Kinetics of Many-Body Reservoir Engineering

Hugo Ribeiro, Florian Marquardt

Recent advances illustrate the power of reservoir engineering in applications to many-body sys-tems, such as quantum simulators based on superconducting circuits. We present a frameworkbased on kinetic equations and noise spectra that can be used to understand both the transientand long-time behavior of many particles coupled to an engineered reservoir in a number-conservingway. For the example of a bosonic array, we show that the non-equilibrium steady state can beexpressed, in a wide parameter regime, in terms of a modified Bose-Einstein distribution with anenergy-dependent temperature.

Field theory of monochromatic optical beams II. Classical and quantum paraxial fields

Andrea Aiello

This work is the second part of an investigation aiming at the study of optical wave equations from a field-theoretic point of view. Here, we study classical and quantum aspects of scalar fields satisfying the paraxial wave equation. First, we determine conservation laws for energy, linear and angular momentum of paraxial fields in a classical context. Then, we proceed with the quantization of the field. Finally, we compare our result with the traditional ones.

Field theory of monochromatic optical beams I. classical fields

Andrea Aiello

We study monochromatic, scalar solutions of the Helmholtz and paraxial wave equations from a field-theoretic point of view. We introduce appropriate time-independent Lagrangian densities for which the Euler-Lagrange equations reproduces either Helmholtz and paraxial wave equations with the $z$-coordinate, associated with the main direction of propagation of the fields, playing the same role of time in standard Lagrangian theory. For both Helmholtz and paraxial scalar fields, we calculate the canonical energy-momentum tensor and determine the continuity equations relating ``energy'' and ``momentum'' of the fields. Eventually, the reduction of the Helmholtz wave equation to a useful first-order Dirac form, is presented. This work sheds some light on the intriguing and not so acknowledged connections between angular spectrum representation of optical wavefields, cosmological models and physics of black holes.

Partial optomechanical refrigeration via multi-mode cold-damping feedback

Christian Sommer, Claudiu Genes

We provide a fully analytical treatment for the partial refrigeration of the thermal motion of a quantum mechanical resonator under the action of feedback. As opposed to standard cavity optomechanics where the aim is to isolate and cool a single mechanical mode, the aim here is to extract the thermal energy from many vibrational modes within a large frequency bandwidth. We consider a standard cold-damping technique, where homodyne readout of the cavity output field is fed into a feedback loop that provides a cooling action directly applied on the mechanical resonator. Analytical and numerical results predict that low final occupancies are achievable independent of the number of modes addressed by the feedback, as long as the cooling rate is smaller than the intermode frequency separation. For resonators exhibiting a few nearly degenerate pairs of modes, cooling is less efficient and a weak dependence on the number of modes is obtained. These scalings hint toward the design of frequency-resolved mechanical resonators, where efficient refrigeration is possible via simultaneous cold-damping feedback.

Quantum state transfer via acoustic edge states in a 2D optomechanical array

Marc-Antoine Lemonde, Vittorio Peano, Peter Rabl, Dimitris G Angelakis

We propose a novel hybrid platform where solid-state spin qubits are coupled to the acoustic modes ofa two-dimensional array of optomechanical(OM)nano cavities. Previous studies of coupled OMcavities have shown that in the presence of strong optical drivingfields, the interplay between thephoton-phonon interaction and their respective inter-cavity hopping allows the generation oftopological phases of sound and light. In particular, the mechanical modes can enter a Chern insulatorphase where the time-reversal symmetry is broken. In this context, we exploit the robust acoustic edgestates as a chiral phononic waveguide and describe a state transfer protocol between spin qubitslocated in distant cavities. We analyze the performance of this protocol as a function of the relevantsystem parameters and show that a high-fidelity and purely unidirectional quantum state transfer canbe implemented under experimentally realistic conditions. As a specific example, we discuss theimplementation of such topological quantum networks in diamond based OM crystals where pointdefects such as silicon-vacancy centers couple to the chiral acoustic channel via strain.

On-the-fly particle metrology in hollow-core photonic crystal fibre

Abhinav Sharma, Shangran Xie, Richard Zeltner, Philip Russell

Efficient monitoring of airborne particulate matter (PM), especially particles with aerodynamic diameter less than 2.5 µm (PM2.5), is crucial for improving public health. Reliable information on the concentration, size distribution and chemical characteristics of PMs is key to evaluating air pollution and identifying its sources. Standard methods for PM2.5 characterization require sample collection from the atmosphere and post-analysis using sophisticated equipment in a laboratory environment, and are normally very time-consuming. Although optical methods based on analysis of scattering of free-space laser beams or evanescent fields are in principle suitable for real-time particle counting and sizing, lack of knowledge of the refractive index in these methods not only leads to inevitable sizing ambiguity but also prevents identification of the particle material. In the case of evanescent wave detection, the system lifetime is strongly limited by adhesion of particles to the surfaces. Here we report a novel technique for airborne particle metrology based on hollow-core photonic crystal fibre. It offers in situ particle counting, sizing and refractive index measurement with effectively unlimited device lifetime, and relies on optical forces that automatically capture airborne particles in front of the hollow core and propel them into the fibre. The resulting transmission drop, together with the time-of-flight of the particles passing through the fibre, provide unambiguous mapping of particle size and refractive index with high accuracy. The technique offers unique advantages over currently available real-time particle metrology systems, and can be directly applied to monitoring air pollution in the open atmosphere as well as precise particle characterization in a local environment such as a closed room or a reaction vessel.

Highly efficient deep UV generation by four-wave mixing in gas-filled hollow-core photonic crystal fiber

Federico Belli, Amir Abdolvand, John Travers, Philip Russell

We report on a highly efficient experimental scheme for the generation of deep-ultraviolet (UV) ultrashort light pulses using four-wave mixing in gas-filled kagomé-style photonic crystal fiber. By pumping with ultrashort, few microjoule pulses centered at 400 nm, we generate an idler pulse at 266 nm and amplify a seeded signal at 800 nm. We achieve remarkably high pump-to-idler energy conversion efficiencies of up to 38%. Although the pump and seed pulse durations are ∼100  fs, the generated UV spectral bandwidths support sub-15 fs pulses. These can be further extended to support few-cycle pulses. Four-wave mixing in gas-filled hollow-core fibers can be scaled to high average powers and different spectral regions such as the vacuum UV (100–200 nm).

Broadly tunable photon pair generation in a suspended-core fiber

J. Hammer, M. V. Chekhova, D. R. Häupl, R. Pennetta, N. Y. Joly

Nowadays fiber biphoton sources are nearly as popular as crystal-based ones. They offer a single spatial mode and easy integrability into optical networks. However, fiber sources lack the broad tunability of crystals, which do not require a tunable pump. Here, we report a broadly tunable biphoton source based on a suspended core fiber. This is achieved by introducing pressurized gas into the fibers hollow channels, changing the step index. The mechanism circumvents the need for a tunable pump laser, making this a broadly tunable fiber biphoton source with a convenient tuning mechanism, comparable to crystals. We report a continuous shift of 0.30THz/bar of the sidebands, using up to 25 bar of argon.

Collisional quantum thermometry

Stella Seah, Stefan Nimmrichter, Daniel Grimmer, Jader P. Santos, Valerio Scarini, Gabriel T. Landi

We introduce a general framework for thermometry based on collisional models, where ancillas probe thetemperature of the environment through an intermediary system. This allows for the generation of correlatedancillas even if they are initially independent. Using tools from parameter estimation theory, we show through aminimal qubit model that individual ancillas can already outperform the thermal Cramer-Rao bound. In addition,when probed collectively, these ancillas may exhibit superlinear scalings of the Fisher information, especiallyfor weak system-ancilla interactions. Our approach sets forth the notion of metrology in a sequential interactionssetting, and may inspire further advances in quantum thermometry.

Experimental reconstruction of spatial Schmidt modes for a wide-field SU(1,1) interferometer

Gaetano Frascella, Roman V Zakharov, Olga V Tikhonova, Maria Chekhova

We study the spatial mode content at the output of a wide-field SU(1, 1) interferometer, i.e. a nonlinear interferometer comprising two coherently-pumped spatially-multimode optical parametric amplifiers placed in sequence with a focusing element in between. This device is expected to provide a phase sensitivity below the shot-noise limit for multiple modes over a broad angular range. To reconstruct the spatial modes and their weights, we implement a simple method based on the acquisition of only intensity distributions. The eigenmode decomposition of the field is obtained through the measurement of the covariance of intensities at different spatial points. We investigate both the radial and azimuthal (orbital angular momentum) modes and show that their total number is large enough to enable applications of the interferometer in spatially-resolved phase measurements.

Laser refrigeration of gas filled hollow-core fibres

Christian Sommer, Nicolas Y. Joly, Helmut Ritsch, Claudiu Genes

We evaluate prospects, performance and temperature limits of a new approach to macroscopic scale laser refrigeration. The considered refrigeration device is based on exciplex-mediated frequency up-conversion inside hollow-core fibers pressurized with a dopant - buffer gas mixture. Exciplexes are excited molecular states formed by two atoms (dopant and buffer) which do not form a molecule in the ground state but exhibit bound states for electronically excited states. The cooling cycle consists of absorption of laser photons during atomic collisions inducing light assisted exciplex formation followed by blue-shifted spontaneous emission on the atomic line of the bare dopant atoms after molecular separation. This process, closely related to reversing the gain mechanism in excimer lasers, allows for a large fraction of collision energy to be extracted in each cycle. The hollow-core fiber plays a crucial role as it allows for strong light-matter interactions over a long distance, which maximizes the cooling rate per unit volume and the cooling efficiency per injected photon while limiting re-absorption of spontaneously emitted photons channeled into unguided radiation modes. Using quantum optical rate equations and refined dynamical simulations we derive general conditions for efficient cooling of both the gas and subsequently of the surrounding solid state environment. Our analytical approach is applicable to any specific exciplex system considered and reveals the shape of the exciplex potential landscapes as well as the density of the dopant as crucial tuning knobs. The derived scaling laws allow for the identification of optimal exciplex characteristics that help to choose suitable gas mixtures that maximize the refrigeration efficiency for specific applications.

