lower Brillouin frequency shifts compared to spheroids growing within hydrogels. Thus, the spheroids’ mechanical properties were modulated by matrix stiffness and degradability as well as multicellularity, and also to the associated level of compressive stress felt by tumor spheroids. Spheroids generated from a panel of invasive breast, prostate and pancreatic cancer cell lines within degradable stiff hydrogels, showed higher Brillouin frequency shifts and less cell invasion compared to those in compliant hydrogels. Taken together, our findings contribute to a better understanding of the interplay between cancer cells and microenvironment mechanics and degradability, which is relevant to better understand cancer progression.

Arbitrary optical wave evolution with Fourier transforms and phase masks

Victor Lopéz-Pastor, Jeff S. Lundeen, Florian Marquardt

A large number of applications in classical and quantum photonics require the capability of implementing arbitrary linear unitary transformations on a set of optical modes. In a seminal work by Reck et al. it was shown how to build such multiport universal interferometers with a mesh of beam splitters and phase shifters, and this design became the basis for most experimental implementations in the last decades. However, the design of Reck et al. is difficult to scale up to a large number of modes, which would be required for many applications. Here we present a constructive proof that it is possible to realize a multiport universal interferometer on N modes with a succession of 6N Fourier transforms and 6N+1 phase masks, for any even integer N. Furthermore, we provide an algorithm to find the correct succesion of Fourier transforms and phase masks to realize a given arbitrary unitary transformation. Since Fourier transforms and phase masks are routinely implemented in several optical setups and they do not suffer from the scalability issues associated with building extensive meshes of beam splitters, we believe that our design can be useful for many applications in photonics.

Efficient nonlinear compression of a thin-disk oscillator to 8.5 fs at 55 W average power

Gaia Barbiero, Haochuang Wang, Martin Grassl, Sebastian Groebmeyer, Dziugas Kimbaras, Marcel Neuhaus, Vladimir Pervak, Thomas Nubbemeyer, Hanieh Fattahi, et al.

We demonstrate an efficient hybrid-scheme for nonlinear pulse compression of high-power thin-disk oscillator pulses to the sub-10 fs regime. The output of a home-built, 16 MHz, 84 W, 220 fs Yb:YAG thin-disk oscillator at 1030 nm is first compressed to 17 fs in two nonlinear multipass cells. In a third stage, based on multiple thin sapphire plates, further compression to 8.5 fs with 55 W output power and an overall optical efficiency of 65% is achieved. Ultrabroadband mid-infrared pulses covering the spectral range 2.4-8 μm were generated from these compressed pulses by intra-pulse difference frequency generation.

Optimized analysis for sensitive detection and analysis of single proteins via interferometric scattering microscopy

Houman Mirzaalian Dastjerdi, Mahyar Dahmardeh, André Gemeinhardt, Reza Gholami Mahmoodabadi, Harald Köstler, Vahid Sandoghdar

It has been shown that interferometric detection of Rayleigh scattering (iSCAT) can reach an exquisite sensitivity for label-free detection of nano-matter, down to single proteins. The sensitivity of iSCAT detection is intrinsically limited by shot noise, which can be indefinitely improved by employing higher illumination power or longer integration times. In practice, however, a large speckle-like background and technical issues in the experimental setup limit the attainable signal-to-noise ratio. Strategies and algorithms in data analysis are, thus, crucial for extracting quantitative results from weak signals, e.g. regarding the mass (size) of the detected nano-objects or their positions. In this article, we elaborate on some algorithms for processing iSCAT data and identify some key technical as well as conceptual issues that have to be considered when recording and interpreting the data. The discussed methods and analyses are made available in the extensive python-based platform, PiSCAT.

Critical dynamics of an asymmetrically bidirectionally pumped optical microresonator

Jonathan M. Silver, Kenneth T. V. Grattan, Pascal Del'Haye

An optical ring resonator with third-order, or Kerr, nonlinearity will exhibit symmetry breaking between the two counterpropagating circulating powers when pumped with sufficient power in both the clockwise and counterclockwise directions. This is due to the effects of self- and cross-phase modulation on the resonance frequencies in the two directions. The critical point of this symmetry breaking exhibits universal behaviors including divergent responsivity to external perturbations, critical slowing down, and scaling invariance. Here we derive a model for the critical dynamics of this system, first for a symmetrically pumped resonator and then for the general case of asymmetric pumping conditions and self- and cross-phase modulation coefficients. This theory not only provides a detailed understanding of the dynamical response of critical-point-enhanced optical gyroscopes and near-field sensors, but is also applicable to nonlinear critical points in a wide range of systems.

Design of quantum optical experiments with logic artificial intelligence

Alba Cervera-Lierta, M. Krenn, Alan Aspuru-Guzik

Logic artificial intelligence (AI) is a subfield of AI where variables can take two defined arguments, True or False, and are arranged in clauses that follow the rules of formal logic. Several problems that span from physical systems to mathematical conjectures can be encoded into these clauses and be solved by checking their satisfiability (SAT). Recently, SAT solvers have become a sophisticated and powerful computational tool capable, among other things, of solving long-standing mathematical conjectures. In this work, we propose the use of logic AI for the design of optical quantum experiments. We show how to map into a SAT problem the experimental preparation of an arbitrary quantum state and propose a logic-based algorithm, called Klaus, to find an interpretable representation of the photonic setup that generates it. We compare the performance of Klaus with the state-of-the-art algorithm for this purpose based on continuous optimization. We also combine both logic and numeric strategies to find that the use of logic AI improves significantly the resolution of this problem, paving the path to develop more formal-based approaches in the context of quantum physics experiments.

Single-molecule vacuum Rabi splitting: four-wave mixing and optical switching at the single-photon level

André Pscherer, Manuel Meierhofer, Daqing Wang, Hrishikesh Kelkar, Diego-Martin Cano, Tobias Utikal, Stephan Götzinger, Vahid Sandoghdar

A single quantum emitter can possess a very strong intrinsic nonlinearity, but its overall promise for nonlinear effects is hampered by the challenge of efficient coupling to incident photons. Common nonlinear optical materials, on the other hand, are easy to couple to but are bulky, imposing a severe limitation on the miniaturization of photonic systems. In this work, we show that a single organic molecule acts as an extremely efficient nonlinear optical element in the strong coupling regime of cavity quantum electrodynamics. We report on single-photon sensitivity in nonlinear signal generation and all-optical switching. Our work promotes the use of molecules for applications such as integrated photonic circuits, operating at very low powers.

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Nonlinear enhanced microresonator gyroscope

Jonathan M. Silver, Leonardo Del Bino, Michael T. M. Woodley, George N. Ghalanos, Andreas O. Svela, Niall Moroney, Shuangyou Zhang, Kenneth T. V. Grattan, Pascal Del'Haye

Optical gyroscopes based on the Sagnac effect have been the mainstay of inertial navigation in aerospace and shipping for decades. These gyroscopes are typically realized either as ring-laser gyroscopes (RLGs) or fiber-optic gyroscopes (FOGs). With the recent rapid progress in the field of ultrahigh-quality optical whispering-gallery mode and ring microresonators, attention has been focused on the development of microresonator-based Sagnac gyroscopes as a more compact alternative to RLGs and FOGs. One avenue that has been explored is the use of exceptional points in non-Hermitian systems to enhance the responsivity to rotation. We use a similar phenomenon, namely, the critical point of a spontaneous symmetry-breaking transition between counterpropagating light, to demonstrate a microresonator gyroscope with a responsivity enhanced by a factor of around 10(4). We present a proof-of-principle rotation measurement as well as a characterization of the system's dynamical response, which shows the universal critical behaviors of responsivity enhancement and critical slowing down, both of which are beneficial in an optical gyroscope. We believe that this concept could be used to realize simple and cheap chip-based gyroscopes with sensitivities approaching those of today's RLGs and FOGs. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Deep recurrent networks predicting the gap evolution in adiabatic quantum computing

Naeimeh Mohseni, Carlos Navarrete-Benlloch, Tim Byrnes, Florian Marquardt

One of the main challenges in quantum physics is predicting efficiently the dynamics of observables in many-body problems out of equilibrium. A particular example occurs in adiabatic quantum computing, where finding the structure of the instantaneous gap of the Hamiltonian is crucial in order to optimize the speed of the computation. Inspired by this challenge, in this work we explore the potential of deep learning for discovering a mapping from the parameters that fully identify a problem Hamiltonian to the full evolution of the gap during an adiabatic sweep applying different network architectures. Through this example, we find that a limiting factor for the learnability of the dynamics is the size of the input, that is, how the number of parameters needed to identify the Hamiltonian scales with the system size. We demonstrate that a long short-term memory network succeeds in predicting the gap when the parameter space scales linearly with system size. Remarkably, we show that once this architecture is combined with a convolutional neural network to deal with the spatial structure of the model, the gap evolution can even be predicted for system sizes larger than the ones seen by the neural network during training. This provides a significant speedup in comparison with the existing exact and approximate algorithms in calculating the gap.

Dynamical phase transitions in quantum spin models with antiferromagnetic long-range interactions

Jad C. Halimeh, Maarten Van Damme, Lingzhen Guo, Johannes Lang, Philipp Hauke

In recent years, dynamical phase transitions and out-of-equilibrium criticality have been at the forefront of ultracold gases and condensed matter research. Whereas universality and scaling are established topics in equilibrium quantum many-body physics, out-of-equilibrium extensions of such concepts still leave much to be desired. Using exact diagonalization and the time-dependent variational principle in uniform matrix product states, we calculate the time evolution of the local order parameter and Loschmidt return rate in transverse-field Ising chains with antiferromagnetic power law-decaying interactions, and map out the corresponding rich dynamical phase diagram. Anomalous cusps in the return rate, which are ubiquitous at small quenches within the ordered phase in the case of ferromagnetic long-range interactions, are absent within the accessible timescales of our simulations in the antiferromagnetic case, showing that long-range interactions are not a sufficient condition for their appearance. We attribute this to much weaker domain-wall binding in the antiferromagnetic case. For quenches across the quantum critical point, regular cusps appear in the return rate and connect to the local order parameter changing sign, indicating the concurrence of two major concepts of dynamical phase transitions. Our results consolidate conclusions of previous works that a necessary condition for the appearance of anomalous cusps in the return rate after quenches within the ordered phase is for topologically trivial local spin flips to be the energetically dominant excitations in the spectrum of the quench Hamiltonian. Our findings are readily accessible in modern trapped-ion setups and we outline the associated experimental considerations.

