Littérature scientifique sur le sujet « Phase slips, dissipation, Bose Einstein Condensate »

Quantum gases of light, such as photon or polariton condensates in optical microcavities, are collective quantum systems enabling a tailoring of dissipation from, for example, cavity loss. This characteristic makes them a tool to study dissipative phases, an emerging subject in quantum many-body physics. We experimentally demonstrate a non-Hermitian phase transition of a photon Bose-Einstein condensate to a dissipative phase characterized by a biexponential decay of the condensate’s second-order coherence. The phase transition occurs because of the emergence of an exceptional point in the quantum gas. Although Bose-Einstein condensation is usually connected to lasing by a smooth crossover, the observed phase transition separates the biexponential phase from both lasing and an intermediate, oscillatory condensate regime. Our approach can be used to study a wide class of dissipative quantum phases in topological or lattice systems.

Abstract In this work, we introduce a method for the stabilization of spatiotemporal (ST) solitons. These solitons correspond to light bullets in multimode optical fiber lasers, energy-scalable waveguide oscillators and amplifiers, localized coherent patterns in Bose–Einstein condensates, etc. We show that a three-dimensional confinement potential, formed by a spatial transverse (radial) parabolic graded refractive index and dissipation profile, in combination with quadratic temporal phase modulation, may permit the generation of stable ST dissipative solitons. This corresponds to combining phase mode-locking with the distributed Kerr-lens mode-locking. Our study of the soliton characteristics and stability is based on analytical and numerical solutions of the generalized dissipative Gross–Pitaevskii equation. This approach could lead to higher energy (or condensate mass) harvesting in coherent spatio-temporal beam structures formed in multimode fiber lasers, waveguide oscillators, and weakly-dissipative Bose–Einstein condensates.

Dissipative and unitary processes define the evolution of a many-body system. Their interplay gives rise to dynamical phase transitions and can lead to instabilities. In this study, we observe a nonstationary state of chiral nature in a synthetic many-body system with independently controllable unitary and dissipative couplings. Our experiment is based on a spinor Bose gas interacting with an optical resonator. Orthogonal quadratures of the resonator field coherently couple the Bose-Einstein condensate to two different atomic spatial modes, whereas the dispersive effect of the resonator losses mediates a dissipative coupling between these modes. In a regime of dominant dissipative coupling, we observe the chiral evolution and relate it to a positional instability.

Motivated by a recent experiment on Bloch oscillation of Bose–Einstein condensates (BEC) in accelerated optical lattices, we consider the Josephson dynamics of a BEC in an accelerated double-well potential. We show that acceleration suppresses coherent population / phase oscillation between the two wells. Accelerating the double-well renders the Josephson coupling energy EJ time-dependent and this emerges as a source of dissipation. This dissipative mechanism helps to stabilize the system. The results are used to interpret a recent experimental result (M. Jona-Lasinio, O. Morsh, M. Cristiani E. Arimonod and C. Menotti, cond-mat / 0501572).

The Dicke model with a weak dissipation channel is realized by coupling a Bose–Einstein condensate to an optical cavity with ultranarrow bandwidth. We explore the dynamical critical properties of the Hepp–Lieb–Dicke phase transition by performing quenches across the phase boundary. We observe hysteresis in the transition between a homogeneous phase and a self-organized collective phase with an enclosed loop area showing power-law scaling with respect to the quench time, which suggests an interpretation within a general framework introduced by Kibble and Zurek. The observed hysteretic dynamics is well reproduced by numerically solving the mean-field equation derived from a generalized Dicke Hamiltonian. Our work promotes the understanding of nonequilibrium physics in open many-body systems with infinite range interactions.