Full-field characterization of helical Bloch modes guided in twisted coreless photonic crystal fiber

Paul Roth, Gordon Wong, Michael Frosz, Goran Ahmed, Philip Russell

It was recently reported that a photonic crystal fiber (PCF) with no structural core guides light if a permanent chiral twist is introduced by spinning the fiber preform during the draw. The intriguing guidance mechanism behind this novel effect has many remarkable features; for example, it intrinsically supports circularly polarized helical Bloch modes (HBMs) that carry multiple optical vortices, making twisted PCFs of interest in fields such as optical micromanipulation, imaging, quantum optics, and optical communications. Here we report for the first time, to the best of our knowledge, that a twisted coreless PCF supports not just one but a family of guided HBMs, each member of which has a unique transverse field distribution and harmonic spectrum. By making detailed interferometric measurements of the near-field phase and amplitude distributions of HBMs, and expanding them as a series of Bessel beams, we are able to extract the amplitude of each azimuthal and radial HBM harmonic. Good agreement is found with the numerical solutions of Maxwell’s equations. The results shed light on the properties of this curious new optical phenomenon.

Carrier-envelope-phase-stable soliton-based pulse compression to 4.4  fs and ultraviolet generation at the 800  kHz repetition rate

Alexey Ermolov, Christian Heide, Philip Dienstbier, Felix Köttig, Francesco Tani, Peter Hommelhoff, Philip Russell

In this Letter, we report the generation of a femtosecond supercontinuum extending from the ultraviolet to the near-infrared spectrum and detection of its carrier-envelope-phase (CEP) variation by f-to-2f interferometry. The spectrum is generated in a gas-filled hollow-core photonic crystal fiber, where soliton dynamics allows the CEP-stable self-compression of the optical parametric chirped-pulse amplifier pump pulses at 800 nm to a duration of 1.7 optical cycles, followed by dispersive wave emission. The source provides up to 1 μJ of pulse energy at the 800 kHz repetition rate, resulting in 0.8 W of average power, and it can be extremely useful, for example in strong-field physics, pump–probe measurements, and ultraviolet frequency comb metrology.

Non-invasive real-time characterization of hollow-core photonic crystal fibers using whispering gallery mode spectroscopy

Michael Frosz, Riccardo Pennetta, Michael Enders, Goran Ahmed, Philip Russell

Single-ring hollow-core photonic crystal fibers, consisting of a ring of one or two thin-walled glass capillaries surrounding a central hollow core, hold great promise for use in optical communications and beam delivery, and are already being successfully exploited for extreme pulse compression and efficient wavelength conversion in gases. However, achieving low loss over long (km) lengths requires highly accurate maintenance of the microstructure—a major fabrication challenge. In certain applications, for example adiabatic mode transformers, it is advantageous to taper the fibers, but no technique exists for measuring the delicate and complex microstructure without first cleaving the taper at several positions along its length. In this Letter, we present a simple non-destructive optical method for measuring the diameter of individual capillaries. Based on recording the spectrum scattered from whispering gallery modes excited in the capillary walls, the technique is highly robust, allowing real-time measurement of fiber structure during the draw with sub-micron accuracy.

Almost thermal operations: inhomogeneous reservoirs

Angeline Shu, Yu Cai, Stella Seah, Stefan Nimmrichter, Valerio Scarini

The resource theory of thermal operations explains the state transformations that are possible ina very specific thermodynamic setting: there is only one thermal bath, auxiliary systems can onlybe in the corresponding thermal state (free states), and the interaction must commute with the freeHamiltonian (free operation). In this paper we study the mildest deviation: the reservoir particlesare subject to inhomogeneities, either in the local temperature (introducing resource states) or inthe local Hamiltonian (generating a resource operation). For small inhomogeneities, the two modelsgenerate the same channel and thus the same state transformations. However, their thermodynamicsis significantly different when it comes to work generation or to the interpretation of the “secondlaws of thermal operations”.

Towards third-order parametric down-conversion in optical fibers

A. Cavanna, J. Hammer, C. Okoth, R. Ortiz-Ricardo, H. Cruz-Ramirez, K. Garay-Palmett, A. B. U’Ren, M. H. Frosz, X. Jiang, N. Y. Joly, et al.

Optical fibers have been considered an optimal platform for third-order parametric down- conversion since they can potentially overcome the weak third-order nonlinearity by their long interaction length. Here we present, in the first part, a theoretical derivation for the conversion rate both in the case of spontaneous generation and in the presence of a seed beam. Then we review three types of optical fibers and we examine their properties in terms of conversion efficiency and practical feasibility.

Shaping Field Gradients for Nanolocalization

Sergey Nechayev, Jörg Eismann, Martin Neugebauer, Peter Banzer

Deep sub-wavelength localization and displacement sensing of optical nanoantennas have emerged as extensively pursued objectives in nanometrology, where focused beams serve as high-precision optical rulers while the scattered light provides an optical readout. Here, we show that in these schemes using an optical excitation as a position gauge implies that the sensitivity to displacements of a nanoantenna depends on the spatial gradients of the excitation field. Specifically, we explore one of such novel localization schemes based on appearance of transversely spinning fields in strongly confined optical beams, resulting in far-field segmentation of left- and right-hand circular polarizations of the scattered light, an effect known as the giant spin-Hall effect of light. We construct vector beams with augmented transverse spin density gradient in the focal plane and experimentally confirm enhanced sensitivity of the far-field spin-segmentation to lateral displacements of an electric-dipole nanoantenna. We conclude that sculpturing of electromagnetic field gradients and intelligent design of scatterers pave the way towards future improvements in displacement sensitivity.

Optically Addressable Array of Optomechanically Compliant Glass Nanospikes on the Endface of a Soft-Glass Photonic Crystal Fiber

Zheqi Wang, Shangran Xie, Xin Jiang, Fehim Babic, Jiapeng Huang, Riccardo Pennetta, Johannes Köhler, Philip Russell

Arrays of elongated nanoscale structures with suitable optical and mechanical properties can act as probes of numerous physical processes at the nanoscale, with applications in, for example, high-resolution optical imaging and atomic force microscopy. They can also be used to investigate optomechanical phenomena such as synchronization among large assemblies of mechanical oscillators. Here we report a novel and versatile technique for fabricating two-dimensional light-guiding arrays of mechanically compliant glass nanospikes with lengths up to several hundred micrometers. The procedure starts with a multicore fiber made by stacking and drawing capillaries and rods of two different germanate glasses with markedly different acid etching rates. After a suitable etching step, a free-standing nanospike array is created at the fiber endface. The parameters are chosen so that there is evanescent coupling between adjacent nanospikes, which gives rise to strong optomechanical forces that can be exploited to drive and control the mechanical motion of the nanospikes and thus the optical properties.

Error suppression in adiabatic quantum computing with qubit ensembles

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

In the standard approach to adiabatic quantum computing (AQC), quantum information storedon qubits are adiabatically evolved to find the ground state of a problem Hamiltonian. Here weinvestigate a variation of AQC where qubit ensembles are used in place of qubits. We identify twodistinct regimes for a given problem Hamiltonian under thismapping as a function of the ensemblesizeN. At a critical ensemble sizeNc, the nature of the first excited state changes from beingmacroscopically distinct spin configuration to a single particle perturbation of the ground state.AboveNcthe minimum gap for large ensembles is well predicted by mean-field theory and theAQC performance improves withN, realizing error-suppression due to duplication of the quantuminformation. While belowNcthe performance is mixed, and can increase withN. For randomlychosen problem instances Nc tends to be smaller than realistic ensemble sizes, hence we expect theensemble version of AQC to work well in a great majority of cases. Our approach shows it is possibleto perform AQC without the necessity of controlling individual qubits, allowing an alternative routetowards implementing AQC.

Wide-field SU(1,1) interferometer

Gaetano Frascella, E. E. Mikhailov, N. Takanashi, R. V. Zakharov, O. V. Tikhonova, Maria Chekhova

An SU(1,1) interferometer uses a sequence of two optical parametric amplifiers for achieving sub-shot-noise sensitivity to a phase shift introduced in between. We present the first realization of a wide-field SU(1,1) interferometer, where the use of a focusing element enables spatially multimode operation within a broad angle. Over this angle, the interference phase is found to be flat. This property is important for the high sensitivity to the phase front disturbance. Further, -4.3 +/- 0.7 dB quadrature squeezing, an essential requirement to the high sensitivity, is experimentally demonstrated for plane-wave modes inside the interferometer. Such an interferometer is useful not only for quantum metrology, but also in remote sensing, enhanced sub-shot-noise imaging, and quantum information processing. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Indefinite-Mean Pareto Photon Distribution from Amplified Quantum Noise

Mathieu Manceau, Kirill Spasibko, Gerd Leuchs, Radim Filip, Maria Chekhova

Extreme events appear in many physics phenomena, whenever the probability distribution has a "heavy tail" differing very much from the equilibrium one. Most unusual are the cases of power-law (Pareto) probability distributions. Among their many manifestations in physics, from "rogue waves" in the ocean to Levy flights in random walks, Pareto dependences can follow very different power laws. For some outstanding cases, the power exponents are less than 2, leading to indefinite values not only for higher moments but also for the mean. Here we present the first evidence of indefinite-mean Pareto distribution of photon numbers at the output of nonlinear effects pumped by parametrically amplified vacuum noise, known as bright squeezed vacuum (BSV). We observe a Pareto distribution with power exponent 1.31 when BSV is used as a pump for supercontinuum generation, and other heavy-tailed distributions (however, with definite moments) when it pumps optical harmonics generation. Unlike in other fields, we can flexibly control the Pareto exponent by changing the experimental parameters. This extremely fluctuating light is interesting for ghost imaging and for quantum thermodynamics as a resource to produce more efficiently nonequilibrium states by single-photon subtraction, the latter of which we demonstrate experimentally.

Accelerated adiabatic quantum gates: optimizing speed versus robustness

Hugo Ribeiro, Aashish A. Clerk

We develop new protocols for high-fidelity single qubit gates that exploit and extend theoretical ideas for accelerated adiabatic evolution. Our protocols are compatible with qubit architectures with highly isolated logical states, where traditional approaches are problematic; a prime example are superconducting fluxonium qubits. By using an accelerated adiabatic protocol we can enforce the desired adiabatic evolution while having gate times that are comparable to the inverse adiabatic energy gap (a scale that is ultimately set by the amount of power used in the control pulses). By modelling the effects of decoherence, we explore the tradeoff between speed and robustness that is inherent to shortcuts-to-adiabaticity approaches.