Engineering long-lived vibrational states for an organic molecule

Burak Gürlek, Vahid Sandoghdar, Diego-Martin Cano

The optomechanical character of molecules was discovered by Raman about one century ago. Today, molecules are promising contenders for high-performance quantum optomechanical platforms because their small size and large energy-level separations make them intrinsically robust against thermal agitations. Moreover, the precision and throughput of chemical synthesis can ensure a viable route to quantum technological applications. The challenge, however, is that the coupling of molecular vibrations to environmental phonons limits their coherence to picosecond time scales. Here, we improve the optomechanical quality of a molecule by several orders of magnitude through phononic engineering of its surrounding. By dressing a molecule with long-lived high-frequency phonon modes of its nanoscopic environment, we achieve storage and retrieval of photons at millisecond time scales and allow for the emergence of single-photon strong coupling in optomechanics. Our strategy can be extended to the realization of molecular optomechanical networks.

Matrix stiffness mechanosensing modulates the expression and distribution of transcription factors in Schwann cells

Gonzalo Rosso, Daniel Wehner, Christine Schweitzer, Stephanie Möllmert, Elisabeth Sock, Jochen Guck, Victor Shahin

After peripheral nerve injury, mature Schwann cells (SCs) de-differentiate and undergo cell reprogramming to convert into a specialized cell repair phenotype that promotes nerve regeneration. Reprogramming of SCs into the repair phenotype is tightly controlled at the genome level and includes downregulation of pro-myelinating genes and activation of nerve repair-associated genes. Nerve injuries induce not only biochemical but also mechanical changes in the tissue architecture which impact SCs. Recently, we showed that SCs mechanically sense the stiffness of the extracellular matrix and that SC mechanosensitivity modulates their morphology and migratory behavior. Here, we explore the expression levels of key transcription factors and myelin-associated genes in SCs, and the outgrowth of primary dorsal root ganglion (DRG) neurites, in response to changes in the stiffness of generated matrices. The selected stiffness range matches the physiological conditions of both utilized cell types as determined in our previous investigations. We find that stiffer matrices induce upregulation of the expression of transcription factors Sox2, Oct6, and Krox20, and concomitantly reduce the expression of the repair-associated transcription factor c-Jun, suggesting a link between SC substrate mechanosensing and gene expression regulation. Likewise, DRG neurite outgrowth correlates with substrate stiffness. The remarkable intrinsic physiological plasticity of SCs, and the mechanosensitivity of SCs and neurites, may be exploited in the design of bioengineered scaffolds that promote nerve regeneration upon injury.

Dynamical Backaction Magnomechanics

Clinton A. Potts, Emil Varga, Victor A. S. V. Bittencourt, Silvia Viola-Kusminskiy, John P. Davis

Dynamical backaction resulting from radiation pressure forces in optomechanical systems has proven to be a versatile tool for manipulating mechanical vibrations. Notably, dynamical backaction has resulted in the cooling of a mechanical resonator to its ground state, driving phonon lasing, the generation of entangled states, and observation of the optical-spring effect. In certain magnetic materials, mechanical vibrations can interact with magnetic excitations (magnons) via the magnetostrictive interaction, resulting in an analogous magnon-induced dynamical backaction. In this article, we directly observe the impact of magnon-induced dynamical backaction on a spherical magnetic sample’s mechanical vibrations. Moreover, dynamical backaction effects play a crucial role in many recent theoretical proposals; thus, our work provides the foundation for future experimental work pursuing many of these theoretical proposals.

All-Optical Generation of Antiferromagnetic Magnon Currents via the Magnon Circular Photogalvanic Effect

Emil Viñas Boström, Tahereh S. Parvini, James W. McIver, Angel Rubio, Silvia Viola-Kusminskiy, Michael A. Sentef

We introduce the magnon circular photogalvanic effect enabled by two-magnon Raman scattering. This provides an all-optical pathway to the generation of directed magnon currents with circularly polarized light in honeycomb antiferromagnetic insulators. The effect is the leading order contribution to magnon photocurrent generation via optical fields. Control of the magnon current by the polarization and angle of incidence of the laser is demonstrated. Experimental detection by sizable inverse spin Hall voltages in platinum contacts is proposed.

Certification of Genuine Multipartite Entanglement with General and Robust Device-independent Witnesses

Chao Zhang, Wen-Hao Zhang, Pavel Sekatski, Jean-Daniel Bancal, Michael Zwerger, Peng Yin, Gong-Chu Li, Xing-Xiang Peng, Lei Chen, et al.

Genuine multipartite entanglement represents the strongest type of entanglement, which is an essential resource for quantum information<br>processing. Standard methods to detect genuine multipartite entanglement, e.g., entanglement witnesses, state tomography, or quantum state verification, require full knowledge of the Hilbert space dimension and precise calibration of measurement devices, which are usually difficult to acquire in an experiment. The most radical way to overcome these problems is to detect<br>entanglement solely based on the Bell-like correlations of measurement outcomes collected in the experiment, namely, device-independently (DI). However, it is difficult to certify genuine entanglement of practical multipartite states in<br>this way, and even more difficult to quantify it, due to the difficulty to identify optimal multipartite Bell inequalities and protocols tolerant to state impurity. In this work, we explore a general and robust DI method which can be applied to various realistic multipartite quantum state in arbitrary finite dimension, while merely relying on bipartite Bell inequalities. Our method allows us both to certify the presence of genuine multipartite entanglement and to quantify it. Several important classes of entangled states are tested with this method, leading to the detection of genuinely entangled states. We also certify genuine multipartite entanglement in weakly-entangled GHZ states, thus showing that the method applies equally well to less standard states.

Channel discord and distortion

Wei-Wei Zhang, Yuval R. Sanders, Barry C. Sanders

Discord, originally notable as a signature of bipartite quantum correlation, in fact can be nonzero<br>classically, i.e. arising from noisy measurements by one of the two parties. Here we redefine<br>classical discord to quantify channel distortion, in contrast to the previous restriction of classical<br>discord to a state, and we then show a monotonic relationship between classical (channel) discord<br>and channel distortion. We show that classical discord is equivalent to (doubly stochastic) channel<br>distortion by numerically discovering a monotonic relation between discord and total-variation<br>distance for a bipartite protocol with one party having a noiseless channel and the other party<br>having a noisy channel. Our numerical method includes randomly generating doubly stochastic<br>matrices for noisy channels and averaging over a uniform measure of input messages. Connecting<br>discord with distortion establishes discord as a signature of classical, not quantum, channel<br>distortion.

Perturbation theory of nearly spherical dielectric optical resonators

Julius Gohsrich, Tirth Shah, Andrea Aiello

Dielectric spheres of various sizes may sustain electromagnetic whispering-gallery modes resonating at optical frequencies with very narrow linewidths. Arbitrary small deviations from the spherical shape typically shift and broaden such resonances. Our goal is to determine these shifted and broadened resonances. A boundary-condition perturbation theory for the acoustic vibrations of nearly circular membranes was developed by Rayleigh more than a century ago. We extend this theory to describe the electromagnetic excitations of nearly spherical dielectric cavities. This approach permits us to avoid dealing with decaying quasinormal modes. We explicitly find the frequencies and the linewidths of the optical resonances for arbitrarily deformed nearly spherical dielectric cavities, as power series expansions by a small parameter, up to and including second-order terms. We thoroughly discuss the physical conditions for the applicability of perturbation theory.

Optical signatures of the coupled spin-mechanics of a levitated magnetic microparticle

Vanessa Wachter, Victor A. S. V. Bittencourt, Shangran Xie, Sanchar Sharma, Nicolas Joly, Philip Russell, Florian Marquardt, Silvia Viola-Kusminskiy

We propose a platform that combines the fields of cavity optomagnonics and levitated optome-<br>chanics in order to control and probe the coupled spin-mechanics of magnetic dielectric particles. We theoretically study the dynamics of a levitated Faraday-active dielectric microsphere serving as an optomagnonic cavity, placed in an external magnetic field and driven by an external laser. We find that the optically driven magnetization dynamics induces angular oscillations of the particle with low associated damping. Further, we show that the magnetization and angular motion dynamics<br>can be probed via the power spectrum of the outgoing light. Namely, the characteristic frequencies attributed to the angular oscillations and the spin dynamics are imprinted in the light spectrum by two main resonance peaks. Additionally, we demonstrate that a ferromagnetic resonance setup with an oscillatory perpendicular magnetic field can enhance the resonance peak corresponding to<br>the spin oscillations and induce fast rotations of the particle around its anisotropy axis.

Fiber-based biphoton source with ultrabroad frequency tunability

Santiago López-Huidrobro, Markus Lippl, Nicolas Joly, Maria Chekhova

Tunable biphotons are highly important for a wide range of quantum applications. For some applications, especially interesting are cases where two photons of a pair are far apart in frequency. Here, we report a tunable biphoton source based on a xenon-filled hollow-core photonic crystal fiber. Tunability is achieved by adjusting the pressure of the gas inside the fiber. This allows us to tailor the dispersion landscape of the fiber, overcoming the principal limitations of solid-core fiber-based biphoton sources. We report a maximum tunability of 120 THz for a pressure range of 4 bar with a continuous shift of 30 THz/bar. At 21 bar, the photons of a pair are separated by more than one octave. Despite the large separation, both photons have large bandwidths. At 17 bar, they form a very broad (110 THz) band around the frequency of the pump.

Molecular polaritonics in dense mesoscopic disordered ensembles

Christian Sommer, Michael Reitz, Francesca Mineo, Claudiu Genes

We study the dependence of the vacuum Rabi splitting (VRS) on frequency disorder, vibrations, near-field effects, and density in molecular polaritonics. In the mesoscopic limit, static frequency disorder alone can already introduce a loss mechanism from polaritonic states into a dark state reservoir, which we quantitatively describe, providing an analytical scaling of the VRS with the level of disorder. Disorder additionally can split a molecular ensemble into donor-type and acceptor-type molecules and the combination of vibronic coupling, dipole-dipole interactions, and vibrational relaxation induces an incoherent FRET (Förster resonance energy transfer) migration of excitations within the collective molecular state. This is equivalent to a dissipative disorder and has the effect of saturating and even reducing the VRS in the mesoscopic, high-density limit. Overall, this analysis allows to quantify the crucial role played by dark states in cavity quantum electrodynamics with mesoscopic, disordered ensembles.

