Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 67, Number 7
Monday–Friday, May 30–June 3 2022; Orlando, Florida
Session K08: Nonlinear Optics and Spin SqueezingRecordings Available

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Chair: Jonathan Wrubel, Creighton University Room: Salon 7/8 
Wednesday, June 1, 2022 10:30AM  10:42AM 
K08.00001: Theoretical study of early time superradiance for atom clouds and arrays Francis J Robicheaux We present results from a recent publication where we explore conditions for Dicke superradiance in a cloud of atoms by examining the Taylor series expansion of the photon emission rate at t=0. By defining superradiance as an increasing photon emission rate for $t\sim 0$, we calculate the conditions for superradiance for a variety of cases. We investigate superradiance as defined for photon emission into all angles as well as directional superradiance where the photon emission is only detected in a particular direction. Although all of the examples are for two level atoms that are fully inverted at t=0, the publication contains equations for partially inverted two level atoms and for fully inverted multilevel atoms. The publication also contains an algorithm for efficiently evaluating these equations for atom arrays and uses this algorithm to determine superradiance conditions for large atom number, N → ∞. The method used to explore this system was inspired by S. J. Masson and A. AsenjoGarcia, "Universality of Dicke superradiance in atomic arrays," arXiv preprint arXiv:2106.02042 (2021). 
Wednesday, June 1, 2022 10:42AM  10:54AM 
K08.00002: Twomode squeezing in cold atomic ensembles Arina Tashchilina, Barry C Sanders, Evgeny Moiseev, Alexander Lvovsky, Sergey Moiseev, Xianxin Guo Twomode quadrature squeezed light is a valuable resource for fundamental quantum entanglement study, quantum teleportation, and quantum metrology. In this work we demonstrate two schemes for twomode squeezing generation; both schemes are based on two strong laser fields driving transitions in a Λtype atomic system. 
Wednesday, June 1, 2022 10:54AM  11:06AM 
K08.00003: Squeezing in a refractive indexenhanced system Shay Inbar, Deniz D Yavuz We consider a system of twophoton Raman transitions capable of refraction index enhancement while maintaining low absorption. In this system, the interference of the gain and absorption resonances of the probe beam results in interesting photon statistics properties including significant squeezing of the quantum states, of both photonnumber and quadrature types, without the use of an explicitly nonlinear material. We investigate the conditions under which this happens. 
Wednesday, June 1, 2022 11:06AM  11:18AM 
K08.00004: Spin squeezing in driven, superradiant gas: an integrated method to describe cooperative effects Hanzhen Ma, Susanne F Yelin Driving a superradiant system can produce spin squeezing. To show this, we introduce an integrated method to describe cooperative radiation in manybody systems, and apply the method to an optically dense gas of twolevel atoms. For a nondriven system, physical observables such as superradiant decay rate and the manybody induced broadening scale with the optical depth of the sample. When the gas is driven by an external field, steady states are found, which give rise to spin squeezing in the atoms. Different optical depths and driving strengths are examined in order to find the maximum squeezed state. 
Wednesday, June 1, 2022 11:18AM  11:30AM 
K08.00005: Dicke superradiance in arrays of multilevel atoms Stuart J Masson, Ana AsenjoGarcia We investigate Dicke superradiance with multilevel atoms. Dicke superradiance is a dissipative manybody phenomenon where excited atoms collectively emit a short intense burst of light. While this is well understood for twolevel systems in a cavity, our work involves extended arrays in free space. In previous work, we showed that Dicke superradiance generically occurs for twolevel systems in arrays, provided that the distance between them is sufficiently small. Here, we consider atoms that can decay into multiple states and show that a superradiant burst can be emitted on all transitions. We develop constraints on the relative rates of each transition and on the geometry of the atomic ensemble, and show that the intensity emitted in certain directions features a superradiant burst in arrays of large interatomic spacing. Our results provide a guide to explore this manybody phenomenon in stateoftheart experimental setups. 
Wednesday, June 1, 2022 11:30AM  11:42AM 
K08.00006: Dicke superradiance in ordered lattices: role of geometry and dimensionality. Eric Sierra Garzo, Stuart J Masson, Ana AsenjoGarcia We investigate the role of dimensionality and geometry on the collective decay of large atomic arrays. A fully inverted ensemble of atoms at a single point is well known emits a short burst of light that initially grows in intensity and photons are emitted in a burst, socalled Dicke superradiance. For mesoscopic systems, the nature of the decay can be characterized from the statistics of the first two photons, which we evaluate in linear time. For 1D arrays, Dicke superradiance occurs due to suppression of decay channels, is bounded, and cannot occur for any atom number above a particular distance. In 2D and 3D, it occurs due to constructive interference. In 2D, the maximum interatomic distance for it scales sublogarithmically with atom number, and as a power law for 3D. In contrast to Dicke's original work, we show that superradiance survives in arrays where the smallest interparticle distance is larger than a wavelength. 