Macroscopicity of quantum mechanical superposition tests via hypothesis falsification

Björn Schrinski, Stefan Nimmrichter, Benjamin A. Stickler, Klaus Hornberger

We establish an objective scheme to determine the macroscopicity of quantum mechanical super-position tests, which is based on the Bayesian hypothesis falsification of macrorealistic modificationsof quantum theory. The measure uses the raw data gathered in an experiment, taking into accountall measurement uncertainties, and can be used to directly assess any conceivable quantum test.We determine the resulting macroscopicity for three recent tests of quantum physics: double-wellinterference of Bose-Einstein condensates, Leggett-Garg tests with atomic random walks, and en-tanglement generation and read-out of nanomechanical oscillators.

Nonlinear dynamics of weakly dissipative optomechanical systems

Thales Figueiredo Roque, Florian Marquardt, Oleg M. Yevtushenko

Optomechanical systems attract a lot of attention because they provide a novel platform for quantum measurements, transduction, hybrid systems, and fundamental studies of quantum physics. Their classical nonlinear dynamics is surprisingly rich and so far remains underexplored. Works devoted to this subject have typically focussed on dissipation constants which are substantially larger than those encountered in current experiments, such that the nonlinear dynamics of weakly dissipative optomechanical systems is almost uncharted waters. In this work, we fill this gap and investigate the regular and chaotic dynamics in this important regime. To analyze the dynamical attractors, we have extended the "Generalized Alignment Index" method to dissipative systems. We show that, even when chaotic motion is absent, the dynamics in the weakly dissipative regime is extremely sensitive to initial conditions. We argue that reducing dissipation allows chaotic dynamics to appear at a substantially smaller driving strength and enables various routes to chaos. We identify three generic features in weakly dissipative classical optomechanical nonlinear dynamics: the Neimark-Sacker bifurcation between limit cycles and limit tori (leading to a comb of sidebands in the spectrum), the quasiperiodic route to chaos, and the existence of transient chaos.

Towards fully integrated photonic displacement sensors

Ankan Bag, Martin Neugebauer, Uwe Mick, Silke Christiansen, Sebastian A Schulz, Peter Banzer

The field of optical metrology with its high precision position, rotation and wavefront sensors represents the basis for lithography and high resolution microscopy. However, the on-chip integration - a task highly relevant for future nanotechnological devices - necessitates the reduction of the spatial footprint of sensing schemes by the deployment of novel concepts. A promising route towards this goal is predicated on the controllable directional emission of the fundamentally smallest emitters of light, i.e. dipoles, as an indicator. Here we realize an integrated displacement sensor based on the directional emission of Huygens dipoles excited in an individual dipolar antenna. The position of the antenna relative to the excitation field determines its directional coupling into a six-way crossing of photonic crystal waveguides. In our experimental study supported by theoretical calculations, we demonstrate the first prototype of an integrated displacement sensor with a standard deviation of the position accuracy below λ/300 at room temperature and ambient conditions.

Route from single-pulse to multi-pulse states in a mid-infrared soliton fiber laser

Jiapeng Huang, Meng Pang, Xin Jiang, Wenbin He, Philip Russell

State-of-the-art ultrafast mid-IR fiber lasers deliver optical solitons with durations of several hundred femtoseconds. The Er- or Ho-doped fluoride gain fibers generally used in these lasers have strong anomalous dispersion at ∼3 µm, which generally forces them to operate in the soliton regime. Here we report that a pulse-energy clamping effect, caused by the buildup of intracavity nonlinearities, limits the shortest obtainable pulse durations in these mid-infrared soliton fiber lasers. Excessive intra-cavity energy results in soliton instability, collapse and fragmentation into a variety of stable multi-pulse states, including phase-locked soliton molecules and harmonically mode-locked states. We report that the spectral evolution of the mid-IR laser pulses can be recorded between roundtrips through stretching their second-harmonic signal in a 25-km-length of single-mode fiber. Using a modified dispersive Fourier transform set-up, we were able to perform for the first time spectro-temporal measurements of mid-IR laser pulses both in the pulsed state and during pulse collapse and fragmentation. The results provide insight into the complex nonlinear dynamics of mid-IR soliton fiber lasers and open up new opportunities for obtaining a variety of stable multi-pulse mode-locked states at mid-IR wavelengths.

Kommt der künstliche Physiker?

Thomas Fösel, Florian Marquardt, Talitha Weiß

2016 besiegte das Computerprogramm AlphaGo einen der weltbesten Go‐Spieler. Damit rückte eine technische Revolution ins Bewusstsein der breiten Öffentlichkeit: Selbstlernende künstliche neuronale Netze sind zunehmend in der Lage, Menschen bei bestimmten Aufgaben zu schlagen. Zahlreiche Anwendungen, von der Bilderkennung bis zur automatischen Übersetzung, revolutionieren momentan die Technik – und auch Physik und Astronomie bieten viele potenzielle Einsatzmöglichkeiten. In der Astronomie können neuronale Netze das automatische Klassifizieren von Galaxien übernehmen. In der Statistischen Physik sind Magnetisierungsmuster von ferro‐ oder paramagnetischen Zuständen ein Beispiel. Ein anderes Beispiel ist die Suche nach Quantenfehler‐Korrekturstrategien in zukünftigen Quantencomputern. Unsere Forschung konnte zeigen, dass künstliche neuronale Netze mittels Reinforcement Learning hier bereits eigenständig neue Korrekturstrategien entwickeln können.

Interferometric Scattering (iSCAT) Microscopy & Related Techniques

Richard W. Taylor, Vahid Sandoghdar, Vasily Astratov

Interferometric scattering (iSCAT) microscopy is a powerful tool for label-free sensitive detection and imaging of nanoparticles to high spatiotemporal resolution. As it was born out of detection principles central to conventional microscopy, we begin by surveying the historical development of the microscope to examine how the exciting possibility for interferometric scattering microscopy with sensitivities sufficient to observe single molecules has become a reality. We discuss the theory of interferometric detection and also issues relevant to achieving a high detection sensitivity and speed. A showcase of numerous applications and avenues of novel research across various disciplines that iSCAT microscopy has opened up is also presented.

Coherent nonlinear optics of quantum emitters in nanophotonic waveguides

Pierre Türschmann, Hanna Le Jeannic, Signe F. Simonsen, Harald Haakh, Stephan Götzinger, Vahid Sandoghdar, Peter Lodahl, Nir Rotenberg

Coherent quantum optics, where the phase of a photon is not scrambled as it interacts with an emitter, lies at the heart of many quantum optical effects and emerging technologies. Solid-state emitters coupled to nanophotonic waveguides are a promising platform for quantum devices, as this element can be integrated into complex photonic chips. Yet, preserving the full coherence properties of the coupled emitter-waveguide system is challenging because of the complex and dynamic electromagnetic landscape found in the solid state. Here, we review progress toward coherent light-matter interactions with solid-state quantum emitters coupled to nanophotonic waveguides. We first lay down the theoretical foundation for coherent and nonlinear light-matter interactions of a two-level system in a quasi-one-dimensional system, and then benchmark experimental realizations. We discuss higher order nonlinearities that arise as a result of the addition of photons of different frequencies, more complex energy level schemes of the emitters, and the coupling of multiple emitters via a shared photonic mode. Throughout, we highlight protocols for applications and novel effects that are based on these coherent interactions, the steps taken toward their realization, and the challenges that remain to be overcome.

Maxwell's lesser demon

Stella Seah, Stefan Nimmrichter, Valerio Scarani

We discuss a self-contained spin-boson model for a measurement-driven engine, in which a demongenerates work from random thermal excitations of a quantum spin via measurement and feedbackcontrol. Instead of granting it full direct access to the spin state and to Landauer’s erasure strokes foroptimal performance, we restrict this lesser demon’s action to pointer measurements, i.e. random orcontinuous interrogations of a damped mechanical oscillator that assumes macroscopically distinctpositions depending on the spin state. The engine could reach simultaneously high output powersand efficiencies and can operate in temperature regimes where quantum Otto engines would fail.

Perturbation theory of optical resonances of deformed dielectric spheres

Andrea Aiello, Jack G. E. Harris, Florian Marquardt

We analyze the optical resonances of a dielectric sphere whose surface has been slightly deformed in an arbitrary way. Setting up a perturbation series up to second order, we derive both the frequency shifts and modified linewidths. Our theory is applicable, for example, to freely levitated liquid drops or solid spheres, which are deformed by thermal surface vibrations, centrifugal forces or arbitrary surface waves. A dielectric sphere is effectively an open system whose description requires the introduction of non-Hermitian operators characterized by complex eigenvalues and not normalizable eigenfunctions. We avoid these difficulties using the Kapur-Peierls formalism which enables us to extend the popular Rayleigh-Schrödinger perturbation theory to the case of electromagnetic Debye's potentials describing the light fields inside and outside the near-spherical dielectric object. We find analytical formulas, valid within certain limits, for the deformation-induced first- and second-order corrections to the central frequency and bandwidth of a resonance. As an application of our method, we compare our results with preexisting ones finding full agreement.

Non-exponential decay of a giant artificial atom

Gustav Andersson, Baladitya Suri, Lingzhen Guo, Thomas Aref, Per Delsing

In quantum optics, light–matter interaction has conventionally been studied using small atoms interacting with electromagnetic fields with wavelength several orders of magnitude larger than the atomic dimensions1,2. In contrast, here we experimentally demonstrate the vastly different ‘giant atom’ regime, where an artificial atom interacts with acoustic fields with wavelength several orders of magnitude smaller than the atomic dimensions. This is achieved by coupling a superconducting qubit3 to surface acoustic waves at two points with separation on the order of 100 wavelengths. This approach is comparable to controlling the radiation of an atom by attaching it to an antenna. The slow velocity of sound leads to a significant internal time-delay for the field to propagate across the giant atom, giving rise to non-Markovian dynamics4. We demonstrate the non-Markovian character of the giant atom in the frequency spectrum as well as non-exponential relaxation in the time domain.