Physical phenotype of blood cells is altered in COVID-19

Markéta Kubánková, Bettina Hohberger, Jakob Hoffmanns, Julia Fürst, Martin Herrmann, Jochen Guck, Martin Kräter

Clinical syndrome coronavirus disease 2019 (COVID-19) induced by severe acute respiratory syndrome coronavirus 2 is characterized by rapid spreading and high mortality worldwide. Although the pathology is not yet fully understood, hyperinflammatory response and coagulation disorders leading to congestions of microvessels are considered to be key drivers of the still-increasing death toll. Until now, physical changes of blood cells have not been considered to play a role in COVID-19 related vascular occlusion and organ damage. Here, we report an evaluation of multiple physical parameters including the mechanical features of five frequent blood cell types, namely erythrocytes, lymphocytes, monocytes, neutrophils, and eosinophils. More than four million blood cells of 17 COVID-19 patients at different levels of severity, 24 volunteers free from infectious or inflammatory diseases, and 14 recovered COVID-19 patients were analyzed. We found significant changes in lymphocyte stiffness, monocyte size, neutrophil size and deformability, and heterogeneity of erythrocyte deformation and size. Although some of these changes recovered to normal values after hospitalization, others persisted for months after hospital discharge, evidencing the long-term imprint of COVID-19 on the body.

Analytic Design of Accelerated Adiabatic Gates in Realistic Qubits: General Theory and Applications to Superconducting Circuits

F Setiawan, Peter Groszkowski, Hugo Ribeiro, Aashish A Clerk

Shortcuts to adiabaticity (STA) is a general methodology for speeding up adiabatic quantumprotocols, and has many potential applications in quantum information processing. Unfortunately,analytically constructing STAs for systems having complex interactions and more than a few levelsis a challenging task. This is usually overcome by assuming an idealized Hamiltonian (e.g., only alimited subset of energy levels are retained, and the rotating-wave approximation (RWA) is made).Here, we develop ananalyticapproach that allows one to go beyond these limitations. Our methodis general and results in analytically-derived pulse shapes that correct both non-adiabatic errorsas well as non-RWA errors. We also show that our approach can yield pulses requiring a smallerdriving power than conventional non-adiabatic protocols. We show in detail how our ideas can beused to analytically design high-fidelity single-qubit “tripod” gates in a realistic superconductingfluxonium qubit.

suggested by editors

Specialty Photonic Crystal Fibers and Their Applications

David Novoa, Nicolas Joly

This year not only commemorates the 60th anniversary of nonlinear optics with the seminal experiment of second harmonic generation, but it is also the 30th anniversary of the invention of the photonic crystal fiber (PCF). Following their first practical demonstration in 1996, PCFs have rapidly evolved into an established platform for applications in both academic and industrial environments. Their unique ability to confine light in a far more versatile way than possible with conventional optical fibers facilitated the expansion of the multifaceted world of PCF to cover not only nonlinear optics, but also many other disparate fields such as interferometry, beam delivery, laser science, telecommunications, quantum optics, sensing, microscopy, and many others.

Quantum technologies in space

Rainer Kaltenbaek, Antonio Acin, Laszlo Bacsardi, Paolo Bianco, Philippe Bouyer, Eleni Diamanti, Christoph Marquardt, Yasser Omar, Valerio Pruneri, et al.

Recently, the European Commission supported by many European countries has announced large investments towards the commercialization of quantum technology (QT) to address and mitigate some of the biggest challenges facing today’s digital era – e.g. secure communication and computing power. For more than two decades the QT community has been working on the development of QTs, which promise landmark breakthroughs leading to commercialization in various areas. The ambitious goals of the QT community and expectations of EU authorities cannot be met solely by individual initiatives of single countries, and therefore, require a combined European effort of large and unprecedented dimensions comparable only to the Galileo or Copernicus programs. Strong international competition calls for a coordinated European effort towards the development of QT in and for space, including research and development of technology in the areas of communication and sensing. Here, we aim at summarizing the state of the art in the development of quantum technologies which have an impact in the field of space applications. Our goal is to outline a complete framework for the design, development, implementation, and exploitation of quantum technology in space.

HIF2α is a Direct Regulator of Neutrophil Motility

Sundary Sormendi, Mathieu Deygas, Anupam Sinha, Anja Krüger, Ioannis Kourtzelis, Gregoire Le Lay, Mathilde Bernard, Pablo J. Sáez, Michael Gerlach, et al.

Orchestrated recruitment of neutrophils to inflamed tissue is essential during initiation of inflammation. Inflamed areas are usually hypoxic, and adaptation to reduced oxygen pressure is typically mediated by hypoxia pathway proteins. However, it is still unclear how these factors influence the migration of neutrophils to and at the site of inflammation either during their transmigration through the blood-endothelial cell barrier, or their motility in the interstitial space. Here, we reveal that activation of the Hypoxia Inducible Factor-2 (HIF2α) due to deficiency of HIF-prolyl hydroxylase domain protein-2 (PHD2) boosts neutrophil migration specifically through highly confined microenvironments. In vivo, the increased migratory capacity of PHD2-deficient neutrophils resulted in massive tissue accumulation in models of acute local inflammation. Using systematic RNAseq analyses and mechanistic approaches, we identified RhoA, a cytoskeleton organizer, as the central downstream factor that mediates HIF2α-dependent neutrophil motility. Thus, we propose that the here identified novel PHD2-HIF2α-RhoA axis is vital to the initial stages of inflammation as it promotes neutrophil movement through highly confined tissue landscapes.

Picosecond acoustic dynamics in stimulated Brillouin scattering

Johannes Piotrowski, Mikołaj K Schmidt, Birgit Stiller, Christopher G. Poulton, Michael Steel

Recent experiments demonstrating storage of optical pulses in acoustic phonons via stimulated Brillouin scattering raise questions about the spectral and temporal capacities of such protocols and the limitations of the theoretical frameworks routinely used to describe them. We consider the dynamics of photon-phonon scattering induced by optical pulses with temporal widths comparable to the period of acoustic oscillations. We revisit the widely adopted classical formalism of coupled modes and demonstrate its breakdown. We use a simple extension to the formulation and find potentially measurable consequences in the dynamics of Brillouin experiments involving ultrashort pulses. (C) 2021 Optical Society of America

Rapid Exploration of Topological Band Structures using Deep Learning

Vittorio Peano, Florian Sapper, Florian Marquardt

The design of periodic nanostructures allows to tailor the transport of photons, phonons, and matter waves for specific applications. Recent years have seen a further expansion of this field by engineering topological properties. However, what is missing currently are efficient ways to rapidly explore and optimize band structures and to classify their topological characteristics for arbitrary unit-cell geometries. In this work, we show how deep learning can address this challenge. We introduce an approach where a neural network first maps the geometry to a tight-binding model. The tight-binding model encodes not only the band structure but also the symmetry properties of the Bloch waves. This allows us to rapidly categorize a large set of geometries in terms of their band representations, identifying designs for fragile topologies. We demonstrate that our method is also suitable to calculate strong topological invariants, even when (like the Chern number) they are not symmetry indicated. Engineering of domain walls and optimization are accelerated by orders of magnitude. Our method directly applies to any passive linear material, irrespective of the symmetry class and space group. It is general enough to be extended to active and nonlinear metamaterials.

A unique macrophage subpopulation signals directly to progenitor cells to promote regenerative neurogenesis in the zebrafish spinal cord

Leonardo Cavone, Tess McCann, Louisa K. Drake, Erika A. Aguzzi, Ana-Maria Oprisoreanu, Elisa Pedersen, Soe Sandi, Jathurshan Selvarajah, Themistoklis M. Tsarouchas, et al.

Central nervous system injury re-initiates neurogenesis in anamniotes (amphibians and fishes), but not in mammals. Activation of the innate immune system promotes regenerative neurogenesis, but it is fundamentally unknown whether this is indirect through the activation of known developmental signaling pathways or whether immune cells directly signal to progenitor cells using mechanisms that are unique to regeneration. Using single-cell RNA-seq of progenitor cells and macrophages, as well as cell-type-specific manipulations, we provide evidence for a direct signaling axis from specific lesion-activated macrophages to spinal progenitor cells to promote regenerative neurogenesis in zebrafish. Mechanistically, TNFa from pro-regenerative macrophages induces Tnfrsf1a-mediated AP-1 activity in progenitors to increase regeneration-promoting expression of hdac1 and neurogenesis. This establishes the principle that macrophages directly communicate to spinal progenitor cells via non-developmental signals after injury, providing potential targets for future interventions in the regeneration-deficient spinal cord of mammals.

Synthesis and dissociation of soliton molecules in parallel optical-soliton reactors

W He, M Pang, D.-H. Yeh, J Huang, Philip St. J. Russell

Mode-locked lasers have been widely used to explore interactions between optical solitons, including bound-soliton<br>states that may be regarded as “photonic molecules”. Conventional mode-locked lasers normally, however, host at<br>most only a few solitons, which means that stochastic behaviours involving large numbers of solitons cannot easily be<br>studied under controlled experimental conditions. Here we report the use of an optoacoustically mode-locked fibre<br>laser to create hundreds of temporal traps or “reactors” in parallel, within each of which multiple solitons can be<br>isolated and controlled both globally and individually using all-optical methods. We achieve on-demand synthesis and<br>dissociation of soliton molecules within these reactors, in this way unfolding a novel panorama of diverse dynamics in<br>which the statistics of multi-soliton interactions can be studied. The results are of crucial importance in understanding<br>dynamical soliton interactions and may motivate potential applications for all-optical control of ultrafast light fields in<br>optical resonators.

Scaling rules for high quality soliton self-compression in hollow-core fibers

Daniel Schade, Felix Köttig, Johannes Köhler, Michael H. Frosz, Philip St.J. Russell, Francesco Tani

Soliton dynamics can be used to temporally compress laser pulses to few fs durations in many different spectral regions. Here we study analytically, numerically and experimentally the scaling of soliton dynamics in noble gas-filled hollow-core fibers. We identify an optimal parameter region, taking account of higher-order dispersion, photoionization, self-focusing, and modulational instability. Although for single-shots the effects of photoionization can be reduced by using lighter noble gases, they become increasingly important as the repetition rate rises. For the same optical nonlinearity, the higher pressure and longer diffusion times of the lighter gases can considerably enhance the long-term effects of ionization, as a result of pulse-by-pulse buildup of refractive index changes. To illustrate the counter-intuitive nature of these predictions, we compressed 250 fs pulses at 1030 nm in an 80-cm-long hollow-core photonic crystal fiber (core radius 15 µm) to ∼5 fs duration in argon and neon, and found that, although neon performed better at a repetition rate of 1 MHz, stable compression in argon was still possible up to 10 MHz.

