# Quantum Fluid Properties of Light in Microcavities

*By Elisabeth Giacobino, Centre National de la Recherche Scientifique *

Polaritons are very special quasi-particles, which are mixtures of matter and light. In a semiconductor microcavity exciton-polaritons arise from strong coupling between cavity photons and quantum well excitons (bound electron-hole states). What makes them very attractive is the possibility of combining the coherent properties of photons with the highly interacting features of electronic states. Due to their very low mass (~10-4 times that of the electron, inherited from their photonic component), polaritons also exhibit condensation and quantum fluid properties at temperatures of a few K.

I will present our recent results, demonstrating superfluid motion of polaritons, which manifests itself as the ability to flow without friction when the flow velocity is slower than the speed of sound in the fluid. Cerenkov-like wake patterns, vortices and dark solitons are also observed when the flow velocity becomes larger than the speed of sound. If the polariton superfluid hits a large obstacle quantized vortices and dark solitons are observed. We have also shown the formation of lattices of vortex-antivortex pairs due to colliding flows of polaritons and very recently we have demonstrated the formation of ensembles of same-sign quantized vortices in an ensemble of polaritons where an orbital angular momentum is injected by the laser excitation. These properties of polaritons open the way to a new understanding of quantum fluids of light, and to promising methods for quantum simulation.

# Measure Synchronization in Ultracold Atomic Systems

*By Haibo Qiu, Xi'an University of Posts and Telecommunications*

# Detection of Bell Correlations in a Spin-squeezed Bose-Einstein Condensate

*By Matteo Fadel, University of Basel*

# Entanglement Prethermalization in a Bose Gas

*By Masahito Ueda, The University of Tokyo*

Entanglement is usually fragile and susceptible to perturbations. One may well wonder if entanglement plays a role in thermodynamic environments. We investigate this problem by solving an exact time evolution of a one-dimentional Bose gas based on the Lieb-Liniger model and show that entanglement plays crucial role in prethermalization in a Bose gas. This may be regarded as an example of decoherence-free subspace in the context of thermalization.

# Entanglement between Rydberg Excited State and Ground-state Spin Wave

*By Dong-Sheng Ding, University of Science and Technology of China*

# Few-body Physics with Ultracold Potassium Rubidium Mixtures

*By Ridha Horchani, Dhofar University*

# Scalable Quantum Information Processing with Photons and Atoms

*By Jian-Wei Pan, University of Science and Technology of China*

Over the past three decades, the promises of super-fast quantum computing and secure quantum cryptography have spurred a world-wide interest in quantum information, generating fascinating quantum technologies for coherent manipulation of individual quantum systems. However, the distance of fiber-based quantum communications is limited due to intrinsic fiber loss and decreasing of entanglement quality. Moreover, probabilistic single-photon source and entanglement source demand exponentially increased overheads for scalable quantum information processing. To overcome these problems, we are taking two paths in parallel: quantum repeaters and through satellite. We used the decoy-state QKD protocol to close the loophole of imperfect photon source, and used the measurement-device-independent QKD protocol to close the loophole of imperfect photon detectors—two main loopholes in quantum cryptograph. Based on these techniques, we are now building world’s biggest quantum secure communication backbone, from Beijing to Shanghai, with a distance exceeding 2000 km. Meanwhile, we are developing practically useful quantum repeaters that combine entanglement swapping, entanglement purification, and quantum memory for the ultra-long distance quantum communication. The second line is satellite-based global quantum communication, taking advantage of the negligible photon loss and decoherence in the atmosphere. We realized teleportation and entanglement distribution over 100 km, and later on a rapidly moving platform. We are also making efforts toward the generation of multiphoton entanglement and its use in teleportation of multiple properties of a single quantum particle, topological error correction, quantum algorithms for solving systems of linear equations and machine learning. Finally, I will talk about our recent experiments on quantum simulations on ultracold atoms. On the one hand, by applying an optical Raman lattice technique, we realized a two-dimensional spin-obit (SO) coupling and topological bands with ultracold bosonic atoms. A controllable crossover between 2D and 1D SO couplings is studied, and the SO effects and nontrivial band topology are observe. On the other hand, utilizing a two-dimensional spin-dependent optical superlattice and a single layer of atom cloud, we directly observed the four-body ring-exchange coupling and the Anyonic fractional statistics.

# Fast and Robust Quantum State Control by Shortcuts to Adiabaticity

*By Xi Chen, Shanghai University*

# PT Symmetry in Coupled Active-passive Microcavities and Optically-induced Atomic Lattices

*By Min Xiao, University of Arkansas*

A coupled gain-loss waveguide pair with balanced gain and loss can mimic the parity-time (PT) symmetric Hamiltonian constructed in quantum mechanics [1], which has started an active research field of PT-symmetric optics in the past ten years. Many interesting new phenomena related to such PT-symmetry systems have been theoretically predicted and, some of them, experimentally demonstrated recently.