Generation of 1.5 cycle pulses at 780 nm at oscillator repetition rates with stable carrier-envelope phase

Philip Dienstbier, Francesco Tani, Takuya Higuchi, John Travers, Philip Russell, Peter Hommelhoff

We demonstrate a spectral broadening and compression setup for carrier-envelope phase (CEP) stable sub-10-fs Ti:sapphire oscillator pulses resulting in 3.9 fs pulses spectrally centered at 780 nm. Pulses from the oscillator with 2 nJ energy are launched into a 1 mm long all-normal dispersive solid-core photonic crystal fiber and spectrally broadened to more than one octave. Subsequent pulse compression is achieved with a phase-only 4f pulse shaper. Second harmonic frequency resolved optical gating with a ptychographic reconstruction algorithm is used to obtain the spectral phase, which is fed back as a phase mask to the shaper display for pulse compression. The compressed pulses are CEP stable with a long term standard deviation of 0.23 rad for the CEP noise and 0.32 rad for the integrated rms phase jitter. The high total throughput of 15% results in a remaining pulse energy of about 300 pJ at 80 MHz repetition rate. With these parameters and the ability to tailor the spectral phase, the system is well suited for waveform sensitive photoemission experiments with needle tips or nanostructures and can be easily adapted to other sub-10 fs ultra-broadband Ti:sapphire oscillators.

Generation of broadband circularly polarized supercontinuum light in twisted photonic crystal fibers

Rafal Sopalla, Gordon Wong, Nicolas Joly, Michael Frosz, Xin Jiang, Goran Ahmed, Philip Russell

We compare the properties of the broadband supercontinuum (SC) generated in twisted and untwisted solid-core photonic crystal fibers when pumped by circularly polarized 40 picosecond laser pulses at 1064 nm. In the helically twisted fiber, fabricated by spinning the preform during the draw, the SC is robustly circularly polarized across its entire spectrum whereas, in the straight fiber, axial fluctuations in linear birefringence and polarization-dependent nonlinear effects cause the polarization state to vary randomly with the wavelength. Theoretical modelling confirms the experimental results. Helically twisted photonic crystal fibers permit the generation of pure circularly polarized SC light with excellent polarization stability against fluctuations in input power and environmental perturbations.

Dynamically Generated Synthetic Electric Fields for Photons

Petr Zapletal, Stefan Walter, Florian Marquardt

Static synthetic magnetic fields give rise to phenomena including the Lorentz force and the quantum Hall effect even for neutral particles, and they have by now been implemented in a variety of physical systems. Moving towards fully dynamical synthetic gauge fields allows, in addition, for backaction of the particles' motion onto the field. If this results in a time-dependent vector potential, conventional electromagnetism predicts the generation of an electric field. Here, we show how synthetic electric fields for photons arise self-consistently due to the nonlinear dynamics in a driven system. Our analysis is based on optomechanical arrays, where dynamical gauge fields arise naturally from phonon-assisted photon tunneling. We study open, one-dimensional arrays, where synthetic magnetic fields are absent. However, we show that synthetic electric fields can be generated dynamically, which, importantly, suppress photon transport in the array. The generation of these fields depends on the direction of photon propagation, leading to a novel mechanism for a photon diode, inducing nonlinear nonreciprocal transport via dynamical synthetic gauge fields.

Interaction of light carrying orbital angular momentum with a chiral dipolar scatterer

Pawel Wozniak, Israel De León, Katja Höflich, Gerd Leuchs, Peter Banzer

The capability to distinguish the handedness of circularly polarized light is a well-known intrinsic property of a chiral nanostructure. It is a long-standing controversial debate, however, whether a chiral object can also sense the vorticity, or the orbital angular momentum (OAM), of a light field. Since OAM is a non-local property, it seems rather counter-intuitive that a point-like chiral object could be able to distinguish the sense of the wave-front of light carrying OAM. Here, we show that a dipolar chiral nanostructure is indeed capable of distinguishing the sign of the phase vortex of the incoming light beam. To this end, we take advantage of the conversion of the sign of OAM, carried by a linearly polarized Laguerre-Gaussian beam, into the sign of optical chirality upon tight focusing. Our study provides for a deeper insight into the discussion of chiral light-matter interactions and the respective role of OAM.

Idealised EPR states from non-phase matched parametric down conversion

C. Okoth, E. Kovlakov, F. Bönsel, A. Cavanna, S. Straupe, S. P. Kulik, M. V. Chekhova

Entanglement of high dimensional states is becoming increasingly important for quantum commu- nication and computing. The most common source of entangled photons is spontaneous parametric down conversion (SPDC), where the degree of frequency and momentum entanglement is deter- mined by the non-linear interaction volume. Here we show that by reducing the length of a highly non-linear material to the micrometer scale it is possible to relax the longitudinal phase matching condition and reach record levels of transverse wavevector entanglement. From a micro-sized layer of lithium niobate we estimate the number of entangled angular modes to be over 1200. The entan- glement is measured both directly using correlation measurements and indirectly using stimulated emission tomography. The high entanglement of the state generated can be used to massively in- crease the quantum information capacity of photons, but it also opens up the possibility to improve the resolution of many quantum imaging techniques.

Interferometric Scattering Microscopy: Seeing Single Nanoparticles and Molecules via Rayleigh Scattering

Richard W. Taylor, Vahid Sandoghdar

Fluorescence microscopy has been the workhorse for investigating optical phenomena at the nanometer scale but this approach confronts several fundamental limits. As a result, there have been a growing number of activities toward the development of fluorescent-free imaging methods. In this Mini Review, we demonstrate that elastic scattering, the most ubiquitous and oldest optical contrast mechanism, offers excellent opportunities for sensitive detection and imaging of nanoparticles and molecules at very high spatiotemporal resolution. We present interferometric scattering (iSCAT) microscopy as the method of choice, explain its theoretical foundation, discuss its experimental nuances, elaborate on its deep connection to bright-field imaging and other established microscopies, and discuss its promise as well as challenges. A showcase of numerous applications and avenues made possible by iSCAT demonstrates its rapidly growing impact on various disciplines concerned with nanoscopic phenomena.

High-Throughput Microfluidic Characterization of Erythrocyte Shapes and Mechanical Variability

Felix Reichel, Johannes Mauer, Ahmad Ahsan Nawaz, Gerhard Gompper, Jochen Guck, Dmitry A. Fedosov

The motion of red blood cells (RBCs) in microchannels is important for microvascular blood flow and biomedical applications such as blood analysis in microfluidics. The current understanding of the complexity of RBC shapes and dynamics in microchannels is mainly based on several simulation studies, but there are a few systematic experimental investigations. Here, we present a combined study that systematically characterizes RBC behavior for a wide range of flow rates and channel sizes. Even though simulations and experiments generally show good agreement, experimental observations demonstrate that there is no single well-defined RBC state for fixed flow conditions but rather a broad distribution of states. This result can be attributed to the inherent variability in RBC mechanical properties, which is confirmed by a model that takes the variation in RBC shear elasticity into account This represents a significant step toward a quantitative connection between RBC behavior in microfluidic devices and their mechanical properties, which is essential for a high-throughput characterization of diseased cells.

Quench Dynamics in 1D Optomechanical Arrays

Sadegh Raeisi, Florian Marquardt

Non-equilibrium dynamics induced by rapid changes of external parameters is relevant for a widerange of scenarios across many domains of physics. For waves in spatially periodic systems, quencheswill alter the bandstructure and generate new excitations. In the case of topological bandstructures,defect modes at boundaries can be generated or destroyed when quenching through a topologicalphase transition. Here, we demonstrate that optomechanical arrays are a promising platform forstudying such dynamics, as their bandstructure can be tuned temporally by a control laser. Westudy the creation of nonequilibrium optical and mechanical excitations in 1D arrays, including abosonic version of the Su-Schrieffer-Heeger model. These ideas can be transferred to other systemssuch as driven nonlinear cavity arrays.

Coherent coupling of single molecules to on-chip ring resonators

Dominik Rattenbacher, Alexey Shkarin, Jan Renger, Tobias Utikal, Stephan Götzinger, Vahid Sandoghdar

We report on cryogenic coupling of organic molecules to ring microresonators obtained by looping subwavelength waveguides (nanoguides). We discuss fabrication and characterization of the chip-based nanophotonic elements which yield a resonator finesse in the order of 20 when covered by molecular crystals. Our observed extinction dips from single molecules reach 22%, consistent with an expected enhancement factor of up to 11 for the molecular emission into the nanoguide. Future efforts will aim at efficient coupling of a handful of molecules via their interaction with a ring microresonator mode, setting the ground for the realization of quantum optical cooperative effects.

Emission of circularly polarized light by a linear dipole

Martin Neugebauer, Peter Banzer, Sergey Nechayev

Controlling the polarization state and the propagation direction of photons is a fundamental prerequisite for many nanophotonic devices and a precursor for future on-chip communication, where the emission properties of individual emitters are particularly relevant. Here, we report on the emission of partially circularly polarized photons by a linear dipole. The underlying effect is linked to the near-field part of the angular spectrum of the dipole, and it occurs in any type of linear dipole emitter, ranging from atoms and quantum dots to molecules and dipole-like antennas. We experimentally observe it by near-field to far-field transformation at a planar dielectric interface and numerically demonstrate the utility of this phenomenon by coupling the circularly polarized light to the individual paths of crossing waveguides.

Cavity Quantum Electrodynamics with Frequency-Dependent Reflectors

Ondrej Černotík, Aurelien Dantan, Claudiu Genes

We present a general framework for cavity quantum electrodynamics with strongly frequency-dependent mirrors. The method is applicable to a variety of reflectors exhibiting sharp internal resonances as can be realized, for example, with photonic-crystal mirrors or with two-dimensional atomic arrays around subradiant points. Our approach is based on a modification of the standard input-output formalism to explicitly include the dynamics of the mirror’s internal resonance. We show how to directly extract the interaction parameters from the comparison with classical transfer matrix theory and how to treat the non-Markovian dynamics of the cavity field mode introduced by the mirror’s internal resonance. As an application within optomechanics, we illustrate how a non-Markovian Fano-resonance cavity with a flexible photonic-crystal mirror can provide both sideband resolution as well as strong heating suppression in optomechanical cooling. This approach, amenable to a wide range of systems, opens up possibilities for using hybrid frequency-dependent reflectors in cavity quantum electrodynamics for engineering novel forms of light-matter interactions.

Vectorial vortex generation and phase singularities upon Brewster reflection

Rene Barczyk, Sergey Nechayev, Muhammad Abdullah Butt, Gerd Leuchs, Peter Banzer

We experimentally demonstrate the emergence of a purely azimuthally polarized vectorial vortex beam with a phase singularity upon Brewster reflection of focused circularly polarized light from a dielectric substrate. The effect originates from the polarizing properties of the Fresnel reflection coefficients described in Brewster’s law. An astonishing consequence of this effect is that the reflected field’s Cartesian components acquire local phase singularities at Brewster’s angle. Our observations are crucial for polarization microscopy and open new avenues for the generation of exotic states of light based on spin-to-orbit coupling, without the need for sophisticated optical elements.