Online Monitoring of Microscale Liquid-Phase Catalysis Using in-Fiber Raman Spectroscopy

Florian Schorn, Manfred Aubermann, Richard Zeltner, Marco Haumann, Nicolas Y. Joly

We report on the use of hollow-core photonic crystal fibers to monitor the evolution of chemical reactions. The combination of tight confinement and long interaction length allows single-pass spectroscopic measurements using less than a microliter volume of chemicals with good accuracy. As a proof of principle, we used here nonlinear Raman spectroscopy for a reaction screening of the acidic catalyzed esterification of methanol and acetic acid.

Know How to Regrow-Axon Regeneration in the Zebrafish Spinal Cord

Vasiliki Tsata, Daniel Wehner

The capacity for long-distance axon regeneration and functional recovery after spinal cord injury is poor in mammals but remarkable in some vertebrates, including fish and salamanders. The cellular and molecular basis of this interspecies difference is beginning to emerge. This includes the identification of target cells that react to the injury and the cues directing their pro-regenerative responses. Among existing models of successful spinal cord regeneration, the zebrafish is arguably the most understood at a mechanistic level to date. Here, we review the spinal cord injury paradigms used in zebrafish, and summarize the breadth of neuron-intrinsic and -extrinsic factors that have been identified to play pivotal roles in the ability of zebrafish to regenerate central nervous system axons and recover function.

Machine Learning and Quantum Devices

Florian Marquardt

These brief lecture notes cover the basics of neural networks and deep learning as well as their applications in the quantum domain, for physicists without prior knowledge. In the first part, we describe training using back-propagation, image classification, convolutional networks and autoencoders.The second part is about advanced techniques like reinforcement learning (for discovering control strategies), recurrent neural networks (for analyzing timetraces), and Boltzmann machines (for learning probability distributions). In the third lecture, we discuss first recent applications to quantum physics, with an emphasis on quantum information processing machines. Finally, the fourth lecture is devoted to the promise of using quantum effects to accelerate machine learning.

Efficient and gentle delivery of molecules into cells with different elasticity via Progressive Mechanoporation

Alena Uvizl, Ruchi Goswami, Shanil Durgeshkumar Gandhi, Martina Augsburg, Frank Buchholz, Jochen Guck, Jörg Mansfeld, Salvatore Girardo

On Quantum Efficiency Measurements and Plasmonic Nano-Antennas

Korenobu Matsuzaki, Hsuan-Wei Liu, Stephan Götzinger, Vahid Sandoghdar

Quantum efficiency is a key quantity that describes the performance of light-emitting materials and is, thus, an important metric for assessing novel nanophotonic systems. This Perspective provides a concise discussion of the difficulties encountered in the characterization of quantum efficiencies, especially for studies that involve single emitters. In particular, we review various approaches that have been recently used for determining quantum efficiencies of emitters coupled to plasmonic antennas and highlight the subtleties and challenges that hinder precise measurements.

Renormalized Mutual Information for Artificial Scientific Discovery

Leopoldo Sarra, Andrea Aiello, Florian Marquardt

We derive a well-defined renormalized version of mutual information that allows to estimate the dependence between continuous random variables in the important case when one is deterministically dependent on the other. This is the situation relevant for feature extraction, where the goal is to produce a low-dimensional effective description of a high-dimensional system. Our approach enables the discovery of collective variables in physical systems, thus adding to the toolbox of artificial scientific discovery, while also aiding the analysis of information flow in artificial neural networks.

Rapid computational cell-rotation around arbitrary axes in 3D with multi-core fiber

Jiawei Sun, Nektarios Koukourakis, Jochen Guck, Jürgen W. Czarske

Optical trapping is a vital tool in biology, allowing precise optical manipulation of nanoparticles, micro-robots, and cells. Due to the low risk of photodamage and high trap stiffness, fiber-based dual-beam traps are widely used for optical manipulation of large cells. Besides trapping, advanced applications like 3D refractive index tomography need a rotation of cells, which requires precise control of the forces, for example, the acting-point of the forces and the intensities in the region of interest (ROI). A precise rotation of large cells in 3D about arbitrary axes has not been reported yet in dual-beam traps. We introduce a novel dual-beam optical trap in which a multi-core fiber (MCF) is transformed to a phased array, using wavefront shaping and computationally programmable light. The light-field distribution in the trapping region is holographically controlled within 0.1 s, which determines the orientation and the rotation axis of the cell with small retardation. We demonstrate real-time controlled rotation of HL60 cells about all 3D axes with a very high degree of freedom by holographic controlled light through an MCF with a resolution close to the diffraction limit. For the first time, the orientation of the cell can be precisely controlled about all 3D axes in a dual-beam trap. MCFs provide much higher flexibility beyond the bulky optics, enabling lab-on-a-chip applications and can be easily integrated for applications like contactless cell surgery, refractive index tomography, cell-elasticity measurement, which require precise 3D manipulation of cells.

Overcoming detection loss and noise in squeezing-based optical sensing

Gaetano Frascella, Sascha Agne, Farid Ya. Khalili, Maria V. Chekhova

Among the known resources of quantum metrology, one of the most practical and efficient is squeezing. Squeezed states of atoms and light improve the sensing of the phase, magnetic field, polarization, mechanical displacement. They promise to considerably increase signal-to-noise ratio in imaging and spectroscopy, and are already used in real-life gravitational-wave detectors. But despite being more robust than other states, they are still very fragile, which narrows the scope of their application. In particular, squeezed states are useless in measurements where the detection is inefficient or the noise is high. Here, we experimentally demonstrate a remedy against loss and noise: strong noiseless amplification before detection. This way, we achieve loss-tolerant operation of an interferometer fed with squeezed and coherent light. With only 50% detection efficiency and with noise exceeding the level of squeezed light more than 50 times, we overcome the shot-noise limit by 6 dB. Sub-shot-noise phase sensitivity survives up to 87% loss. Application of this technique to other types of optical sensing and imaging promises a full use of quantum resources in these fields.

Intrinsic Sensitivity Limits for Multiparameter Quantum Metrology

Aaron Z. Goldberg, Luis Sanchez-Soto, Hugo Ferretti

The quantum Cramér-Rao bound is a cornerstone of modern quantum metrology, as it provides the ultimate precision in parameter estimation. In the multiparameter scenario, this bound becomes a matrix inequality, which can be cast to a scalar form with a properly chosen weight matrix. Multiparameter estimation thus elicits tradeoffs in the precision with which each parameter<br>can be estimated. We show that, if the information is encoded in a unitary transformation, we can naturally choose the weight matrix as the metric tensor<br>linked to the geometry of the underlying algebra su(n). This ensures an intrinsic bound that is independent of the choice of parametrization.<br>

Error suppression in adiabatic quantum computing with qubit ensembles

Naeimeh Mohseni, Marek Narozniak, Alexey N Pyrkov, Valentin Ivannikov, Jonathan P Dowling

Incorporating protection against quantum errors into adiabatic quantum computing (AQC) is an important task due to the inevitable presence of decoherence. Here, we investigate an error-protected encoding of the AQC Hamiltonian, where qubit ensembles are used in place of qubits. Our Hamiltonian only involves total spin operators of the ensembles, offering a simpler route towards error-corrected quantum computing. Our scheme is particularly suited to neutral atomic gases where it is possible to realize large ensemble sizes and produce ensemble-ensemble entanglement. We identify a critical ensemble size Nc where the nature of the first excited state becomes a single particle perturbation of the ground state, and the gap energy is predictable by mean-field theory. For ensemble sizes larger than Nc, the ground state becomes protected due to the presence of logically equivalent states and the AQC performance improves with N, as long as the decoherence rate is sufficiently low.

Single organic molecules for photonic quantum technologies

C. Toninelli, I. Gerhardt, A.S. Clark, A. Reserbat-Plantey, Stephan Götzinger, Z. Ristanovic, M. Colautti, P. Lombardi, K.D. Major, et al.

Isolating single molecules in the solid state has allowed fundamental experiments in basic and applied sciences. When cooled down to liquid helium temperature, certain molecules show transition lines, that are tens of megahertz wide, limited only by the excited state lifetime. The extreme flexibility in the synthesis of organic materials provides, at low costs, a wide palette of emission wavelengths and supporting matrices for such single chromophores. In the last decades, the controlled coupling to photonic structures has led to an optimized interaction efficiency with light. Molecules can hence be operated as single photon sources and as non-linear elements with competitive performance in terms of coherence, scalability and compatibility with diverse integrated platforms. Moreover, they can be used as transducers for the optical read-out of fields and material properties, with the promise of single-quanta resolution in the sensing of charges and motion. We show that quantum emitters based on single molecules hold promise to play a key role in the development of quantum science and technologies.

Photon Pairs from Resonant Metasurfaces

Tomas Santiago-Cruz, Anna Fedotova, Vitaliy Sultanov, Maximilian A. Weissflog, Dennis Arslan, Mohammadreza Younesi, Thomas Pertsch, Isabelle Staude, Frank Setzpfandt, et al.

All-dielectric optical metasurfaces are a workhorse in nano-optics, because of both their ability to manipulate light in different degrees of freedom and their excellent performance at light frequency conversion. Here, we demonstrate first-time generation of photon pairs via spontaneous parametric-down conversion in lithium niobate quantum optical metasurfaces with electric and magnetic Mie-like resonances at various wavelengths. By engineering the quantum optical metasurface, we tailor the photon-pair spectrum in a controlled way. Within a narrow bandwidth around the resonance, the rate of pair production is enhanced up to 2 orders of magnitude, compared to an unpatterned film of the same thickness and material. These results enable flat-optics sources of entangled photons—a new promising platform for quantum optics experiments.

Multimode optical parametric amplification in the phase-sensitive regime

Gaetano Frascella, R. V. Zakharov, O. V. Tikhonova, Maria V. Chekhova

Phase-sensitive optical parametric amplification of squeezed states helps to overcome detection loss and noise and thus increases the robustness of sub-shot-noise sensing. Because such techniques, e.g., imaging and spectroscopy, operate with multimode light, multimode amplification is required. Here we find the optimal methods for multimode phase-sensitive amplification and verify them in an experiment where a pumped second-order nonlinear crystal is seeded with a Gaussian coherent beam. Phase-sensitive amplification is obtained by tightly focusing the seed into the crystal, rather than seeding with close-to-plane waves. This suggests that phase-sensitive amplification of sub-shot-noise images should be performed in the near field. A similar recipe can be formulated for the time and frequency, which makes this work relevant for quantum-enhanced spectroscopy.

