Due to their driven-dissipative nature, photonic quantum fluids present new challenges in understanding superfluidity. Some associated effects have been observed, and notably the report of nearly dissipationless flow for coherently driven microcavity-polaritons was taken as a smoking gun for superflow. Here we show that the superfluid response --- the difference between responses to longitudinal and transverse forces --- is zero for coherently driven polaritons. This is a consequence of the gapped excitation spectrum caused by external phase locking. Furthermore, while a normal component exists at finite pump momentum, the remainder forms a rigid state that is unresponsive to either longitudinal or transverse perturbations. Interestingly, the total response almost vanishes when the real part of the excitation spectrum has a linear dispersion, which was the regime investigated experimentally. This suggests that the observed suppression of scattering should be interpreted as a sign of this new rigid state and not a superfluid.
Superfluidity and Bose-Einstein condensation are usually considered as two closely related phenomena. Indeed, in most macroscopic quantum systems, like liquid helium, ultracold atomic Bose gases, and exciton-polaritons, condensation and superfluidity occur in parallel. In photon Bose-Einstein condensates realized in the dye microcavity system, thermalization does not occur by direct interaction of the condensate particles as in the above described systems, i.e. photon-photon interactions, but by absorption and re-emission processes on the dye molecules, which act as a heat reservoir. Currently…
Emission of time-ordered photon pairs from a coherently-driven Kerr microcavity
Majulab International Research Laboratory, French National Centre for Scientific Research, National University of Singapore, Nanyang Technological University, Sorbonne Université, Université Côte d’Azur（马朱国际研究实验室，法国国家科学研究中心，新加坡国立大学，南洋理工大学，索邦大学，蔚蓝海岸大学）；Centre for Quantum technologies, National University of Singapore（新加坡国立大学量子技术中心）；Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies（巴黎萨克雷大学，法国国家科学研究中心，纳米科学和纳米技术中心）；Laboratoire Kastler Brossel, Sorbonne Université, CNRS, ENS-PSL Research University, Collège de France（卡斯特尔-布罗塞尔实验室，索邦大学，法国国家科学研究中心，巴黎高等师范学院-PSL研究大学，法兰西学院）；Univ. Grenoble Alpes, CNRS, LPMMC（格勒诺布尔阿尔卑斯大学，法国国家科学研究中心，凝聚态物理和物质实验室）；Pitaevskii BEC Center, INO-CNR and Dipartimento di Fisica, Università di Trento（皮塔耶夫斯基玻色-爱因斯坦凝聚中心，意大利国家研究委员会光学研究所和特伦托大学物理系）
Superfluidity---the suppression of scattering in a quantum fluid at velocities below a critical value---is one of the most striking manifestations of the collective behaviour typical of Bose-Einstein condensates. This phenomenon, akin to superconductivity in metals, has until now only been observed at prohibitively low cryogenic temperatures. For atoms, this limit is imposed by the small thermal de Broglie wavelength, which is inversely related to the particle mass. Even in the case of ultralight quasiparticles such as exciton-polaritons, superfluidity has only been demonstrated at liquid heli…
We theoretically study Hawking radiation processes from an analog acoustic black hole in a flowing superfluid of exciton-polaritons in a one-dimensional semiconductor microcavity. Polaritons are coherently injected into the microcavity by a laser pump with a suitably tailored spot profile. An event horizon with a large analog surface gravity is created by inserting a defect in the polariton flow along the cavity plane. Experimentally observable signatures of the analog Hawking radiation are identified in the scattering of phonon wavepackets off the horizon, as well as in the spatial correlatio…
Polaritons, in particular microcavity exciton-polaritons, have attracted much attention in recent years, as the phenomena of Bose-Einstein condensation and superfluidity have been observed for these quasiparticles. While the basic physics of these systems is well understood, there has been confusion over the connection of these systems to other phenomena, namely the Jaynes-Cummings Hamiltonian, Rabi flopping, and the vacuum Rabi splitting of atoms in a cavity. This paper reviews the basic theory of polaritons and shows these connections explicitly.