Electrically driven single-photon superradiance from molecular chains in a plasmonic nanocavity

Yang Luo, Gong Chen, Yang Zhang, Li Zhang, Yunjie Yu, Fanfang Kong, Xiaojun Tian, Yao Zhang, Chongxin Shan, Yi Luo, et al.

We demonstrate single-photon superradiance from artificially constructed nonbonded zinc-phthalocyanine molecular chains of up to 12 molecules. We excite the system via electron tunneling in a plasmonic nanocavity and quantitatively investigate the interaction of the localized plasmon with single-exciton superradiant states resulting from dipole-dipole coupling. Dumbbell-like patterns obtained by subnanometer resolved spectroscopic imaging disclose the coherent nature of the coupling associated with superradiant states while second-order photon correlation measurements demonstrate single-photon emission. The combination of spatially resolved spectral measurements with theoretical considerations reveals that nanocavity plasmons dramatically modify the linewidth and intensity of emission from the molecular chains, but they do not dictate the intrinsic coherence of the superradiant states. Our studies shed light on the optical properties of molecular collective states and their interaction with nanoscopically localized plasmons.

Optical traps and anti-traps for glass nanoplates in hollow waveguides

Mehmet Can Günendi, Shangran Xie, David Novoa, Philip Russell

We study theoretically the optical forces acting on glass nanoplates introduced into hollow waveguides, and show that, depending on the sign of the laser detuning relative to the nanoplate resonance, optomechanical back-action between nanoplate and hollow waveguide can create both traps and anti-traps at intensity nodes and anti-nodes in the supermode field profile, behaving similarly to those experienced by cold atoms when the laser frequency is red or blue detuned of an atomic resonance. This arises from dramatic distortions to the mode profile in the hollow waveguide when the nanoplate is off-resonant, producing gradient forces that vary strongly with nanoplate position. In a planar system, we show that when the nanoplate is constrained by an imaginary mechanical spring, its position exhibits strong bistability as the base position is varied. We then treat a two-dimensional system consisting of an anti-resonant nanoplate in the hollow core of a photonic crystal fiber, and predict the stable dark trapping of nanoplate at core center against both translational and rotational motion. The results show that spatial and angular position of nano-scale objects in hollow waveguides can be optically controlled by launching beams with appropriately synthesized transverse field profiles.

Experimental demonstration of linear and spinning Janus dipoles for polarisation and wavelength selective near-field coupling

M. F. Picardi, Martin Neugebauer, Jörg Eismann, Gerd Leuchs, Peter Banzer, F. J. Rodriguez-Fortuno, A. V. Zayats

The electromagnetic field scattered by nano-objects contains a broad range of wave vectors and can be efficiently coupled to waveguided modes. The dominant ontribution to scattering from subwavelength dielectric and plasmonic nanoparticles is determined by electric and magnetic dipolar responses. Here, we experimentally demonstrate spectral and phase selective excitation of Janus dipoles, sources with electric and magnetic dipoles oscillating out of phase, in order to control near-field interference and directional coupling to waveguides. We show that by controlling the polarisation state of the dipolar excitations and the excitation wavelength to adjust their relative contributions, directionality and coupling strength can be fully tuned. Furthermore, we introduce a novel spinning Janus dipole featuring cylindrical symmetry in the near and far field, which results in either omnidirectional coupling or noncoupling. Controlling the propagation of guided light waves via fast and robust near-field interference between polarisation components of a source is required in many applications in nanophotonics and quantum optics.

Mimicking Chiral Light-Matter Interaction

Sergey Nechayev, Peter Banzer

We demonstrate that electric-dipole scatterers can mimic chiral light-matter interaction by generating far-field circular polarization upon scattering, even though the optical chirality of the incident field as well as that of the scattered light is zero. The presented effect originates from the fact that electric-dipole scatterers respond selectively only to the incident electric field, which eventually results in depolarization of the transmitted beam and in generation of far-field circular polarization. To experimentally demonstrate this effect we utilize a cylindrical vector beam with spiral polarization and a spherical gold nanoparticle positioned on the optical axis -- the axis of rotational symmetry of the system. Our experiment and a simple theoretical model address the fundamentals of duality symmetry in optics and chiral light-matter interactions, accentuating their richness and ubiquity yet in highly symmetric configurations.

Resonance inversion in a superconducting cavity coupled to artificial atoms and a microwave background

Juha Leppäkangas, Jan David Brehm, Ping Yang, Lingzhen Guo, Michael Marthaler, Alexey V. Ustinov, Martin Weides

We demonstrate how heating of an environment can invert the line shape of a driven cavity. We consider a superconducting coplanar cavity coupled to multiple artificial atoms. The measured cavity transmission is characterized by Fano-type resonances with a shape that is continuously tunable by bias current through nearby (magnetic flux) control lines. In particular, the same dispersive shift of the microwave cavity can be observed as a peak or a dip. We find that this Fano-peak inversion is possible due to a tunable interference between a microwave transmission through a background, with reactive and dissipative properties, and through the cavity, affected by bias-current induced heating. The background transmission occurs due to crosstalk with the multiple control lines. We show how such background can be accounted for by a Jaynes- or Tavis-Cummings model with modified boundary conditions between the cavity and transmission-line microwave fields. A dip emerges when cavity transmission is comparable with background transmission and dissipation. We find generally that resonance positions determine system energy levels, whereas resonance shapes give information on system fluctuations and dissipation.

MRI-guided robotic arm drives optogenetic fMRI with concurrent Ca2+ recording

Y Chen, P Pais-Roldán, X Chen, Michael Frosz, X Yu

Optical fiber-mediated optogenetic activation and neuronal Ca2+ recording in combination with fMRI provide a multi-modal fMRI platform. Here, we developed an MRI-guided robotic arm (MgRA) as a flexible positioning system with high precision to real-time assist optical fiber brain intervention for multi-modal animal fMRI. Besides the ex vivo precision evaluation, we present the highly reliable brain activity patterns in the projected basal forebrain regions upon MgRA-driven optogenetic stimulation in the lateral hypothalamus. Also, we show the step-wise optical fiber targeting thalamic nuclei and map the region-specific functional connectivity with whole-brain fMRI accompanied by simultaneous calcium recordings to specify its circuit-specificity. The MgRA also guides the real-time microinjection to specific deep brain nuclei, which is demonstrated by an Mn-enhanced MRI method. The MgRA represents a clear advantage over the standard stereotaxic-based fiber implantation and opens a broad avenue to investigate the circuit-specific functional brain mapping with the multi-modal fMRI platform.

Prospects of reinforcement learning for simultaneous damping of many mechanical modes

Christian Sommer, Muhammad Asjad, Claudiu Genes

We apply adaptive feedback for the refrigeration of a mechanical resonator, i.e. with the aim of simultaneously cooling the classical thermal motion of more than one vibrational degree of freedom. The feedback is obtained from a neural network trained via a reinforcement learning strategy to choose the correct sequence of actions from a finite set in order to reduce the total energy of all modes of vibration. The actions are realized either as optical modulations of spring constants or as radiation pressure induced momentum kicks. As a proof of principle we numerically show simultaneous cooling of four independent modes with an overall strong reduction of the total system temperature.

Towards Polarization-based Excitation Tailoring for Extended Raman Spectroscopy

Simon Grosche, Richard Hünermann, George Sarau, Silke Christiansen, Robert W. Boyd, Gerd Leuchs, Peter Banzer

Undoubtedly, Raman spectroscopy is one of the most elaborated spectroscopy tools in materials science, chemistry, medicine and optics. However, when it comes to the analysis of nanostructured specimens, accessing the Raman spectra resulting from an exciting electric field component oriented perpendicularly to the substrate plane is a difficult task and conventionally can only be achieved by mechanically tilting the sample, or by sophisticated sample preparation. Here, we propose a novel experimental method based on the utilization of polarization tailored light for Raman spectroscopy of individual nanostructures. As a proof of principle, we create three-dimensional electromagnetic field distributions at the nanoscale using tightly focused cylindrical vector beams impinging normally onto the specimen, hence keeping the conventional beam-path of commercial Raman systems. Using this excitation scheme, we experimentally show that the recorded Raman spectra of individual gallium-nitride nanostructures of sub-wavelength diameter used as a test platform depend sensitively on their location relative to the focal vector field. The observed Raman spectra can be attributed to the interaction with transverse or longitudinal electric field components. This novel technique may pave the way towards a characterization of Raman active nanosystems using full information of all Raman modes.

Spin-Orbit Coupling and the Evolution of Transverse Spin

Jörg Eismann, Peter Banzer, Martin Neugebauer

We investigate the evolution of transverse spin in tightly focused circularly polarized beams of light, where spin-orbit coupling causes a local rotation of the polarization ellipses upon propagation through the focal volume. The effect can be explained as a relative Gouy-phase shift between the circularly polarized transverse field and the longitudinal field carrying orbital angular momentum. The corresponding rotation of the electric transverse spin density is observed experimentally by utilizing a recently developed reconstruction scheme, which relies on transverse-spin-dependent directional scattering of a nano-probe.

Thresholdless deep and vacuum ultraviolet Raman frequency conversion in hydrogen-filled photonic crystal fiber

Manoj K. Mridha, David Novoa, Pooria Hosseini, Philip St. J. Russell

Coherent ultraviolet light has many uses, for example, in the study of molecular species relevant in biology and chemistry. Very few, if any, laser materials offer ultraviolet transparency along with damage-free operation at high-photon energies and laser power. Here we report efficient generation of narrowband deep and vacuum ultraviolet light using hydrogen-filled hollow-core photonic crystal fiber. Pumping above the stimulated Raman threshold at 532 nm, coherent molecular vibrations are excited in the gas, permitting thresholdless wavelength conversion in the ultraviolet with efficiencies close to 60%. The system is uniquely pressure tunable, allows spatial structuring of the out-coupled radiation, and shows excellent performance in the vacuum ultraviolet. As the underlying scattering process is effectively linear, our approach can also in principle operate at the single-photon level, when all other alternatives are extremely inefficient.