Doppler optical frequency domain reflectometry for remote fiber sensing

Max Koeppel, Abhinav Sharma, Jasper Podschus, Sanju Sundaramahalingam, Nicolas Y. Joly, Shangran Xie, Philip Russell, Bernhard Schmauss

Coherent optical frequency domain reflectometry has been widely used to locate static reflectors with high spatial resolution. Here, we present a new type of Doppler optical frequency domain reflectometry that offers simultaneous measurement of the position and speed of moving objects. The system is exploited to track optically levitated "flying" particles inside a hollow-core photonic crystal fiber. As an example, we demonstrate distributed temperature sensing with sub-mm-scale spatial resolution and a standard deviation of similar to 10 degrees C up to 200 degrees C. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Depth encoded input polarisation independent swept source cross-polarised optical coherence tomography probe

Katharina Blessing, Judith Schirmer, Asha Parmar, Kanwarpal Singh

Within the last decades, several studies have been published that prove the benefit of polarisation sensitive optical coherence (psOCT) tomography for the field of biomedical diagnostics. However, polarisation sensitive imaging typically requires careful control of the polarisation state of the input illumination, which leads to bulky and delicate systems. While psOCT provides quantitative information, it is mostly sufficient to analyse the images qualitatively in the field of biomedical diagnostics. Therefore, a reduced form of this technique, cross-polarised optical coherence tomography (cpOCT), moves into the focus of interest that serves to visualise the birefringence properties of a sample. Despite the low requirements for the illumination's polarisation, most of the proposed systems still include complex illumination control mechanisms. Here, we propose a common path probe based endoscopic system with an lateral resolution of 30 µm and a sensitivity of 103 dB comprising a commercially available swept-source OCT system and a free-space module which does not require any polarisation controlling elements. A Faraday mirror substitutes the complex polarisation control apparatus. We demonstrate the independence of the approach from the polarisation state of the light source by monitoring the illumination power in the orthogonal channels while varying the source polarisation. Furthermore, we validate the ability of the system to reveal the birefringence properties of different samples, starting from a quarter-wave plate, since its properties are fully characterised. Additionally, we present imaging results from several tissues to demonstrate its feasibility for the field of biomedical diagnostics.

Toward deep biophysical cytometry: prospects and challenges

Kelvin C.M. Lee, Jochen Guck, Keisuke Goda, Kevin K. Tsia

The biophysical properties of cells reflect their identities, underpin their homeostatic<br>state in health, and define the pathogenesis of disease. Recent leapfrogging<br>advances in biophysical cytometry now give access to this information,<br>which is obscured in molecular assays, with a discriminative power that was<br>once inconceivable. However, biophysical cytometry should go 'deeper' in<br>terms of exploiting the information-rich cellular biophysical content, generating<br>a molecular knowledge base of cellular biophysical properties, and standardizing<br>the protocols for wider dissemination. Overcoming these barriers, which<br>requires concurrent innovations in microfluidics, optical imaging, and computer<br>vision, could unleash the enormous potential of biophysical cytometry not only<br>for gaining a new mechanistic understanding of biological systems but also for<br>identifying new cost-effective biomarkers of disease.

Efficient self-compression of ultrashort near-UV pulses in air-filled hollow-core photonic crystal fibers

Jie Luan, Philip St. J. Russell, David Novoa

We report generation of ultrashort near-UV pulses by soliton self-compression in kagomé-style hollow-core photonic crystal fibers filled with ambient air. Pump pulses with the energy of 2.6 µJ and duration of 54 fs at 400 nm were compressed temporally by a factor of 5, to a duration of ∼11 fs. The experimental results are supported by numerical simulations, showing that both Raman and Kerr effects play a role in the compression dynamics. The convenience of using ambient air and the absence of glass windows that would distort the compressed pulses makes the setup highly attractive as the basis of an efficient table-top UV pulse compressor.

Microsphere kinematics from the polarization of tightly focused nonseparable light

Stefan Berg-Johansen, Martin Neugebauer, Andrea Aiello, Gerd Leuchs, Peter Banzer, Christoph Marquardt

Recently, it was shown that vector beams can be utilized for fast kinematic sensing via measurements of their global polarization state [Optica 2, 864 (2015)]. The method relies on correlations between the spatial and polarization degrees of freedom of the illuminating field which result from its nonseparable mode structure. Here, we extend the method to the nonparaxial regime. We study experimentally and theoretically the far-field polarization state generated by the scattering of a dielectric microsphere in a tightly focused vector beam as a function of the particle position. Using polarization measurements only, we demonstrate position sensing of a Mie particle in three dimensions. Our work extends the concept of back focal plane interferometry and highlights the potential of polarization analysis in optical tweezers employing structured light.

Microsphere kinematics from the polarization of tightly focused nonseparable light

Stefan Berg-Johansen, Martin Neugebauer, Andrea Aiello, Gerd Leuchs, Peter Banzer, Christoph Marquardt

Recently, it was shown that vector beams can be utilized for fast kinematic sensing via measurements of their global polarization state [Optica 2(10), 864 (2015)]. The method relies on correlations between the spatial and polarization degrees of freedom of the illuminating field which result from its nonseparable mode structure. Here, we extend the method to the nonparaxial regime. We study experimentally and theoretically the far-field polarization state generated by the scattering of a dielectric microsphere in a tightly focused vector beam as a function of the particle position. Using polarization measurements only, we<br>demonstrate position sensing of a Mie particle in three dimensions. Our work extends the concept of back focal plane interferometry and highlights the potential of polarization analysis in optical tweezers employing structured light.<br>

Portable Optical Coherence Elastography System With Flexible and Phase Stable Common Path Optical Fiber Probe

Asha Parmar, Gargi Sharma, Shivani Sharma, Kanwarpal Singh

Biomechanical properties drive the functioning of cells and tissue. Measurement of such properties in the clinic is quite challenging, however. Optical coherence elastography is an emerging technique in this field that can measure the biomechanical properties of the tissue. Unfortunately, such systems have been limited to benchtop configuration with limited clinical applications. A truly portable system with a flexible probe that could probe different sample sites with ease is still missing. In this work, we report a portable optical coherence elastography system based on a flexible common path optical fiber probe. The common path approach allows us to reduce the undesired phase noise in the system by an order of magnitude less than the standard non-common path systems. The flexible catheter makes it possible to probe different parts of the body with ease. Being portable, our system can be easily transported to and from the clinic. We tested the efficacy of the system by measuring the mechanical properties of the agar-based tissue phantoms. We also measured the mechanical properties (Young’s Modulus) of the human skin at different sites. The measured values for the agar phantom and the skin were found to be comparable with the previously reported studies. Ultra-high phase stability and flexibility of the probe along with the portability of the whole system makes an ideal combination for the faster clinical adoption of the optical coherence elastography technique.

The Xenopus spindle is as dense as the surrounding cytoplasm

Abin Biswas, Kyoohyun Kim, Gheorghe Cojoc, Jochen Guck, Simone Reber

The mitotic spindle is a self-organizing molecular machine, where hundreds of different molecules continuously interact to maintain a dynamic steady state. While our understanding of key molecular players in spindle assembly is significant, it is still largely unknown how the spindle’s material properties emerge from molecular interactions. Here, we use correlative fluorescence imaging and label-free three-dimensional optical diffraction tomography (ODT) to measure the Xenopus spindle’s mass density distribution. While the spindle has been commonly referred to as a denser phase of the cytoplasm, we find that it has the same density as its surrounding, which makes it neutrally buoyant. Molecular perturbations suggest that spindle mass density can be modulated by tuning microtubule nucleation and dynamics. Together, ODT provides direct, unbiased, and quantitative information of the spindle’s emergent physical properties—essential to advance predictive frameworks of spindle assembly and function.

Nanoscopic charge fluctuations in a gallium phosphide waveguide measured by single molecules

Alexey Shkarin, Dominik Rattenbacher, Jan Renger, Simon Hönl, Tobias Utikal, Paul Seidler, Stephan Götzinger, Vahid Sandoghdar

We present efficient coupling of single organic molecules to a gallium phosphide subwavelengthwaveguide (nanoguide). By examining and correlating the temporal dynamics of various single-molecule resonances at different locations along the nanoguide, we reveal light-induced fluctuationsof their Stark shifts. Our observations are consistent with the predictions of a simple model basedon the optical activation of a small number of charges in the GaP nanostructure.

Comment on “An encryption protocol for NEQR images based on one-particle quantum walks on a circle”

Markus Grassl

Rotation sensing at the ultimate limit

Aaron Z. Goldberg, Andrei B. Klimov, Gerd Leuchs, Luis Sanchez-Soto

Conventional classical sensors are approaching their maximum sensitivity<br>levels in many areas. Yet these levels still are far from the ultimate limits<br>dictated by quantum mechanics. Quantum sensors promise a substantial step ahead<br>by taking advantage of the salient sensitivity of quantum states to the<br>environment. Here, we focus on sensing rotations, a topic of broad application.<br>By resorting to the basic tools of estimation theory, we derive states that<br>achieve the ultimate sensitivities in estimating both the orientation of an<br>unknown rotation axis and the angle rotated about it. The critical enhancement<br>obtained with these optimal states should make of them an indispensable<br>ingredient in the next generation of rotation sensors that is now blossoming.<br>

Brillouin scattering - theory and experiment: tutorial

C. Wolff, M.J.A. Smith, Birgit Stiller, C. G. Poulton

Brillouin scattering is an important and interesting nonlinear effect involving the interaction between optical and acoustic fields in optical waveguides. It is increasingly useful in the field of photonics, where it supplies a tunable ultra-narrow linewidth response that can be used for applications including sensing, filtering, and lasing, as well as the acoustic storage of optical pulses. This tutorial gives an overview of the fundamentals of Brillouin scattering aimed at newcomers to the field, and covers the physics underlying the interaction, the mathematical theory, and setup details of foundational Brillouin experiments. (C) 2021 Optical Society of America

Axial superlocalization with vortex beams

D. Koutny, Z. Hradil, J. Rehacek, Luis Sanchez-Soto

Improving axial resolution is of paramount importance for three-dimensional optical imaging systems. Here, we investigate the ultimate precision in axial<br>localization using vortex beams. For Laguerre-Gauss beams, this limit can be achieved with just an intensity scan. The same is not true for superpositions<br>of Laguerre-Gauss beams, in particular for those with intensity profiles that rotate on defocusing. Microscopy methods based on rotating vortex beams may thus benefit from replacing traditional intensity sensors with advanced mode-sorting techniques.