In this talk, two experimental systems will be discussed that show interesting PT-symmetry properties. The first one is consisted of a pair of coupled active-passive high-Q microtoroid cavities. With carefully balanced gain and loss in the active and passive microcavities, an exceptional (phase transition) point emerges in the transmitted spectral plot (the two spectral peaks merge into one) as the coupling strength between the two microtoroids decreases beyond a certain point (i.e. when the loss equals to gain in the two microtoroids, and becomes larger than the coupling strength between the microtoroids), indicating a PT symmetry breaking to occur in the system [2]. By exploring the gain-saturation nonlinearity in the active microtoroid, nonreciprocal light transmission can be realized in this coupled active-passive microcavity [2]. Such nonreciprocal light transmission can work for balanced & unbalanced gain/loss conditions and under PT-symmetry & broken PT-symmetry cases [2], indicating that the nonreciprocal light transmission is mainly due to the gain-saturation nonlinearity in the system. Actually, nonreciprocal light propagation and optical circulation can be realized in a single active high-Q microtoroid cavity [3,4], which is a much easier system to implement for practical applications.

The second system showing PT-symmetric behavior is done in optically-induced atomic lattices [5,6]. By setting up two sets of coupling and pump standing-wave laser beams in a four-level N-type atomic medium (inside a heated vapor cell) to form optical lattices with controllable gain/loss ratio in the adjacent channels, symmetric refractive index and anti-symmetric gain/loss profiles can be achieved for the injected signal beam [5-7]. Due to the large available parametric spaces, the refractive index and gain/loss (via Raman gain and modified absorption due to the electromagnetically induced transparency) profiles induced by the two sets of the atomic lattices can be easily tuned/controlled and reconfigured in this four-level atomic configuration. The presence of a well-defined exceptional point (or breaking-phase threshold) under balanced gain/loss condition was experimentally verified by observing an abrupt change of relative phase difference between the gain and loss channels [7]. Results from numerical simulations can be used to qualitatively explain the observed phenomenon.

1. R. El-Ganainy, K.G. Makris, D.N. Christodoulides, and Z.H. Musslimani, Opt. Lett. 32, 2632 (2007).

2. L. Chang, X. Jiang, S. Hua, C. Yang, J. Wen, L. Jiang, G. Li, G. Wang, and M. Xiao, Nature Photonics 8, 524 (2014).

3. J. Wen, X.S. Jiang, M. Zhang, L. Jiang, S. Hua, H. Wu, C. Yang, and M. Xiao, Photonics 2, 498 (2015).

4. X. S. Jiang, C. Yang, H. Wu, S. Hua, J. Wen, L. Jiang, L. Chang, Y. Ding, M. Zhang, Q. Hua, and M. Xiao, “On-chip optical nonreciprocity using an active microcavity”, submitted, 2015.

5. J. Sheng, M-A. Miri, D. N. Christodoulides, and Min Xiao, Phys. Rev. A 88, 041803(R) (2013).

6. J. Sheng, J. Wang, M-A. Miri, D. N. Christodoulides, and M. Xiao, Optics Express 23, 19777 (2015).

7. Z.Y. Zhang, Y.Q. Zhang, J. Sheng, L. Yang, M.A. Miri, D. N. Christodoulides, B. He, Y.P. Zhang and M. Xiao, “Observation of parity–time symmetry in optically induced atomic lattices”, submitted, 2016.

# Strongly Interacting Fermi Gases: Birth, Life, and Fate of Quasiparticles

*By Rudolf Grimm, University of Innsbruck*

An impurity immersed in a Fermi sea represents an elementary building block for quantum many-body physics. The behavior of the resulting quasiparticles (Fermi polarons) lies at the heart of interesting phases of quantum matter, with broad interest ranging from strongly interacting Fermi gases in the normal phase and high-Tc superconductors to Kondo physics. A polaron’s life strongly depends on the conditions of its interactions with its environment and exhibits a variety of intriguing phenomena.

Our model system is a small sample of 40K atoms immersed in a large Fermi sea of 6Li atoms[1]. We control the interaction via an interspecies Feshbach resonance, with the additional possibility of extremely fast optical control. An interferometric detection scheme in the time domain[2] enables us to unveil the full story of the polaron’s life. The birth is observed on a short time scale of a few microseconds[3], directly set by the Fermi time. The polaron then lives with a well-defined quasiparticle energy, but undergoes a loss of coherence by thermal excitations[4]. A new chapter beyond single-impurity physics is opened up by interpolaronic interactions mediated by the Fermi sea[3].

We briefly discuss further new developments, like bosonic 41K impurities in the 6Li Fermi sea, and the creation of a novel Fermi-Fermi mixture of dysprosium and potassium atoms.

- C. Kohstall, M. Zaccanti, M. Jag, A. Trenkwalder, P. Massignan, G. Bruun, R. Grimm, Nature 485, 615 (2012).
- M. Knap, A. Shashi, Y. Nishida, A. Imambekov, D. A. Abanin, E. Demler, Phys. Rev. X S. 2, 041020 (2012).
- M. Cetina, M. Jag, R. S. Lous, I. Fritsche, J. T. M. Walraven, R. Grimm, J. Levinsen, M. M. Parish, R. Schmidt, M. Knap, E. Demler, Ultrafast many-body interferometry of impurities coupled to a Fermi sea, to be published.
- M. Cetina, M. Jag, R. S. Lous, J. T. M. Walraven, R. Grimm, R. Christensen, G. Bruun, Phys. Rev. Lett. 115, 135302 (2015).