Wednesday, June 1, 2022 11:42AM  11:54AM 
K08.00007: Rapid Quantum Squeezing by Jumping the Harmonic Oscillator Frequency Mingjie Xin, Wui Seng Leong, Zilong Chen, ShauYu Lan Quantum sensing and quantum information processing use quantum advantages such as squeezedstates that encode a quantity of interest with higher precision and generate quantum correlations tooutperform classical methods. In harmonic oscillators, the rate of generating squeezing is set by aquantum speed limit. Therefore, the degree to which a quantum advantage can be used in practiceis limited by the time needed to create the state relative to the rate of unavoidable decoherence.Alternatively, a sudden change of harmonic oscillator's frequency projects a ground state into asqueezed state which can circumvent the time constraint. Here, we create squeezed states of atomicmotion by sudden changes of the harmonic oscillation frequency of atoms in an optical lattice.Building on this protocol, we demonstrate rapid quantum amplification of a displacement operatorthat could be used for detecting motion. Our results can speed up quantum gates and enablequantum sensing and quantum information processing in noisy environments. 
Wednesday, June 1, 2022 11:54AM  12:06PM 
K08.00008: Experimental observation of nonclassical correlations in photon pairs generated from an ensemble of pure twolevel atoms Daniel Felinto P Barbosa, Michelle O Araujo, Lucas S Marinho We report the experimental verification of nonclassical correlations for an unfiltered spontaneous fourwavemixing process in an ensemble of cold twolevel rubidium atoms, obtaining $R = (1.98\pm0.03) \nleq 1$ for the violation of a CauchySchwarz inequality in the system, confirming theoretical predictions by Du {\it et al.} in 2007. Quantum correlations are observed in a nanoseconds timescale, in the interference between the sidebands dislocated by the detuning to the atomic resonance. They prevail over the noise background coming from Rayleigh scattering from the same optical transition. These correlations are fragile with respect to processes that disturb the phase of the atomic excitation but are robust to variations in number of atoms and to increasing light intensities. 
Wednesday, June 1, 2022 12:06PM  12:18PM 
K08.00009: Phase transition in a nonlinear optical Ising machine Zhaotong Li, Chunlei Qu Many optimization problems such as the traveling salesman problem and number partitioning can be usually solved by finding the ground state of the classical spin model. Realizing an optical Ising machine and demonstrating its optical advantage are currently attracting broad attention in optical physics. We have explored the phase transition in a recently engineered nonlinear optical Ising machine. By tuning the nonlinear interaction, we find that the ferromagnetic to paramagnetic phase transition changes from secondorder to firstorder due to the competition of different types of spin interactions. Our results provide a deeper theoretical understanding of the nonlinear optical Ising machine and its potential application for addressing optimization problems. 
Wednesday, June 1, 2022 12:18PM  12:30PM 
K08.00010: Highorder (N=4–6) multiphoton absorption in GaP, ZnSe, GaSe, and ZGP crystals Konstantin L Vodopyanov, Taiki Kawamori, Vitaly Gruzdev, Peter Schunemann With the ability to generate fewcycle optical pulses from modelocked lasers, focused power densities in excess of 100 GW/cm^{2} are easily achievable even at full (~100 MHz) repetition rates using oscillator outputs. The development of ultrafast sources in the midinfrared range (MIR, λ > 2 μm) made it possible to demonstrate highfield effects, such as the generation of high harmonics in solids, the generation of a multioctave supercontinuum, and the creation of an ultrawideband subharmonic frequency combs and singlecycle MIR transient generation via optical rectification. Therefore, it is important to know the behavior of nonlinear materials in terms of highorder detrimental effects such as multiphoton absorption. Here we report on our study of highorder multiphoton absorption (MPA) and its anisotropy in four notable midinfrared χ^{(2)} crystals: GaP, ZnSe, GaSe and ZGP using 2.35 µm femtosecond pulses with peak intensity up to 200 GW/cm^{2} in combination with the Z scan method. We found that, at the intensities used, the nonlinear absorption obeys a perturbation model with multiphoton absorption (MPA) orders from N = 4 to 6, in good agreement with the bandgaps of the crystals. A study of the role of free carrier absorption, performed by increasing the pulse duration from 30 to 70 fs while maintaining a constant peak intensity, showed that, at our intensity levels, free carriers absorb much stronger than would be expected from their linear absorption cross section. Possible mechanisms of this enhanced absorption by freecarriers include highly nonthermalized distribution of carriers within the lowest conduction band combined with fieldinduced intravalley scattering and direct absorption to higher conduction bands. 
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