Compliant Substrates Enhance Macrophage Cytokine Release and NLRP3 Inflammasome Formation During Their Pro-Inflammatory Response

Joan-Carles Escolano, Anna V. Taubenberger, Shada Abuhattum, Christine Schweitzer, Aleeza Farrukh, Aránzazu del Campo, Clare E. Bryant, Jochen Guck

Immune cells process a myriad of biochemical signals but their function and behavior are also determined by mechanical cues. Macrophages are no exception to this. Being present in all types of tissues, macrophages are exposed to environments of varying stiffness, which can be further altered under pathological conditions. While it is becoming increasingly clear that macrophages are mechanosensitive, it remains poorly understood how mechanical cues modulate their inflammatory response. Here we report that substrate stiffness influences the expression of pro-inflammatory genes and the formation of the NLRP3 inflammasome, leading to changes in the secreted protein levels of the cytokines IL-1β and IL-6. Using polyacrylamide hydrogels of tunable elastic moduli between 0.2 and 33.1 kPa, we found that bone marrow-derived macrophages adopted a less spread and rounder morphology on compliant compared to stiff substrates. Upon LPS priming, the expression levels of the gene encoding for TNF-α were higher on more compliant hydrogels. When additionally stimulating macrophages with the ionophore nigericin, we observed an enhanced formation of the NLRP3 inflammasome, increased levels of cell death, and higher secreted protein levels of IL-1β and IL-6 on compliant substrates. The upregulation of inflammasome formation on compliant substrates was not primarily attributed to the decreased cell spreading, since spatially confining cells on micropatterns led to a reduction of inflammasome-positive cells compared to well-spread cells. Finally, interfering with actomyosin contractility diminished the differences in inflammasome formation between compliant and stiff substrates. In summary, we show that substrate stiffness modulates the pro-inflammatory response of macrophages, that the NLRP3 inflammasome is one of the components affected by macrophage mechanosensing, and a role for actomyosin contractility in this mechanosensory response. Thus, our results contribute to a better understanding of how microenvironment stiffness affects macrophage behavior, which might be relevant in diseases where tissue stiffness is altered and might potentially provide a basis for new strategies to modulate inflammatory responses.

Precision single-particle localization using radial variance transform

Anna Kashkanova, Alexey Shkarin, Reza Gholami Mahmoodabadi, Martin Blessing, Yazgan Tuna, André Gemeinhardt, Vahid Sandoghdar

We introduce an image transform designed to highlight features with high degree of radial symmetry for identification and subpixel localization of particles in microscopy images. The transform is based on analyzing pixel value variations in radial and angular directions. We compare the subpixel localization performance of this algorithm to other common methods based on radial or mirror symmetry (such as fast radial symmetry transform, orientation alignment transform, XCorr, and quadrant interpolation), using both synthetic and experimentally obtained data. We find that in all cases it achieves the same or lower localization error, frequently reaching the theoretical limit.

Design of an optomagnonic crystal: Towards optimal magnon-photon mode matching at the microscale

Jasmin Graf, Sanchar Sharma, Hans Hübl, Silvia Viola-Kusminskiy

We put forward the concept of an optomagnonic crystal: a periodically patterned structure at the microscale based on a magnetic dielectric, which can co-localize magnon and photon modes. The co-localization in small volumes can result in large values of the photon-magnon coupling at the single quanta level, which opens perspectives for quantum information processing and quantum conversion schemes with these systems. We study theoretically a simple geometry consisting of a one-dimensional array of holes with an abrupt defect, considering the ferrimagnet yttrium iron garnet (YIG) as the basis material. We show that both magnon and photon modes can be localized at the defect, and use symmetry arguments to select an optimal pair of modes in order to maximize the coupling. We show that an optomagnonic coupling in the kHz range is achievable in this geometry, and discuss possible optimization routes in order to improve both coupling strengths and optical losses.

Chromatic Dispersion Based Wide-Band, Fiber-Coupled, Tunable Light Source for Hyperspectral Imaging

Gargi Sharma, Sheeza Kainat Naveed, Asha Parmar, Kanwarpal Singh

Hyperspectral imaging is a powerful label-free imaging technique that provides topological and spectral information at once. In this work, we have designed and characterized a hyperspectral source based on the chromatic dispersion property of off-the-shelf lenses and converted a supercontinuum laser light source into a hyperspectral imaging light source for 490 nm to 900 nm wavelength range with a spectral resolution of 3.5 nm to 18 nm respectively. The potential of the source was demonstrated by imaging two color dots with different absorption bands. Further, we generated the hypercube of the lily ovary and dense connective tissue and measured their spectral signature as a function of wavelength. We also imaged the lower tongue of a healthy volunteer at 540 nm, 630 nm, and white light. Our simple hyperspectral light source design can easily be incorporated in a standard endoscope or microscope to perform hyperspectral imaging.

Polarization of Light: In Classical, Quantum, and Nonlinear Optics

Maria V. Chekhova, Peter Banzer

This book starts with the description of polarization in classical optics, including also a chapter on crystal optics, which is necessary to understand the use of nonlinear crystals. In addition, spatially non-uniform polarization states are introduced and described. Further, the role of polarization in nonlinear optics is discussed. The final chapters are devoted to the description and applications of polarization in quantum optics and quantum technologies.

Broadband mid-infrared supercontinuum generation in dispersion-engineered As2S3-silica nanospike waveguides pumped by 2.8 μm femtosecond laser

Pan Wang, Jiapeng Huang, Shangran Xie, Johann Troles, Philip St. J. Russell

Broadband mid-infrared (IR) supercontinuum laser sources are essential for spectroscopy in the molecular fingerprint region. Here, we report generation of octave-spanning and coherent mid-IR supercontinua in As2S3-silica nanospike hybrid waveguides pumped by a custom-built 2.8 μm femtosecond fiber laser. The waveguides are formed by pressure-assisted melt-filling of molten As2S3 into silica capillaries, allowing the dispersion and nonlinearity to be precisely tailored. Continuous coherent spectra spanning from 1.1 μm to 4.8 μm (30 dB level) are observed when the waveguide is designed so that 2.8 μm lies in the anomalous dispersion regime. Moreover, linearly tapered millimeter-scale As2S3-silica waveguides are fabricated and investigated for the first time, to the best of our knowledge, showing much broader supercontinua than uniform waveguides, with improved spectral coherence. The waveguides are demonstrated to be long-term stable and water-resistant due to the shielding of the As2S3 by the fused silica sheath. They offer an alternative route to generating broadband mid-IR supercontinua, with applications in frequency metrology and molecular spectroscopy, especially in humid and aqueous environments.

Effects of coherence on temporal resolution

Syamsundar De, Jano Gil-Lopez, Benjamin Brecht, Christine Silberhorn, Luis Sanchez-Soto, Z. Hradil, J. Rehacek

Measuring small separations between two optical sources, either in space or in time, constitute an important metrological challenge as standard<br>intensity-only measurements fail for vanishing separations. Contrarily, it has been established that appropriate coherent mode projections can appraise<br>arbitrarily small separations with quantum-limited precision. However, the question of whether the optical coherence brings any metrological advantage to<br>mode projections is still a point of debate. Here, we elucidate this problem by experimentally investigating the effect of varying coherence on estimating the<br>temporal separation between two single-photon pulses. We show that, for an accurate interpretation, special attention must be paid to properly normalize<br>the quantum Fisher information to account for the strength of the signal. Our experiment demonstrates that coherent mode projections are optimal for any<br>degree of coherence.<br>

AIDeveloper: deep learning image classification in life science and beyond

Martin Kräter, Shada Abuhattum Hofemeier, Despina Soteriou, Angela Jacobi, Thomas Krüger, Jochen Guck, Maik Herbig

Artificial intelligence (AI)‐based image analysis has increased drastically in recent years. However, all applications use individual solutions, highly specialized for a particular task. Here, an easy‐to‐use, adaptable, and open source software, called AIDeveloper (AID) to train neural nets (NN) for image classification without the need for programming is presented. AID provides a variety of NN‐architectures, allowing to apply trained models on new data, obtain performance metrics, and export final models to different formats. AID is benchmarked on large image datasets (CIFAR‐10 and Fashion‐MNIST). Furthermore, models are trained to distinguish areas of differentiated stem cells in images of cell culture. A conventional blood cell count and a blood count obtained using an NN are compared, trained on >1.2 million images, and demonstrated how AID can be used for label‐free classification of B‐ and T‐cells. All models are generated by non‐programmers on generic computers, allowing for an interdisciplinary use.

Frenet–Serret analysis of helical Bloch modes in N-fold rotationally symmetric rings of coupled spiraling optical waveguides

Yang Chen, Philip St.J. Russell

The behavior of electromagnetic waves in chirally twisted structures is a topic of enduring interest, dating back at least to the 1940s invention of the microwave travelling-wave-tube amplifier and culminating in contemporary studies of chiral metamaterials, metasurfaces, and photonic crystal fibers (PCFs). Optical fibers with chiral microstructures, drawn from a spinning preform, have many useful properties, exhibiting, for example, circular birefringence and circular dichroism. It has recently been shown that chiral fibers with N-fold rotationally symmetric (symmetry group CN) transverse microstructures support families of helical Bloch modes (HBMs), each of which consists of a superposition of azimuthal Bloch harmonics (or optical vortices). An example is a fiber with N coupled cores arranged in a ring around its central axis (N-core single-ring fiber). Although this type of fiber can be readily modeled using scalar coupled-mode theory, a full description of its optical properties requires a vectorial analysis that takes account of the polarization state of the light, which is particularly important in studies of circular and vortical birefringence. In this paper, we develop, using an orthogonal 2D helicoidal coordinate system embedded in a cylindrical surface at constant radius, a rigorous vector coupled-mode description of the fields using local Frenet–Serret frames that rotate and twist with each of the N cores. The analysis places on a firm theoretical footing a previous HBM theory in which a heuristic approach was taken, based on physical intuition of the properties of Bloch waves. After a detailed review of the polarization evolution in a single spiraling core, analysis of the N-core single-ring system is carefully developed step by step. Accuracy limits of the analysis are assessed by comparison with the results of finite element modeling, focusing in particular on the dispersion, polarization states, and transverse field profiles of the HBMs. We believe this study provides clarity into what can sometimes be a rather difficult field and will facilitate further exploration of real-world applications of these fascinating waveguiding systems.

