Properties of bright squeezed vacuum at increasing brightness
P. R. Sharapova, G Frascella, A. M. Perez, O. V. Tikhonova, S. Lemieux, R. W. Boyd, G. Leuchs, M. V. Chekhova
Physical Review Research
2(1)
013371
(2020)

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A bright squeezed vacuum (BSV) is a nonclassical macroscopic state of light, which is generated through highgain parametric downconversion or fourwave mixing. Although the 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 with gainindependent shapes fails to explain the spectral broadening observed in the experiment as the mean number of photons increases. Meanwhile, the semiclassical description accounting for the broadening does not allow us to decouple the intermodal photonnumber correlations. In this work, we present a new generalized theoretical approach to describe the spatial properties of a multimode BSV. In the multimode case, one has to take into account the complicated interplay between all involved modes: each planewave mode interacts with all other modes, which complicates the problem significantly. The developed approach is based on exchanging the (k, t ) and (ω, z) representations and solving a system of integrodifferential equations. Our approach predicts correctly the dynamics of the Schmidt modes and the broadening of the angular distribution with the increase in the BSV mean photon number due to a stronger pumping. Moreover, the model correctly 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 interference 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 the frequency spectrum, frequency Schmidt modes, and other experimental configurations.
Broadly tunable photonpair generation in a suspendedcore fiber
Jonas Hammer, Maria V. Chekhova, Daniel Häupl, Riccardo Pennetta, Nicolas Y. Joly
Physical Review Research
2(1)
012079(R)
(2020)

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Nowadays fiber biphoton sources are nearly as popular as crystalbased 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 0.30 THz/bar of the sidebands, using up to 25 bar of argon.
Engineering Fast HighFidelity Quantum Operations With Constrained Interactions
Thales Figueiredo Roque, Aashish A Clerk, Hugo Ribeiro
Understanding how to tailor quantum dynamics to achieve a desired evolution is a crucial problemin almost all quantum technologies. We present a very general method for designing highefficiencycontrol sequences that are always fully compatible with experimental constraints on available interactions and their tunability. Our approach reduces in the end to finding control fields by solvinga set of timeindependent linear equations. We illustrate our method by applying it to a numberof physicallyrelevant problems: the strongdriving limit of a twolevel system, fast squeezing in aparametrically driven cavity, the leakage problem in transmon qubit gates, and the acceleration ofSNAP gates in a qubitcavity system.
Ensembleinduced strong lightmatter coupling of a single quantum emitter
Stefan Schütz, Johannes Schachenmayer, David Hagenmüller, Gavin K. Brennen, Thomas Volz, 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 nonlinearities 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, and provide realistic parameters for a possible implementation with
SiV− defects in diamond. Our scheme can find applications, amongst others, in quantum information processing or in the field of cavityassisted quantum chemistry.
Robust excitation and Raman conversion of guided vortices in a chiral gasfilled photonic crystal fiber
Sona Davtyan, Yang Chen, Michael Frosz, Philip Russell, David Novoa
The unique ringshaped intensity patterns and helical phase fronts of optical vortices make them useful in many applications. Here we report for the first time, to the best of our knowledge, efficient Raman frequency conversion between vortex modes in a twisted hydrogenfilled singlering hollow core photonic crystal fiber (SRPCF). Highfidelity transmission of optical vortices in an untwisted SRPCF becomes
more and more difficult as the orbital angular momentum (OAM) order increases, due to scattering at structural imperfections in the fiber microstructure. In a helically twisted
SRPCF, however, the degeneracy between left and righthanded versions of the same mode is lifted, with the result
that they are topologically protected from such scattering. With launch efficiencies of ∼75%, a high damage threshold and broadband guidance, these fibers are ideal for performing nonlinear experiments that require the polarization
state and azimuthal order of the interacting modes to be preserved over long distances. Vortex coherence waves of internal molecular motion carrying angular momentum are excited in the gas, permitting the polarization and OAM of the Raman bands to be tailored, even in spectral regions where conventional solidcore waveguides are opaque or susceptible to optical damage.
Efficient singlecycle pulse compression of an ytterbium fiber laser at 10 MHz repetition rate
Felix Köttig, Daniel Schade, Johannes Köhler, Philip Russell, Francesco Tani
Over the past years, ultrafast lasers with average powers in the 100 W range have become a mature technology, with a multitude of applications in science and technology. Nonlinear temporal compression of these lasers to few or even singlecycle duration is often essential, yet still hard to achieve, in particular at high repetition rates. Here we report a twostage system for compressing pulses from a 1030 nm ytterbium fiber laser to singlecycle durations with 5 µJ output pulse energy at 9.6 MHz repetition rate. In the first stage, the laser pulses are compressed from 340 to 25 fs by spectral broadening in a kryptonfilled singlering photonic crystal fiber (SRPCF), subsequent phase compensation being achieved with chirped mirrors. In the second stage, the pulses are further compressed to singlecycle duration by solitoneffect selfcompression in a neonfilled SRPCF. We estimate a pulse duration of ∼3.4 fs at the fiber output by numerically backpropagating the measured pulses. Finally, we directly measured a pulse duration of 3.8 fs (1.25 optical cycles) after compensating (using chirped mirrors) the dispersion introduced by the optical elements after the fiber, more than 50% of the total pulse energy being in the main peak. The system can produce compressed pulses with peak powers >0.6 GW and a total transmission exceeding 66%.
The Ising model in a lightinduced quantized transverse field
Jonas Rohn, Max Hörmann, Claudiu Genes, Kai Phillip Schmidt
We investigate the influence of lightmatter interactions on correlated quantum matter by studying
the paradigmatic Ising model subject to a quantum Rabi coupling. This type of coupling to a
confined, spatially delocalized bosonic light mode, such as provided by an optical resonator, resembles
a quantized transverse magnetic field of tunable strength. As a consequence, the symmetrybroken
magnetic state breaks down for strong enough lightmater interactions to a paramagnetic state.
The nonlocal character of the bosonic mode can change the quantum phase transition in a drastic
manner, which we analyze quantitatively for the simplest case of a chain geometry (DickeIsing
chain). The results show a direct transition between a magnetically ordered phase with zero photon
density and a magnetically polarized phase with lasing behaviour of the light. Our predictions are
equally valid for the dual quantized Ising chain in a conventional transverse magnetic field.
Maxwell's lesser demon: A Quantum Engine Driven by Pointer Measurements
Stella Seah, Stefan Nimmrichter, Valerio Scarani
Physical Review Letters
124
100603