Microcavity polaritons are composite half-light half-matter quasi-particles, which have recently been demonstrated to exhibit rich physical properties, such as non-equilibrium Bose-Einstein condensation, parametric scattering and superfluidity. At the same time, polaritons have some important advantages over photons for information processing applications, since their excitonic component leads to weaker diffraction and stronger inter-particle interactions, implying, respectively, tighter localization and lower powers for nonlinear functionality. Here we present the first experimental observati…
Superfluidity, the ability of a liquid or gas to flow with zero viscosity, is one of the most remarkable implications of collective quantum coherence. In equilibrium systems like liquid 4He and ultracold atomic gases, superfluid behaviour conjugates diverse yet related phenomena, such as persistency of metastable flow in multiply connected geometries and the existence of a critical velocity for frictionless flow when hitting a static defect. The link between these different aspects of superfluid behaviour is far less clear in driven-dissipative systems displaying collective coherence, such as …
We theoretically study the superfluidity properties of a non-equilibrium Bose-Einstein condensate of exciton-polaritons in a semiconductor microcavity. The dynamics of the condensate is described at mean-field level in terms of a modified Complex Ginzburg Landau equation. A generalized Landau criterion is formulated which estimates the onset of the drag force on a small moving defect. Metastability of supercurrents in multiply connected geometries persists up to higher flow speeds.
We consider a polariton microcavity resonantly driven by two external lasers which simultaneously pump both lower and upper polariton branches at normal incidence. In this setup, we study the occurrence of instabilities of the pump-only solutions towards the spontaneous formation of patterns. Their appearance is a consequence of the spontaneous symmetry breaking of translational and rotational invariance due to interaction induced parametric scattering. We observe the evolution between diverse patterns which can be classified as single-pump, where parametric scattering occurs at the same energ…
We study the properties of a binary microcavity polariton superfluid coherently injected by two lasers. The crossover from the supersonic to subsonic regime, where motion is frictionless, is described by evaluating the Bogoliubov spectra. We show that according to the Landau criteria, the coupling between the two components precludes the existence of superfluidity just for one component but not for the other. By analysing the drag force exerted on a defect, we give a recipe to experimentally address the crossover from the supersonic to the subsonic regime.
Quantum vortices are the quantized version of classical vortices. Their center is a phase singularity or vortex core around which the flow of particles as a whole circulates and is typical in superfluids, condensates and optical fields. However, the exploration of the motion of the phase singularities in coherently-coupled systems is still underway. We theoretically analyze the propagation of an interference dislocation in the regime of strong coupling between light and matter, with strong mass imbalance, corresponding to the case of microcavity exciton-polaritons. To this end, we utilize comb…
A quantum fluid passing an obstacle behaves differently from a classical one. When the flow is slow enough, the quantum gas enters a superfluid regime and neither whirlpools nor waves form around the obstacle. For higher flow velocities, it has been predicted that the perturbation induced by the defect gives rise to the turbulent emission of quantised vortices and to the nucleation of solitons. Using an interacting Bose gas of exciton-polaritons in a semiconductor microcavity, we report the transition from superfluidity to the hydrodynamic formation of oblique dark solitons and vortex streets …
In the linear response approximation, coherently driven microcavity polaritons in the pump-only configuration are expected to satisfy the Landau criterion for superfluidity at either strong enough pump powers or small flow velocities. Here, we solve non-perturbatively the time dependent Gross-Pitaevskii equation describing the resonantly-driven polariton system. We show that, even in the limit of asymptotically large densities, where in linear response approximation the system satisfies the Landau criterion, the fluid always experiences a residual drag force when flowing through the defect. We…
We study the spin vortices and skyrmions coherently imprinted into an exciton-polariton condensate on a planar semiconductor microcavity. We demonstrate that the presence of a polarization anisotropy can induce a complex dynamics of these structured topologies, leading to the twist of their circuitation on the Poincaré sphere of polarizations. The theoretical description of the results carries the concept of generalized quantum vortices in two-component superfluids, which are conformal with polarization loops around an arbitrary axis in the pseudospin space.
The appearance of quantised vortices in the classical ``rotating bucket'' experiments of liquid helium and ultracold dilute gases provides the means for fundamental and comparative studies of different superfluids. Here, we realize the ``rotating bucket'' experiment for optically trapped quantum fluid of light based on exciton-polariton Bose-Einstein condensate in semiconductor microcavity. We utilise the beating note of two frequency-stabilized single-mode lasers to generate an asymmetric time-periodic rotating, non-resonant excitation profile that both injects and stirs the condensate throug…