Properties of bright squeezed vacuum at increasing brightness

P. R. Sharapova, Gaetano Frascella, M. Riabinin, A. M. Perez, O. V. Tikhonova, S. Lemieux, R. W. Boyd, G Leuchs, Maria Chekhova

Bright squeezed vacuum (BSV) is a non-classical macroscopic state of light, which can be gen- erated through high-gain parametric down-conversion or four-wave mixing. Although BSV is an important tool in quantum optics and has a lot of applications, its theoretical description is still not complete. In particular, the existing description in terms of Schmidt modes fails to explain the spectral broadening observed in experiment as the mean number of photons increases. On the other hand, the semi-classical description accounting for the broadening does not allow to decouple the intermodal photon-number correlations. In this work, we present a new generalized theoretical approach to describe the spatial properties of BSV. This approach is based on exchanging the (k, t) and (ω, z) representations and solving a system of integro-differential equations. Our approach pre- dicts correctly the dynamics of the Schmidt modes and the broadening of the spectrum with the increase in the BSV mean photon number due to a stronger pumping. Moreover, the model succes- fully describes various properties of a widely used experimental configuration with two crystals and an air gap between them, namely an SU(1,1) interferometer. In particular, it predicts the narrowing of the intensity distribution, the reduction and shift of the side lobes, and the decline in the inter- ference visibility as the mean photon number increases due to stronger pumping. The presented experimental results confirm the validity of the new approach. The model can be easily extended to the case of frequency spectrum, frequency Schmidt modes and other experimental configurations.

Fabrication and non-destructive characterization of tapered single-ring hollow-core photonic crystal fiber

Riccardo Pennetta, Michael T. Enders, Michael H. Frosz, Francesco Tani, Philip St. J. Russell

We report on the properties of tapered single-ring hollow-core photonic-crystal fibers, with a particular emphasis on applications in nonlinear optics. The simplicity of these structures allows the use of non-invasive side-illumination to assess the quality of the tapering process, by observing the scattered far-field spectrum originating from excitation of whispering-gallery modes in the cladding capillaries. We investigate the conditions that ensure adiabatic propagation in the up- and down-tapers, and the scaling of loss-bands (created by anti-crossings between the core mode and modes in the capillary walls) with taper ratio. We also present an analytical model for the pressure profile along a tapered hollow fiber under differential pumping

Langevin Approach to Quantum Optics with Molecules

Michael Reitz, Christian Sommer, Claudiu Genes

We investigate the interaction between light and molecular systems modeled as quantum emitters coupled to a multitude of vibrational modes via a Holstein-type interaction. We follow a quantum Langevin equations approach that allows for analytical derivations of absorption and fluorescence profiles of molecules driven by classical fields or coupled to quantized optical modes. We retrieve analytical expressions for the modification of the radiative emission branching ratio in the Purcell regime and for the asymmetric cavity transmission associated with dissipative cross talk between upper and lower polaritons in the strong coupling regime. We also characterize the Förster resonance energy transfer process between donor-acceptor molecules mediated by the vacuum or by a cavity mode.

Large-Area 3D Plasmonic Crescents with Tunable Chirality

Eric S. A. Goerlitzer, Reza Mohammadi, Sergey Nechayev, Peter Banzer, Nicolas Vogel

Abstract Chiral plasmonic nanostructures hold promise for enhanced chiral sensing and circular dichroism spectroscopy of chiral molecules. It is therefore of interest to fabricate chiral plasmonic nanostructures with tailored chiroptical properties over large areas with reasonable effort. Here, a colloidal lithography approach is used to produce macroscopic arrays of sub-micrometer 3D chiral plasmonic crescent structures over areas >1 cm2. The chirality originates from symmetry breaking by the introduction of a step within the crescent structure. This step is produced by an intermediate layer of silicon dioxide onto which the metal crescent structure is deposited. It is experimentally demonstrated that the chiroptical properties in such structures can be tailored by changing the position of the step within the crescent. These experiments are complemented by finite element simulations and the application of a multipole expansion to elucidate the physical origin of the circular dichroism of the crescent structures.

Pump-probe multi-species CARS in a hollow-core PCF with a 20 ppm detection limit under ambient conditions

Rinat Tyumenev, Luisa Späth, Barbara M. Trabold, Goran Ahmed, Michael H. Frosz, Philip St. J. Russell

We report coherent anti-Stokes Raman spectroscopy (CARS) in a gas-filled single-ring hollow-core photonic crystal fiber (SR-PCF) using a pump-probe configuration. The long collinear path length offered by an SR-PCF strongly enhances the efficiency of the Raman interactions. Pressure tuning the zero-dispersion wavelength (ZDW) of the SR-PCF allows the Raman coherence prepared by seeded pumping at 515 nm to be used in the visible for phase-matched generation of an anti-Stokes signal from a probe in the ultraviolet. The unique dispersion profile in the vicinity of the ZDW enables simultaneous phase matching of all known Raman transitions. We demonstrate that simultaneous multi-species CARS with a detection limit of 20 ppm is possible with only 20 kW of peak pump power delivered by a single laser source.

Multi-twist polarization ribbon topologies in highly-confined optical fields

Thomas Bauer, Peter Banzer, Frédéric Bouchard, Sergej Orlov, Lorenzo Marrucci, Enrico Santamato, Robert W Boyd, Ebrahim Karimi, Gerd Leuchs

Electromagnetic plane waves, solutions to Maxwell's equations, are said to be 'transverse' in vacuum. Namely, the waves' oscillatory electric and magnetic fields are confined within a plane transverse to the waves' propagation direction. Under tight-focusing conditions however, the field can exhibit longitudinal electric or magnetic components, transverse spin angular momentum, or non-trivial topologies such as Möbius strips. Here, we show that when a suitably spatially structured beam is tightly focused, a 3-dimensional polarization topology in the form of a ribbon with two full twists appears in the focal volume. We study experimentally the stability and dynamics of the observed polarization ribbon by exploring its topological structure for various radii upon focusing and for different propagation planes.

Substrate-Induced Chirality in an Individual Nanostructure

Sergey Nechayev, Rene Barczyk, Uwe Mick, Peter Banzer

We experimentally investigate the chiral optical response of an individual nanostructure consisting of three equally sized spherical nanoparticles made of different materials and arranged in \ang{90} bent geometry. Placing the nanostructure on a substrate converts its morphology from achiral to chiral. Chirality leads to pronounced differential extinction, i.e., circular dichroism and optical rotation, or equivalently, circular birefringence, which would be strictly forbidden in the absence of a substrate or heterogeneity. This first experimental observation of the substrate-induced break of symmetry in an individual heterogeneous nanostructure sheds new light on chiral light-matter interactions at substrate-nanostructure interfaces.

Spatio-temporal measurement of ionization-induced modal index changes in gas-filled PCF by prism-assisted side-coupling

Barbara M. Trabold, Mallika I. Suresh, Johannes R. Köhler, Michael H. Frosz, Francesco Tani, Philip St. J. Russell

We report the use of prism-assisted side-coupling to investigate the spatio-temporal dynamics of photoionization in an Ar-filled hollow-core photonic crystal fiber. By launching four different LP core modes we are able to probe temporal and spatial changes in the modal refractive index on timescales from a few hundred picoseconds to several hundred microseconds after the ionization event. We experimentally analyze the underlying gas density waves and find good agreement with quantitative and qualitative hydrodynamic predictions. Moreover, we observe periodic modulations in the MHz-range lasting for a few microseconds, indicating nanometer-scale vibrations of the fiber structure, driven by gas density waves.

Investigating the Optical Properties of a Laser Induced 3D Self‐Assembled Carbon–Metal Hybrid Structure

Muhammad Abdullah Butt, Antonino Calà Lesina, Martin Neugebauer, Thomas Bauer, Lora Ramunno, Alessandro Vaccari, Pierre Berini, Yuriy Petrov, Denis Danilov, Alina Manshina, et al.

Carbon‐based and carbon–metal hybrid materials hold great potential for applications in optics and electronics. Here, a novel material made of carbon and gold–silver nanoparticles is discussed, fabricated using a laser‐induced self‐assembly process. This self‐assembled metamaterial manifests itself in the form of cuboids with lateral dimensions on the order of several micrometers and a height of tens to hundreds of nanometers. The carbon atoms are arranged following an orthorhombic unit cell, with alloy nanoparticles intercalated in the crystalline carbon matrix. The optical properties of this metamaterial are analyzed experimentally using a microscopic Müller matrix measurement approach and reveal a high linear birefringence across the visible spectral range. Theoretical modeling based on local‐field theory applied to the carbon matrix links the birefringence to the orthorhombic unit cell, while finite‐difference time‐domain simulations of the metamaterial relates the observed optical response to the distribution of the alloy nanoparticles and the optical density of the carbon matrix.

Chimera states in small disordered optomechanical arrays

Karl Pelka, Vittorio Peano, Andre Xuereb

Synchronization of weakly-coupled non-linear oscillators is a ubiquitous phenomenon that has been observedacross the natural sciences. We study the dynamics of optomechanical arrays—networks of mechanically com-pliant structures that interact with the radiation pressure force—which are driven to self-oscillation. Thesesystems offer a convenient platform to study synchronization phenomena and have potential technological ap-plications. We demonstrate that this system supports the existence of long-lived chimera states, where parts ofthe array synchronize whilst others do not. Through a combined numerical and analytical analysis we show thatthese chimera states can only emerge in the presence of disorder.

Enhanced collective Purcell effect of coupled quantum emitter systems

David Plankensteiner, Christian Sommer, Michael Reitz, Helmut Ritsch, Claudiu Genes

Cavity-embedded quantum emitters show strong modifications of free space radiation properties such as an enhanced decay known as the Purcell effect. The central parameter is the cooperativity C - the ratio of the square of the coherent cavity coupling strength over the product of cavity and emitter decay rates. For a single emitter, C is independent of the transition dipole moment and dictated by geometric cavity properties such as finesse and mode waist. In a recent work Phys. Rev. Lett. 119, 093601 (2017) we have shown that collective excitations in ensembles of dipole-dipole coupled quantum emitters show a disentanglement between the coherent coupling to the cavity mode and spontaneous free space decay. This leads to a strong enhancement of the cavity cooperativity around certain collective subradiant antiresonances. Here, we present a quantum Langevin equations approach aimed at providing results beyond the classical coupled dipoles model. We show that the subradiantly enhanced cooperativity imprints its effects onto the cavity output field quantum correlations while also strongly increasing the cavity-emitter system's collective Kerr nonlinear effect.