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We discuss a selfcontained spinboson model for a measurementdriven 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.
Prospects of reinforcement learning for the simultaneous damping of many mechanical modes
Christian Sommer, Muhammad Asjad, Claudiu Genes
Scientific Reports
10(2623)
(2020)

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We apply adaptive feedback for the partial refrigeration of a mechanical resonator, i.e. with the aim to
simultaneously cool the classical thermal motion of more than one vibrational degree of freedom. The
feedback is obtained from a neural network parametrized policy trained via a reinforcement learning
strategy to choose the correct sequence of actions from a fnite set in order to simultaneously reduce
the energy of many modes of vibration. The actions are realized either as optical modulations of the
spring constants in the socalled quadratic optomechanical coupling regime or as radiation pressure
induced momentum kicks in the linear coupling regime. As a proof of principle we numerically illustrate
efcient simultaneous cooling of four independent modes with an overall strong reduction of the total
system temperature.
Swept source crosspolarized optical coherence tomography for any input polarized light
Gargi Sharma, Shivani Sharma, Katharina Blessing, Georg Hartl, Maximilian Waldner, Kanwarpal Singh
Cross polarized optical coherence tomography offers enhanced contrast in certain
pathological conditions. Traditional crosspolarized optical coherence tomography systems
require a defined input polarization and thus require several polarization controlling elements
increasing the overall complexity of the system. Our proposed system requires a single
quarter wave plate as a polarization controller thus simplifying the system significantly.
Majority of Crosspolarized optical coherence tomography systems are spectrometer based
which suffers from slow speed and low signal to noise ratio. In this work, we present a swept
source based crosspolarized optical coherence tomography system that works for any input
polarization state. The system was tested against known birefringent materials such as quarter
wave plate. Furthermore, biological samples such as finger, nail and chicken breast were
imaged to demonstrate the potential of our technique.
Quantum metamaterials with magnetic response at optical frequencies
Rasoul Alaee Khanghah, Burak Gürlek, Mohammad Albooyeh, DiegoMartin Cano, Vahid Sandoghdar
We propose novel quantum antennas and metamaterials with strong magnetic response at optical frequencies. Our design is based on the arrangement of natural atoms with only electric dipole transition moments at distances smaller than a wavelength of light but much larger than their physical size. In particular, we show that an atomic dimer can serve as a magnetic antenna at its antisymmetric mode to enhance the decay rate of a magnetic transition in its vicinity by several orders of magnitude. Furthermore, we study metasurfaces composed of atomic bilayers with and without cavities and show that they can fully reflect the electric and magnetic fields of light, thus, forming nearly perfect electric/magnetic mirrors. The proposed quantum metamaterials can be fabricated with available stateoftheart technologies and promise several applications both in classical optics and quantum engineering.
MagnonPhonon Quantum Correlation Thermometry
C. A. Potts, Victor A. S. V. Bittencourt, Silvia ViolaKusminskiy, J. P. Davis
A large fraction of quantum science and technology requires lowtemperature
environments such as those afforded by dilution refrigerators. In these
cryogenic environments, accurate thermometry can be difficult to implement,
expensive, and often requires calibration to an external reference. Here, we
theoretically propose a primary thermometer based on measurement of a hybrid
system consisting of phonons coupled via a magnetostrictive interaction to
magnons. Thermometry is based on a crosscorrelation measurement in which the
spectrum of backaction driven motion is used to scale the thermomechanical
motion, providing a direct measurement of the phonon temperature independent of
experimental parameters. Combined with a simple lowtemperature compatible
microwave cavity readout, this primary thermometer is expected to become a
popular thermometer for experiments below 1 K.
Nonlinear dynamics of weakly dissipative optomechanical systems
Thales Figueiredo Roque, Florian Marquardt, Oleg M. Yevtushenko
New Journal of Physics (22)
013049
(2020)