Quantum circuit optimization with deep reinforcement learning

Thomas Fösel, Murphy Yuezhen Niu, Florian Marquardt, Li Li (李力)

A central aspect for operating future quantum computers is quantum circuit optimization, i.e., the search for efficient realizations of quantum algorithms given the device capabilities. In recent years, powerful approaches have been developed which focus on optimizing the high-level circuit structure. However, these approaches do not consider and thus cannot optimize for the hardware details of the quantum architecture, which is especially important for near-term devices. To address this point, we present an approach to quantum circuit optimization based on reinforcement learning. We demonstrate how an agent, realized by a deep convolutional neural network, can autonomously learn generic strategies to optimize arbitrary circuits on a specific architecture, where the optimization target can be chosen freely by the user. We demonstrate the feasibility of this approach by training agents on 12-qubit random circuits, where we find on average a depth reduction by 27% and a gate count reduction by 15%. We examine the extrapolation to larger circuits than used for training, and envision how this approach can be utilized for near-term quantum devices.

Self-learning Machines based on Hamiltonian Echo Backpropagation

Victor Lopez-Pastor, Florian Marquardt

A physical self-learning machine can be defined as a nonlinear dynamical system that can be trained on data (similar to artificial neural networks), but where the update of the internal degrees of freedom that serve as learnable parameters happens autonomously. In this way, neither external processing and feedback nor knowledge of (and control of) these internal degrees of freedom is required. We introduce a general scheme for self-learning in any time-reversible Hamiltonian system. We illustrate the training of such a self-learning machine numerically for the case of coupled nonlinear wave fields.

Mechanical properties of cell- and microgel bead-laden oxidized alginate-gelatin hydrogels

Thomas Distler, Lena Kretzschmar, Dominik Schneidereit, Salvatore Girardo, Ruchi Goswami, Oliver Friedrich, Rainer Detsch, Jochen Guck, Aldo R. Boccaccini, et al.

3D-printing technologies, such as biofabrication, capitalize on the homogeneous distribution and growth of cells inside biomaterial hydrogels, ultimately aiming to allow for cell differentiation, matrix remodeling, and functional tissue analogues. However, commonly, only the mechanical properties of the bioinks or matrix materials are assessed, while the detailed influence of cells on the resulting mechanical properties of hydrogels remains insufficiently understood. Here, we investigate the properties of hydrogels containing cells and spherical PAAm microgel beads through multi-modal complex mechanical analyses in the small- and large-strain regimes. We evaluate the individual contributions of different filler concentrations and a non-fibrous oxidized alginate-gelatin hydrogel matrix on the overall mechanical behavior in compression, tension, and shear. Through material modeling, we quantify parameters that describe the highly nonlinear mechanical response of soft composite materials. Our results show that the stiffness significantly drops for cell- and bead concentrations exceeding four million per milliliter hydrogel. In addition, hydrogels with high cell concentrations (≥6 mio ml−1) show more pronounced material nonlinearity for larger strains and faster stress relaxation. Our findings highlight cell concentration as a crucial parameter influencing the final hydrogel mechanics, with implications for microgel bead drug carrier-laden hydrogels, biofabrication, and tissue engineering.

Spin cat states in ferromagnetic insulators

Sanchar Sharma, V. A. S. V. Bittencourt, Alexy D. Karenowska, Silvia Viola-Kusminskiy

Generating nonclassical states in macroscopic systems is a long-standing challenge. A promising platform in the context of this quest are novel hybrid systems based on magnetic dielectrics, where photons can couple strongly and coherently to magnetic excitations, although a nonclassical state therein is yet to be observed. We propose a scheme to generate a magnetization cat state, i.e., a quantum superposition of two distinct magnetization directions, using a conventional setup of a macroscopic ferromagnet in a microwave cavity. Our scheme uses the ground state of an ellipsoid shaped magnet, which displays anisotropic quantum fluctuations akin to a squeezed vacuum. The magnetization collapses to a cat state by either a single photon or a parity measurement of the microwave cavity state. We find that a cat state with two components separated by ∼5ℏ is feasible and briefly discuss potential experimental setups that can achieve it.

Benchmarking quantum tomography completeness and fidelity with machine learning

Yong Siah Teo, Seongwook Shin, Hyunseok Jeong, Yosep Kim, Yoon-Ho Kim, Gleb I. Struchalin, Egor V. Kovlakov, Stanislav S. Straupe, Sergei P. Kulik, et al.

We train convolutional neural networks to predict whether or not a set of measurements is informationally complete to uniquely reconstruct any given quantum state with no prior information. In addition, we perform fidelity benchmarking based on this measurement set without explicitly carrying out state tomography. The networks are trained to recognize the fidelity and a<br>reliable measure for informational completeness through collective encoding of quantum measurements, data and target states into grayscale images. By<br>gradually accumulating measurements and data, these convolutional networks can efficiently certify a low-measurement-cost quantum-state characterization<br>scheme. We confirm the potential of this machine-learning approach by presenting experimental results for both spatial-mode and multiphoton systems<br>of large dimensions. These predictions are further shown to improve with noise recognition when the networks are trained with additional bootstrapped training sets from real experimental data.<br>

Entanglement-assisted quantum communication beating the quantum Singleton bound

Markus Grassl

Brun, Devetak, and Hsieh [Science 314, 436 (2006)] demonstrated that preshared entanglement between the sender and receiver enables quantum communication protocols that have better parameters than schemes without the assistance of entanglement. Subsequently, the same authors derived a version of the so-called quantum Singleton bound that relates the parameters of the entanglement-assisted quantum-error-correcting codes proposed by them. We present an entanglement-assisted quantum communication scheme with parameters violating this bound in certain ranges. For a fixed transmission rate, our scheme allows one to correct a larger fraction of errors.

Agile and versatile quantum communication: Signatures and secrets

Stefan Richter, Matthew Thornton, Imran Khan, Hamish Scott, Kevin Jaksch, Ulrich Vogl, Birgit Stiller, Gerd Leuchs, Christoph Marquardt, et al.

Agile cryptography allows for a resource-efficient swap of a cryptographic core in case the security of an underlying classical cryptographic algorithm becomes compromised. Conversely, versatile cryptography allows the user to switch the cryptographic task without requiring any knowledge of its inner workings.<br>In this paper, we suggest how these related principles can be applied to the field of quantum cryptography by explicitly demonstrating two quantum cryptographic protocols, quantum digital signatures (QDS) and quantum secret sharing (QSS), on the same hardware sender and receiver platform. Crucially, the protocols differ only in their classical postprocessing. The system is also suitable for quantum key distribution (QKD) and is highly compatible with deployed telecommunication infrastructures, since it uses standard quadrature phase-shift keying encoding and heterodyne detection. For the first time, QDS protocols are modified to allow for postselection at the receiver, enhancing protocol performance. The cryptographic primitives QDS and QSS are inherently multipartite, and we prove that they are secure not only when a player internal to the task is dishonest, but also when (external) eavesdropping on the quantum channel is allowed. In our first proof-of-principle demonstration of an agile and versatile quantum communication system, the quantum states are distributed at GHz rates. A 1-bit message may be securely signed using our QDS protocols in less than 0.05 ms over a 2-km fiber link and in less than 0.2 s over a 20-km fiber link. To our knowledge, this also marks the first demonstration of a continuous-variable direct QSS protocol.

Floquet theory for temporal correlations and spectra in time-periodic open quantum systems: Application to squeezed parametric oscillation beyond the rotating-wave approximation

Carlos Navarrete-Benlloch, Rafael Garcés, Naeimeh Mohseni, German J. de Valcarcel

Open quantum systems can display periodic dynamics at the classical level either due to external periodic modulations or to self-pulsing phenomena typically following a Hopf bifurcation. In both cases, the quantum fluctuations around classical solutions do not reach a quantum-statistical stationary state, which prevents adopting the simple and reliable methods used for stationary quantum systems. Here we put forward a general and efficient method to compute two-time correlations and corresponding spectral densities of time-periodic open quantum systems within the usual linearized (Gaussian) approximation for their dynamics. Using Floquet theory, we show how the quantum Langevin equations for the fluctuations can be efficiently integrated by partitioning the time domain into one-period duration intervals, and relating the properties of each period to the first one. Spectral densities, like squeezing spectra, are computed similarly, now in a two-dimensional temporal domain that is treated as a chessboard with one-period × one-period cells. This technique avoids cumulative numerical errors as well as efficiently saving computational time. As an illustration of the method, we analyze the quantum fluctuations of a damped parametrically driven oscillator (degenerate parametric oscillator) below threshold and far away from rotating-wave approximation conditions, which is a relevant scenario for modern low-frequency quantum oscillators. Our method reveals that the squeezing properties of such devices are quite robust against the amplitude of the modulation or the low quality of the oscillator, although optimal squeezing can appear for parameters that are far from the ones predicted within the rotating-wave approximation.

suggested by editors

Excitation transport with collective radiative decay

Francesca Mineo, Claudiu Genes

We investigate a one-dimensional quantum emitter chain where transport of excitations and correlations takes place via nearest neighbor, dipole-dipole interactions. In the presence of collective radiative emission, we show that a phase imprinting wavepacket initialization procedure can lead to subradiant transport and can preserve quantum correlations. In the context of cavity mediated transport, where emitters are coupled to a common delocalized optical mode, we analyze the effect of frequency disorder and nonidentical photon-emitter couplings on excitation transport.

Special Topic: Quantum sensing with correlated light sources

Alex S. Clark, Maria V. Chekhova, Jonathan C F Matthews, John G. Rarity, Rupert F. Oulton

Engineering Fast High-Fidelity Quantum Operations With Constrained Interactions

Thales Figueiredo Roque, Aashish A Clerk, Hugo Ribeiro

Understanding how to tailor quantum dynamics to achieve a desired evolution is a crucial problemin almost all quantum technologies. We present a very general method for designing high-efficiencycontrol sequences that are always fully compatible with experimental constraints on available inter-actions and their tunability. Our approach reduces in the end to finding control fields by solvinga set of time-independent linear equations. We illustrate our method by applying it to a numberof physically-relevant problems: the strong-driving limit of a two-level system, fast squeezing in aparametrically driven cavity, the leakage problem in transmon qubit gates, and the acceleration ofSNAP gates in a qubit-cavity system.

Analytical model of the deformation-induced inertial dynamics of a magnetic vortex

Myoung-Woo Yoo, Francesca Mineo, Joo-Von Kim

We present an analytical model to account for the deformation-induced inertial dynamics of a magnetic vortex. The model is based on a deformation of the vortex core profile based on the Döring kinetic field, whereby the deformation amplitudes are promoted to dynamical variables in a collective-coordinate approach that provides a natural extension to the Thiele model. This extended model describes complex transients due to inertial effects and the variation of the effective mass with velocity. The model also provides a quantitative description of the inertial dynamics leading up to vortex core reversal, which is analogous to the Walker transition in domain wall dynamics. Our work paves the way for a standard prescription for describing the inertial effects of topological magnetic solitons

Genome-wide CRISPR screens reveal a specific ligand for the glycan-binding immune checkpoint receptor Siglec-7

Simon Wisnovsky, Leonhard Möckl, Stacy A. Malaker, Kayvon Pedram, Gaelen T. Hess, Nicholas M. Riley, Melissa A. Gray, Benjamin A. H. Smith, Michael C. Bassik, et al.