Coherently refreshed acoustic phonons for extended light storage

Birgit Stiller, Moritz Merklein, Christian Wolff, Khu Vu, Pan Ma, Stephen J. Madden, Benjamin J. Eggleton

Acoustic waves can serve as memory for optical information, however, acoustic phonons in the GHz regime decay on the nanosecond timescale. Usually this is dominated by intrinsic acoustic loss due to inelastic scattering of the acoustic waves and thermal phonons. Here we show a way to counteract the intrinsic acoustic decay of the phonons in a waveguide by resonantly reinforcing the acoustic wave via synchronized optical pulses. This scheme overcomes the previous constraints of phonon-based optical signal processing for light storage and memory. We experimentally demonstrate on-chip storage up to 40 ns, four times the intrinsic acoustic lifetime in the waveguide. We confirm the coherence of the scheme by detecting the phase of the delayed optical signal after 40 ns using homodyne detection. Through theoretical considerations we anticipate that this concept allows for storage times up to microseconds within realistic experimental limitations while maintaining a GHz bandwidth of the optical signal. The refreshed phonon-based light storage removes the usual bandwidth-delay product limitations of e.g. slow-light schemes.

Nanoprinting organic molecules at the quantum level

Claudio U. Hail, Christian Höller, Korenobu Matsuzaki, Patrik Rohner, Jan Renger, Vahid Sandoghdar, Dimos Poulikakos, Hadi Eghlidi

Organic compounds present a powerful platform for nanotechnological applications. In particular, molecules suitable for optical functionalities such as single photon generation and energy transfer have great promise for complex nanophotonic circuitry due to their large variety of spectral properties, efficient absorption and emission, and ease of synthesis. Optimal integration, however, calls for control over position and orientation of individual molecules. While various methods have been explored for reaching this regime in the past, none satisfies requirements necessary for practical applications. Here, we present direct non-contact electrohydrodynamic nanoprinting of a countable number of photostable and oriented molecules in a nanocrystal host with subwavelength positioning accuracy. We demonstrate the power of our approach by writing arbitrary patterns and controlled coupling of single molecules to the near field of optical nanostructures. Placement precision, high yield and fabrication facility of our method open many doors for the realization of novel nanophotonic devices.

Ensemble-induced strong light-matter coupling of a single quantum emitter

Stefan Schütz, Johannes Schachenmayer, David Hagenmüller, Vahid Sandoghdar, Thomas W. Ebbesen, Claudiu Genes, Guido Pupillo

We discuss a technique to strongly couple a single target quantum emitter to a cavity mode, which is enabled by virtual excitations of a nearby mesoscopic ensemble of emitters. A collective coupling of the latter to both the cavity and the target emitter induces strong photon non-linearities in addition to polariton formation, in contrast to common schemes for ensemble strong coupling. We demonstrate that strong coupling at the level of a single emitter can be engineered via coherent and dissipative dipolar interactions with the ensemble. Our scheme can find applications, amongst others, in quantum information processing or in the field of cavity-assisted quantum chemistry.

Spheroid Culture of Mesenchymal Stromal Cells Results in Morphorheological Properties Appropriate for Improved Microcirculation

Stefanie Tietze, Martin Kraeter, Angela Jacobi, Anna Taubenberger, Maik Herbig, Rebekka Wehner, Marc Schmitz, Oliver Otto, Catrin List, Berna Kaya, et al.

Human bone marrow mesenchymal stromal cells (MSCs) are used in clinical trials for the treatment of systemic inflammatory diseases due to their regenerative and immunomodulatory properties. However, intravenous administration of MSCs is hampered by cell trapping within the pulmonary capillary networks. Here, it is hypothesized that traditional 2D plastic-adherent cell expansion fails to result in appropriate morphorheological properties required for successful cell circulation. To address this issue, a method to culture MSCs in nonadherent 3D spheroids (mesenspheres is adapted. The biological properties of mesensphere-cultured MSCs remain identical to conventional 2D cultures. However, morphorheological analyses reveal a smaller size and lower stiffness of mesensphere-derived MSCs compared to plastic-adherent MSCs, measured using real-time deformability cytometry and atomic force microscopy. These properties result in an increased ability to pass through microconstrictions in an ex vivo microcirculation assay. This ability is confirmed in vivo by comparison of cell accumulation in various organ capillary networks after intravenous injection of both types of MSCs in mouse. The findings generally identify cellular morphorheological properties as attractive targets for improving microcirculation and specifically suggest mesensphere culture as a promising approach for optimized MSC-based therapies.

Interferometric scattering microscopy reveals microsecond nanoscopic protein motion on a live cell membrane

Richard W. Taylor, Reza Gholami Mahmoodabadi, Verena Rauschenberger, Andreas Giessl, Alexandra Schambony, Vahid Sandoghdar

Much of the biological functions of a cell are dictated by the intricate motion of proteins within its membrane over a spatial range of nanometers to tens of micrometers and time intervals of microseconds to minutes. While this rich parameter space is not accessible to fluorescence microscopy, it can be within reach of interferometric scattering (iSCAT) particle tracking. Being sensitive even to single unlabeled proteins, however, iSCAT is easily accompanied by a large speckle-like background, which poses a substantial challenge for its application to cellular imaging. Here, we show that these difficulties can be overcome and demonstrate tracking of transmembrane epidermal growth factor receptors (EGFR) with nanometer precision in all three dimensions at up to microsecond speeds and tens of minutes duration. We provide unprecedented examples of nanoscale motion and confinement in ubiquitous processes such as diffusion in the plasma membrane, transport on filopodia, and endocytosis.

Polarization-Tailored Raman Frequency Conversion in Chiral Gas-Filled Hollow-Core Photonic Crystal Fibers

Sona Davtyan, David Novoa, Yang Chen, Michael H. Frosz, Philip St. J. Russell

Broadband-tunable sources of circularly polarized light are crucial in fields such as laser science, biomedicine, and spectroscopy. Conventional sources rely on nonlinear wavelength conversion and polarization control using standard optical components and are limited by the availability of suitably transparent crystals and glasses. Although a gas-filled hollow-core photonic crystal fiber provides pressuretunable dispersion, long well-controlled optical path lengths, and high Raman conversion efficiency, it is unable to preserve a circular polarization state, typically exhibiting weak linear birefringence. Here we report a revolutionary approach based on a helically twisted hollow-core photonic crystal fiber, which displays circular birefringence, thus robustly maintaining a circular polarization state against external perturbations. This makes it possible to generate pure circularly polarized Stokes and anti-Stokes signals by rotational Raman scattering in hydrogen. The polarization state of the frequency-shifted Raman bands can be continuously varied by tuning the gas pressure in the vicinity of the gain-suppression point. The results pave the way to a new generation of compact and efficient fiber-based sources of broadband light with a fully controllable polarization state.

Huygens' Dipole for Polarization-Controlled Nanoscale Light Routing

Sergey Nechayev, Jörg Eismann, Martin Neugebauer, Pawel Wozniak, Ankan Bag, Gerd Leuchs, Peter Banzer

Structured illumination allows for satisfying the first Kerker condition of in-phase perpendicular electric and magnetic dipole moments in any isotropic scatterer that supports electric and magnetic dipolar resonances. The induced Huygens' dipole may be utilized for unidirectional coupling to waveguide modes that propagate transverse to the excitation beam. We study two configurations of a Huygens' dipole -- longitudinal electric and transverse magnetic dipole moments or vice versa. We experimentally show that only the radially polarized emission of the first and azimuthally polarized emission of the second configuration are directional in the far-field. This polarization selectivity implies that directional excitation of either TM or TE waveguide modes is possible. Applying this concept to a single nanoantenna excited with structured light, we are able to experimentally achieve scattering directivities of around 23 dB and 18 dB in TM and TE modes, respectively. This strong directivity paves the way for tunable polarization-controlled nanoscale light routing and applications in optical metrology, ocalization microscopy and on-chip optical devices.

Seeded and unseeded high-order parametric down-conversion

Cameron Okoth, Andrea Cavanna, Nicolas Joly, Maria Chekhova

Spontaneous parametric down-conversion (SPDC) has been one of the foremost tools in quantum optics for over five decades. Over that time, it has been used to demonstrate some of the curious features that arise from quantum mechanics. Despite the success of SPDC, its higher-order analogs have never been observed, even though it has been suggested that they generate far more unique and exotic states than SPDC. An example of this is the emergence of non-Gaussian states without the need for postselection. Here we calculate the expected rate of emission for nth-order SPDC with and without external stimulation (seeding). Focusing primarily on third-order parametric down-conversion, we estimate the photon detection rates in a rutile crystal for both the unseeded and seeded regimes.

Study of broadband multimode light via non-phase-matched sum frequency generation

Denis Kopylov, Kirill Spasibko, Tatiana Murzina, Maria Chekhova

We propose non-phase-matched sum frequency generation (SFG) as a method for characterizing broadband multimode light. Both the central wavelength and the bandwidth are in this case not limited by the phase matching condition. As an example, we consider bright squeezed vacuum (BSV) generated through high-gain parametric down conversion (PDC). In the spectrum of SFG from BSV, we observe the coherent peak and the incoherent background. We show that the ratio of their widths is equal to the number of frequency modes in BSV, which in the case of low-gain PDC gives the degree of frequency entanglement for photon pairs. By generating the sum frequency in the near-surface region of a nonlinear crystal, we increase the SFG efficiency and get rid of the modulation caused by chromatic dispersion, known as Maker fringes. This allows one to use non-phasematched SFG as a wavelength-independent autocorrelator. Furthermore, we demonstrate efficient non-phase-matched three- and four-frequency summation of broadband multimode light, hardly possible under phase matching. We show that the latter contains the coherent peak while the former does not.

Pulse-repetition-rate tuning of a harmonically mode-locked fiber laser using a tapered photonic crystal fiber

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

Strong enhancement of optoacoustic interactions in the micrometer-sized core of a photonic crystal fiber (PCF) enables stable, harmonic mode locking of a soliton fiber laser at GHz frequencies. Here we report that by tapering the PCF during the draw, the optoacoustic gain bandwidth can be broadened to ∼47 MHz, more than 3 times wider than in the untapered fiber. This made possible broad pulse-repetition-rate tuning over 66 MHz (from 2.042 to 2.108 GHz) of an optoacoustically mode-locked soliton fiber laser. Within this tuning range, the harmonically mode-locked pulse trains at the laser output were observed to be quite robust, with better than 40 dB supermode suppression ratio, sub-ps pulse timing jitter, and <0.2% relative intensity noise. This gigahertz-rate, near-infrared soliton fiber laser has remarkable pulse-rate tunability and low noise level, and has important potential applications in frequency metrology, high-speed optical sampling, and fiber telecommunications.