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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 NeimarkSacker 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.
Idealized EinsteinPodolskyRosen states from non–phasematched parametric downconversion
Cameron Okoth, E. Kovlakov, F. Bönsel, Andrea Cavanna, S. Straupe, S. P. Kulik, Maria Chekhova
The most common source of entangled photons is spontaneous parametric downconversion (SPDC). The degree of energy and momentum entanglement in SPDC is determined by the nonlinear interaction volume. By reducing the length of a highly nonlinear material, we relax the longitudinal phasematching condition and reach record levels of transverse momentum entanglement. The degree of entanglement is estimated using both correlation measurements and stimulated emission tomography in wavevector space. The high entanglement of the state in wavevector space can be used to massively increase the quantum information capacity of photons, but more interestingly the equivalent state measured in position space is correlated over distances far less than the photon wavelength. This property promises to improve the resolution of many quantum imaging techniques beyond the current state of the art.
Single Photons Emitted by Nanocrystals Optically Trapped in a Deep Parabolic Mirror
Vsevolod Salakhutdinov, Markus Sondermann, Luigi Carbone, Elisabeth Giacobino, Alberto Bramati, Gerd Leuchs
We investigate the emission of single photons from CdSe/CdS dotsinrod which are optically trapped in the focus of a deep parabolic mirror. Thanks to this mirror, we are able to image almost the full 4π emission pattern of nanometersized elementary dipoles and verify the alignment of the rods within the optical trap. From the motional dynamics of the emitters in the trap, we infer that the singlephoton emission occurs from clusters comprising several emitters. We demonstrate the optical trapping of rodshaped quantum emitters in a configuration suitable for efficiently coupling an ensemble of linear dipoles with the electromagnetic field in free space.
ManyBody Dephasing in a TrappedIon Quantum Simulator
Harvey B. Kaplan, Lingzhen Guo, Wen Lin Tan, Arinjoy De, Florian Marquardt, Guido Pagano, Christopher Monroe
How a closed interacting quantum manybody system relaxes and dephases as a function of time is a fundamental question in thermodynamic and statistical physics. In this work, we observe and analyse the persistent temporal fluctuations after a quantum quench of a tunable longrange interacting transversefield Ising Hamiltonian realized with a trappedion quantum simulator. We measure the temporal fluctuations in the average magnetization of a finitesize system of spin1/2 particles and observe the experimental evidence for the theoretically predicted regime of manybody dephasing. We experiment in a regime where the properties of the system are closely related to the integrable Hamiltonian with global spinspin coupling, which enables analytical predictions even for the longtime nonintegrable dynamics. We find that the measured fluctuations are exponentially suppressed with increasing system size, consistent with theoretical predictions.
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