Interaction-shaped vortex-antivortex lattices in polariton fluids
Romain Hivet, E. Cancellieri, Thomas Boulier, Dario Ballarini, D. Sanvitto, Francesca Maria Marchetti, Marzena Hanna Szymańska, Cristiano Ciuti, E. Giacobino, Alberto Bramati
Topological defects such as quantized vortices are one of the most striking manifestations of the superfluid nature of Bose-Einstein condensates and typical examples of quantum mechanical phenomena on a macroscopic scale. Here we demonstrate the formation of a lattice of vortex-antivortex pairs and study its properties in the nonlinear regime at high polariton-density where polariton-polariton interactions dominate the behavior of the system. In this work first we demonstrate that the array of vortex-antivortex pairs can be generated in a controllable way in terms of size of the array and in terms of size and shape of its fundamental unit cell. Then we demonstrate that polariton-polariton repulsion can strongly deform the lattice unit cell and determine the pattern distribution of the vortex-antivortex pairs, reaching a completely new behavior with respect to geometrically generated vortex lattices whose shape is determined only by the geometry of the system.
Topological defects such as quantized vortices are one of the most striking manifestations of the superfluid nature of Bose-Einstein condensates and typical examples of quantum mechanical phenomena on a macroscopic scale. Here we demonstrate the formation of a lattice of vortex-antivortex pairs and study its properties in the nonlinear regime at high polariton-density where polariton-polariton interactions dominate the behavior of the system. In this work first we demonstrate that the array of vortex-antivortex pairs can be generated in a controllable way in terms of size of the array and in terms of size and shape of its fundamental unit cell. Then we demonstrate that polariton-polariton repulsion can strongly deform the lattice unit cell and determine the pattern distribution of the vortex-antivortex pairs, reaching a completely new behavior with respect to geometrically generated vortex lattices whose shape is determined only by the geometry of the system.
Exciton-polaritons, mixed light-matter quasiparticles in semiconductors, have recently shown evidence for Bose-Einstein condensation. Some of the properties of condensates of exciton-polaritons are reviewed in this article. We first discuss the spontaneous appearance of long-range order and the way this can be easily accessed in the case of polariton fluids. We show that the Penrose-Onsager criterion is valid even for such a very special case of condensate. We then describe the experiments that allow observation of topological defects in the fluid: quantized vortices, half vortices, and hyperbolic spin vortices. We demonstrate through the comparison with the gross Pitaevskii equation that the appearance and stability of such vortices are linked with the dissipative nature of the condensate together with the presence of disorder. We then briefly summarize the experiments on superfluid behavior of the polaritons at large-enough densities and expand somewhat more on the dynamical behavior of turbulence in the wake of an obstacle, with the appearance of vortex streets. We finally show that the Bogoliubov transformation has been revealed through four-wave mixing experiments.
Vortex reconnections and interactions play a fundamental role in the dynamics of fluids and turbulent flows, both in the classical and quantum regime. Studying vortices in a clean system like ultracold gases can therefore help, as a bottom-up approach, to understand the physics in a wider context, including superfluid helium, polariton condensates, fluid dynamics and turbulence, neutron stars and cosmological models. So far vortex-vortex interaction was studied in Bose-Einstein condensates either in rotating systems with the observation of regular Abrikosov lattices or in flat condensates across the Berezinskii-Kosterlitz-Thouless transition. In both cases the geometrical constraints allowed to study just a planar interaction among aligned or anti-aligned vortices. The present study instead is carried out in an axisymmetric cigar-shaped Bose-Einstein condensate. Vortices in prolate structures are also known as solitonic vortices. This geometry is especially suitable for investigating vortex interactions. Indeed vortices are oriented perpendicularly to the condensate axis to minimize their length, hence energy, and, because of the cylindrical symmetry, the orientation of a vortex in the radial plane has no constraints. These facts permit interactions to occur with different incoming relative angles between the vortex lines and with different relative velocities. The strong confinement, acting along the radial axis, enhances also interesting effects due to the boundaries.
The possible existence of quantum crystals phase of polariton condensate in two-dimensional microcavity polariton was studied by using mean-field method for bosons at zero temperature. In this study, we observe the supersolid crystallized (hexagonal, square) and a quantized winding number of the phase in a regime of strong- field interaction in rotating exciton–polariton condensates. First, the ground state of the condensate was found; and the solution was further extended for dynamics state to reach the equilibrium steady-state as well as their density profile and energy diagrams. The supersolid crystal is the result of the considerable deviation induced by the interaction of polaritons of both ground and dynamic states of a dressed dipolar Bose–Einstein condensate. Here, the researchers demonstrated the formation of a hexagonal lattice in the nonlinear regime at high polariton-density where polariton–polariton interactions dominate the behavior of the system. It was identified that stability regimes for ground state increases as the polariton–polariton interaction strength increases. The phase diagram for the stable vortex state will be useful for conducting experimental and theoretical studies on rotating dipolar quantum gases and many other exotic systems.
The global spin polarization of hadrons in heavy ion collisions has been measured in the solenoidal tracker at relativistic heavy ion collider experiments, which opens up a new window in the study of the hottest, least viscous, and most vortical fluid that has ever been produced in the laboratory. We present a different approach to spin polarization from conventional ones: a topological realization of spin polarization through quantum vortex formation in collisions of Bose–Einstein condensates. This approach is based on the observation that the vortex is a topological excitation in a superfluid in the presence of local orbital angular momentum and is an analog of spin degrees of freedom. The formation processes of vortices and vortex–antivortex pairs are investigated by solving the Gross–Pitaevskii equation with a large-scale parallel algorithm on a graphics processing unit to very high precision. In a rotating environment, the primary vortex with winding number one is stable against perturbation, which has minimal energy and fixed orbital angular momentum (OAM), but the vortices with larger winding numbers are unstable and will decay into primary vortices through a redistribution of the energy and vorticity. The injection of OAM can also be realized in non-central collisions of self-interacting condensates, part of the OAM of the initial state will induce the formation of vortices through concentration of energy and vorticity density around topological defects. Different from a hydrodynamical description, the interference of the wave function plays an important role in the transport of energy and vorticity, reflecting the quantum nature of the vortex formation process. The study of the vortex formation may shed light on the nature of particle spin and spin–orbit couplings in strong interaction matter produced in heavy-ion collisions.
If a quantum fluid is driven with enough angular momentum, at equilibrium the ground state of the system is given by a lattice of quantized vortices whose density is prescribed by the quantization of circulation. We report on the first experimental study of the Feynman-Onsager relation in a nonequilibrium polariton fluid, free to expand and rotate. Upon initially imprinting a lattice of vortices in the quantum fluid, we track the vortex core positions on picosecond timescales. We observe an accelerated stretching of the lattice and an outward bending of the linear trajectories of the vortices, due to the repulsive polariton interactions. Access to the full density and phase fields allows us to detect a small deviation from the Feynman-Onsager rule in terms of a transverse velocity component, due to the density gradient of the fluid envelope acting on the vortex lattice.
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 quantized 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 wake of a potential barrier. The direct observation of these topological excitations provides key information on the mechanisms of superflow and shows the potential of polariton condensates for quantum turbulence studies.