Overcoming inefficient detection in sub-shot-noise absorption measurement and imaging

Eugene Knyazev, Farid Khalili, Maria Chekhova

Photon-number squeezing and correlations enable measurement of absorption with an accuracy exceeding that of the shot-noise limit. However, sub-shot noise imaging and sensing based on these methods require high detection efficiency, which can be a serious obstacle if measurements are carried out in “difficult” spectral ranges. We show that this problem can be overcome through the phase-sensitive amplification before detection. Here we propose an experimental scheme of sub-shot-noise imaging with tolerance to detection losses.

Initialisation of single spin dressed states using shortcuts to adiabaticity

Johannes Kölbl, Arne Barfuss, Mark Kasperczyk, Lucas Thiel, Aashish Clerk, Hugo Ribeiro, Patrick Maletinsky

We demonstrate the use of shortcuts to adiabaticity protocols for initialisation, readout, and coherent control of dressed states generated by closed-contour, coherent driving of a single spin. Such dressed states have recently been shown to exhibit efficient coherence protection, beyond what their two-level counterparts can offer. Our state transfer protocols yield a transfer fidelity of ~ 99.4(2) % while accelerating the transfer speed by a factor of 2.6 compared to the adiabatic approach. We show bi-directionality of the accelerated state transfer, which we employ for direct dressed state population readout after coherent manipulation in the dressed state manifold. Our results enable direct and efficient access to coherence-protected dressed states of individual spins and thereby offer attractive avenues for applications in quantum information processing or quantum sensing.

A primary radiation standard based on quantum nonlinear optics

Samuel Lemieux, Enno Giese, Robert Fickler, Maria Chekhova, Robert Boyd

The black body remains the most prominent source of light for absolute radiometry. Its main alternative, synchrotron radiation, requires costly and large facilities. Quantum optics offers a new radiometric source: parametric down-conversion (PDC), a nonlinear optical process, in which pairwise photon correlations enable absolute calibration of photodetectors. Since the emission rate crucially depends on the brightness of the electromagnetic field, quantum-mechanical fluctuations of the vacuum can be seen as a seed of spontaneous PDC, and their amplitude is a natural radiometric standard. Thus, they allow for the calibration of the spectral radiance of light sources by measuring the ratio between seeded and unseeded PDC. Here, we directly use the frequency spectrum of the electromagnetic vacuum to trigger spontaneous PDC and employ the generated light to infer the spectral response of a spectrometer over a broad spectral range. Then, we deduce the absolute quantum efficiency from the spectral shape of PDC in the high-gain regime, without relying on a seed or reference detector. Our results compare well with the ones obtained with a reference lamp, demonstrating a promising primary radiation standard.

Non-phase matched parametric down conversion in an ultrathin nonlinear layer

Cameron Okoth, Andrea Cavanna, Tomas Santiago-Cruz, Maria Chekhova

We report, for the first time, the generation of entangled photon pairs via type-0 spontaneous parametric down conversion in an ultra-thin wafer of lithium niobate in which momentum between the pump photon and daughter photons is not conserved. The characteristics of the emission are investigated using several techniques. We find that non-phase matched SPDC is a promising source of ultra-broadband two photon radiation that exhibits remarkably tight correlation widths in both the spatial and temporal domains.

Super- and subradiance of clock atoms in multimode optical waveguides

Laurin Ostermann, Clément Meignant, Claudiu Genes, Helmut Ritsch

The transversely confined propagating modes of an optical fiber mediate virtually infinite range energy exchanges among atoms placed within their field, which adds to the inherent free space dipole–dipole coupling. Typically, the single atom free space decay rate largely surpasses the emission rate into the guided fiber modes. However, scaling up the atom number as well as the system size amounts to entering a collective emission regime, where fiber-induced superradiant spontaneous emission dominates over free space decay. We numerically study this super- and subradiant decay of highly excited atomic states for one or several transverse fiber modes as present in hollow core fibers. As particular excitation scenarios we compare the decay of a totally inverted state to the case of π/2 pulses applied transversely or along the fiber axis as in standard Ramsey or Rabi interferometry. While a mean field approach fails to correctly describe the initiation of superradiance, a second-order approximation accounting for pairwise atom–atom quantum correlations generally proves sufficient to reliably describe superradiance of ensembles from two to a few hundred particles. In contrast, a full account of subradiance requires the inclusion of all higher order quantum correlations. Considering multiple guided modes introduces a natural effective cut-off for the interaction range emerging from the dephasing of different fiber contributions.

Orbital-to-Spin Angular Momentum Conversion Employing Local Helicity

Sergey Nechayev, Jörg Eismann, Gerd Leuchs, Peter Banzer

Spin-orbit interactions in optics traditionally describe an influence of the polarization degree of freedom of light on its spatial properties. The most prominent example is the generation of a spin-dependent optical vortex upon focusing or scattering of a circularly polarized plane-wave by a nanoparticle, converting spin to orbital angular momentum of light. Here, we present a mechanism of conversion of orbital-to-spin angular momentum of light upon scattering of a linearly polarized vortex beam by a spherical silicon nanoparticle. We show that focused linearly polarized Laguerre-Gaussian beams of first order (ℓ=±1) exhibit an ℓ-dependent spatial distribution of helicity density in the focal volume. By using a dipolar scatterer the helicity density can be manipulated locally, while influencing globally the spin and orbital angular momentum of the beam. Specifically, the scattered light can be purely circularly polarized with the handedness depending on the orbital angular momentum of the incident beam. We corroborate our findings with theoretical calculations and an experimental demonstration. Our work sheds new light on the global and local properties of helicity conservation laws in electromagnetism.

Turning a molecule into a coherent two-level quantum system

Daqing Wang, Hrishikesh Kelkar, Diego-Martin Cano, Dominik Rattenbacher, Alexey Shkarin, Tobias Utikal, Stephan Götzinger, Vahid Sandoghdar

The use of molecules in quantum optical applications has been hampered by incoherent internal vibrations and other phononic interactions with their environment. Here we show that an organic molecule placed into an optical microcavity behaves as a coherent two-level quantum system. This allows the observation of 99% extinction of a laser beam by a single molecule, saturation with less than 0.5 photons and non-classical generation of few-photons super-bunched light. Furthermore, we demonstrate efficient interaction of the molecule–microcavity system with single photons generated by a second molecule in a distant laboratory. Our achievements represent an important step towards linear and nonlinear quantum photonic circuits based on organic platforms.

Classically Entangled Light

Andrew Forbes, Andrea Aiello, Bienvenu Ndagano, Taco D. Visser

The concept of entanglement is so synonymous with quantum mechanics that the prefix “quantum” is often deemed unnecessary; there is after all only quantum entanglement. But the hallmark of entangled quantum states is nonseparability, a property that is not unique to the quantum world. On the contrary, nonseparability appears in many physical systems, and pertinently, in classical vector states of light: classical entanglement? Here we outline the concept of classical entanglement, highlight where it may be found, how to control and exploit it, and discuss the similarities and differences between quantum and classical entangled systems. Intriguingly, we show that quantum tools may be applied to classical systems, and likewise that classical light may be used in quantum processes. While we mostly use vectorial structured light throughout the text as our example of choice, we make it clear that the concepts outlined here may be extended beyond this with little effort, which we showcase with a few selected case studies.

Direct characterization of tuneable few-femtosecond dispersive-wave pulses in the deep UV

Christian Brahms, Dane R. Austin, Francesco Tani, Allan S. Johnson, Douglas Garratt, John. C. Travers, John W. G. Tisch, Philip Russell, Jon P. Marangos

Dispersive wave emission (DWE) in gas-filled hollow-core dielectric waveguides is a promising source of tuneable coherentand broadband radiation, but so far the generation of fewfemtosecond pulses using this technique has not been demonstrated. Using in-vacuum frequency-resolved optical gating, we directly characterize tuneable 3 fs pulses in the deep ultraviolet generated via DWE. Through numerical simulations, we identify that the use of a pressure gradient in the waveguide is critical for the generation of short pulses.

Interference effects in hybrid cavity optomechanics

Ondrej Černotík, Claudiu Genes, Aurelien Dantan

Radiation pressure forces in cavity optomechanics allow for efficient cooling of vibrational modes of macroscopic mechanical resonators, the manipulation of their quantum states, as well as generation of optomechanical entanglement. The standard mechanism relies on the cavity photons directly modifying the state of the mechanical resonator. Hybrid cavity optomechanics provides an alternative approach by coupling mechanical objects to quantum emitters, either directly or indirectly via the common interaction with a cavity field mode. While many approaches exist, they typically share a simple effective description in terms of a single force acting on the mechanical resonator. More generally, one can study the interplay between various forces acting on the mechanical resonator in such hybrid mechanical devices. This interplay can lead to interference effects that may, for instance, improve cooling of the mechanical motion or lead to generation of entanglement between various parts of the hybrid device. Here, we provide such an example of a hybrid optomechanical system where an ensemble of quantum emitters is embedded into the mechanical resonator formed by a vibrating membrane. The interference between the radiation pressure force and the mechanically modulated Tavis--Cummings interaction leads to enhanced cooling dynamics in regimes in which neither force is efficient by itself. Our results pave the way towards engineering novel optomechanical interactions in hybrid optomechanical systems.

Weak measurement enhanced spin Hall effect of light for particle displacement sensing

Martin Neugebauer, Sergey Nechayev, Martin Vorndran, Gerd Leuchs, Peter Banzer

A spherical nanoparticle can scatter tightly focused optical beams in a spin-segmented manner, meaning that the far field of the scattered light exhibits laterally separated left- and right-handed circularly polarized components. This effect, commonly referred to as giant spin Hall effect of light, strongly depends on the position of the scatterer in the focal volume. Here, a scheme that utilizes an optical weak measurement in a cylindrical polarization basis is put forward to drastically enhance the spin-segmentation and, therefore, the sensitivity to small displacements of a scatterer. In particular, we experimentally achieve a change of the spin-splitting signal of 5% per nanometer displacement.