Glyco-immune checkpoint receptors, molecules that inhibit immune cell activity following binding to glycosylated cell-surface<br>antigens, are emerging as attractive targets for cancer immunotherapy.<br>Defining biologically relevant ligands that bind and activate such receptors, however, has historically been a significant challenge. Here, we present a CRISPRi genomic screening strategy that allowed unbiased identification of the key genes required for<br>cell-surface presentation of glycan ligands on leukemia cells that bind the glyco-immune checkpoint receptors Siglec-7 and Siglec-9.<br>This approach revealed a selective interaction between Siglec-7 and the mucin-type glycoprotein CD43. Further work identified a specific N-terminal glycopeptide region of CD43 containing clusters of disialylated O-glycan tetrasaccharides that form specific Siglec-7 binding motifs. Knockout or blockade of CD43 in leukemia<br>cells relieves Siglec-7-mediated inhibition of immune killing activity.<br>This work identifies a potential target for immune checkpoint blockade therapy and represents a generalizable approach to dissection of glycan–receptor interactions in living cells.

Entangled photons from subwavelength nonlinear films

Tomas Santiago-Cruz, Vitaliy Sultanov, Haizhong Zhang, Leonid A. Krivitsky, Maria V. Chekhova

Miniaturized entangled photon sources, in particular based on subwavelength metasurfaces, are highly demanded for the development of integrated quantum photonics. Here, as a first step towards the development of quantum optical metasurfaces (QOMs), we demonstrate generation of entangled photons via spontaneous parametric down-conversion (SPDC) from subwavelength films. We achieve photon pair generation with a high coincidence-to-accidental ratio in lithium niobate and gallium phosphide nanofilms. By implementing the fiber spectroscopy of SPDC in nanofilms, we measure a spectrum with a bandwidth of 500 nm, limited only by the overall detection efficiency. The spectrum reveals vacuum field enhancement due to a Fabry–Perot resonance inside the nonlinear films. It also suggests a strategy for observing SPDC from QOM. Our experiments lay the groundwork for future development of flat SPDC sources, including QOM.

Self-Switching Kerr Oscillations of Counterpropagating Light in Microresonators

Michael T. M. Woodley, Lewis Hill, Leonardo Del Bino, Gian-Luca Oppo, Pascal Del'Haye

We report the experimental and numerical observation of oscillatory antiphase switching between counterpropagating light beams in Kerr ring microresonators, where dominance between the intensities of the two beams is periodically or chaotically exchanged. Self-switching occurs in balanced regimes of operation and is well captured by a simple coupled dynamical system featuring only the self- and crossphase Kerr nonlinearities. Switching phenomena are due to temporal instabilities of symmetry-broken states combined with attractor merging, which restores the broken symmetry on average. Self-switching of counterpropagating light is robust for realizing controllable, all-optical generation of waveforms, signal encoding, and chaotic cryptography.

suggested by editors

Optofluidic photonic crystal fiber microreactors for in-situ studies of carbon nanodot-driven photoreduction

Philipp Koehler, Takashi Lawson, Julian Neises, Janina Willkomm, Benjamin C. M. Martindale , Georgina A. M. Hutton, Daniel Antón-García, Ava Lage, Alexander S. Gentleman, et al.

Performing quantitative in situ spectroscopic analysis on minuscule sample volumes is a common difficulty in photochemistry. To address this challenge, we use a hollow-core photonic crystal fiber (HC-PCF) that guides light at the center of a microscale liquid channel and acts as an optofluidic microreactor with a reaction volume of less than 35 nL. The system was used to demonstrate in situ optical detection of photoreduction processes that are key components of many photocatalytic reaction schemes. The photoreduction of viologens (XV2+) to the radical XV•+ in a homogeneous mixture with carbon nanodot (CND) light absorbers is studied for a range of different carbon dots and viologens. Time-resolved absorption spectra, measured over several UV irradiation cycles, are interpreted with a quantitative kinetic model to determine photoreduction and photobleaching rate constants. The powerful combination of time-resolved, low-volume absorption spectroscopy and kinetic modeling highlights the potential of optofluidic microreactors as a highly sensitive, quantitative, and rapid screening platform for novel photocatalysts and flow chemistry in general.

Optical memories and switching dynamics of counterpropagating light states in microresonators

Leonardo Del Bino, Niall Moroney, Pascal Del'Haye

The Kerr nonlinearity can be a key enabler for many digital photonic circuits as it allows access to bistable states needed for all-optical memories and switches. A common technique is to use the Kerr shift to control the resonance frequency of a resonator and use it as a bistable, optically-tunable filter. However, this approach works only in a narrow power and frequency range or requires the use of an auxiliary laser. An alternative approach is to use the asymmetric bistability between counterpropagating light states resulting from the interplay between self- and cross-phase modulation, which allows light to enter a ring resonator in just one direction. Logical HIGH and Low states can be represented and stored as the direction of circulation of light, and controlled by modulating the input power. Here we study the switching speed, operating laser frequency and power range, and contrast ratio of such a device. We reach a bitrate of 2 Mbps in our proof-of-principle device over an optical frequency range of 1 GHz and an operating power range covering more than one order of magnitude. We also calculate that integrated photonic circuits could exhibit bitrates of the order of Gbps, paving the way for the realization of robust and simple all-optical memories, switches, routers and logic gates that can operate at a single laser frequency with no additional electrical power. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License

SU(1, 1) covariant s-parametrized maps

Andrei B. Klimov, Ulrich Seyfarth, Hubert de Guise, Luis Sanchez-Soto

We propose a practical recipe to compute the s-parametrized maps for systems with SU(1, 1) symmetry using a connection between the Q- and P-symbols through the action of an operator invariant under the group. This establishes equivalence relations between s-parametrized SU(1, 1)-covariant maps. The particular case of the self-dual (Wigner) phase-space functions, defined on the upper sheet of the two-sheet hyperboloid (or, equivalently, inside the Poincaré disc) are analysed.

Squeezed comb states

Namrata Shukla, Stefan Nimmrichter, Barry C. Sanders

Continuous-variable codes are an expedient solution for quantum information processing and quantum communication involving optical networks. Here we characterize the squeezed comb, a finite superposition of equidistant squeezed coherent states on a line, and its properties as a continuous-variable encoding choice for a logical qubit. The squeezed comb is a realistic approximation to the ideal code proposed by Gottesman et al. [D. Gottesman, A. Kitaev, and J. Preskill, Phys. Rev. A 64, 012310 (2001)], which is fully protected against errors caused by the paradigmatic types of quantum noise in continuous-variable systems: damping and diffusion. This is no longer the case for the code space of finite squeezed combs, and noise robustness depends crucially on the encoding parameters. We analyze finite squeezed comb states in phase space, highlighting their complicated interference features and characterizing their dynamics when exposed to amplitude damping and Gaussian diffusion noise processes. We find that squeezed comb states are more suitable and less error prone when exposed to damping, which speaks against standard error-correction strategies that employ linear amplification to convert damping into easier-to-describe isotropic diffusion noise.

A switch in pdgfrb+ cell-derived ECM composition prevents inhibitory scarring and promotes axon regeneration in the zebrafish spinal cord

Vasiliki Tsata, Stephanie Möllmert, Christine Schweitzer, Julia Kolb, Conrad Möckel, Benjamin Böhm, Gonzalo Rosso, Christian Lange, Mathias Lesche, et al.

In mammals, perivascular cell-derived scarring after spinal cord injury impedes axonal regrowth. In contrast, the extracellular matrix (ECM) in the spinal lesion site of zebrafish is permissive and required for axon regeneration. However, the cellular mechanisms underlying this interspecies difference have not been investigated. Here, we show that an injury to the zebrafish spinal cord triggers recruitment of pdgfrb+ myoseptal and perivascular cells in a PDGFR signaling-dependent manner. Interference with pdgfrb+ cell recruitment or depletion of pdgfrb+ cells inhibits axonal regrowth and recovery of locomotor function. Transcriptional profiling and functional experiments reveal that pdgfrb+ cells upregulate expression of axon growth-promoting ECM genes (cthrc1a and col12a1a/b) and concomitantly reduce synthesis of matrix molecules that are detrimental to regeneration (lum and mfap2). Our data demonstrate that a switch in ECM composition is critical for axon regeneration after spinal cord injury and identify the cellular source and components of the growth-promoting lesion ECM.

Achieving the ultimate quantum timing resolution

Vahid Ansari, Benjamin Brecht, Jano Gil-López, John M. Donohue, Jaroslav Řeháček, Zdeněk Hradil, Luis Sanchez-Soto, Christine Silberhorn

Accurate time-delay measurement is at the core of many modern technologies.<br>Here, we present a temporal-mode demultiplexing scheme that achieves the<br>ultimate quantum precision for the simultaneous estimation of the temporal<br>centroid, the time offset, and the relative intensities of an incoherent<br>mixture of ultrashort pulses at the single-photon level. We experimentally<br>resolve temporal separations ten times smaller than the pulse duration, as well<br>as imbalanced intensities differing by a factor of 10^2. This represents an<br>improvement of more than an order of magnitude over the best standard methods<br>based on intensity detection.<br>

Cross-phase modulational instability of circularly polarized helical Bloch modes carrying optical vortices in a chiral three-core photonic crystal fiber

Paul Roth, Michael Frosz, Linda Weise, Philip Russell, Gordon Wong

We report the first, to the best of our knowledge, observation of cross-phase modulational instability (XPMI) of circularly polarized helical Bloch modes carrying optical vortices in a twisted photonic crystal fiber with a three-fold symmetric core, formed by spinning the fiber preform during the draw. When the fiber is pumped by a superposition of left-circular polarization (LCP) and right-circular polarization (RCP) modes, a pair of orthogonal circularly polarized sidebands of opposite topological charge is generated. When, on the other hand, a pure LCP (or RCP) mode is launched, the XPMI gain is zero, and no sidebands are seen. This observation has not been seen before in any system and is unique to chiral structures with N-fold rotational symmetry. The polarization state and topological charge of the generated sidebands are measured. By decomposing the helical Bloch modes into their azimuthal harmonics, we are able to deduce the selection rules for the appearance of modulational instability sidebands. We showed that the four waves in the nonlinear mixing process must exhibit the same set of azimuthal harmonic orders.