Abstract Bose-Einstein condensation trapped in quadruple-well potential provides a great useful platform in theoretical and experimental researches. Utilizing analytical and numerical methods, we investigate the properties of quantum dynamics and the reciprocity of quantum transport of Bose-Einstein condensation (BEC) in quadruple-well potential formed by the laser-assisted transition. The tunneling dynamics of BEC is under the control of coupling between wells, dissipation, and the phase caused by Raman coupling laser. By adjusting the coupling phase, the coherent destruction of tunneling can be observed. Meanwhile, by testing the reciprocity of tunneling dynamics of BEC, this system provide a applicable proposal for reciprocal quantum switch by using the tunneling dynamics of BEC.

Photonic materials are a rapidly growing platform for studying condensed matter physics with light, where the exquisite control capability is allowing us to learn about the relation between microscopic dynamics and macroscopic properties. One of the most interesting aspects of condensed matter is the interplay between interactions and the effect of an external magnetic field or rotation, responsible for a plethora of rich phenomena---Hall physics and quantized vortex arrays. At first sight, however, these effects for photons seem vetoed: they do not interact with each other and they are immune to magnetic fields and rotations. Yet in specially devised structures these effects can be engineered. Here, we propose the use of a synthetic magnetic field induced by strain in a honeycomb lattice of resonators to create a non-equilibrium Bose-Einstein condensate of light-matter particles (polaritons) in a rotating state, without the actual need for external rotation nor reciprocity-breaking elements. We show that thanks to the competition between interactions, dissipation and a suitably designed incoherent pump, the condensate spontaneously becomes chiral by selecting a single Dirac valley of the honeycomb lattice, occupying the lowest Landau level and forming a vortex array. Our results offer a new platform where to study the exciting physics of arrays of quantized vortices with light and pave the way to explore the transition from a vortex-dominated phase to the photonic analogue of the fractional quantum Hall regime.

The goal of my thesis is to experimentally investigate the phenomenon of phase slips in one dimensional (1D) Bose-Einstein condensates, by probing the supercurrent in the presence of an obstacle in different ranges of velocities, interactions and temperature. Phase slips are the primary elementary excitations of the order parameter due to thermal or quantum fluctuations in 1D superfluids and superconductors, in the presence of an obstacle for the superflow and supercurrent. They control the dissipation of nominally frictionless systems by inducing a finite resistance and a finite dissipation in 1D superconductors and superfluids, respectively. Due to the fact that 1D systems are more fragile and more vulnerable to the presence of perturbation and fluctuations than higher dimensional systems, phase slips are more easily detected in 1D systems. Several theoretical models concerning the phenomenon of phase slips in ultracold quantum gases have been built, but an experimental exhaustive picture of quantum phase slips in ultracold superfluids has remained elusive. The system I use is a 1D Bose-Einstein condensate of 39K atoms, in the presence of a 1D optical lattice along the axis of the system, which acts as an obstacle. The Bose Einstein condensate, being a superfluid, should flow without dissipation, even in the presence of an obstacle. Anyway, by performing transport measurements, I observe a finite dissipation, due to phase slips that cause the superfluity breakage. During my Ph.D I focused my attention on this dissipation phenomenon. In the first part of my work, I focused my attention on the system dissipation and I investigated the system oscillation for different values of the velocity. More in detail, I tuned the system velocity in a range between vC/5 and vC, where vC is the critical velocity for breaking superfliduity via the so-called dynamical instability. Depending on the velocity, the systems behaves differently: close to the occurrence of the dynamical instability, I observed an overdamped motion, which is a consequence of the divergence of phase slips. For velocity smaller than vc, instead, I observed how phase slips act on the system: in this situation, the system oscillates with a damping due to the presence of phase slips. I measured the damping rate G due to the presence of phase slips for different values of interaction, velocity and temperature, far from the dynamical instability. In this situation the system never enters in the unstable regime but keeps oscillating with a finite dissipation. The damping rate G, which is related to the phase slips nucleation rate, according to the theory should behave differently depending on the nature of phase slips: in the presence of phaseslips due to thermal fluctuations, the damping rate depends on the temperature, whereas it depends on velocity if the phase slips are due to quantum fluctuations. These observations appear consistent with the theoretically predicted crossover from a regime where the nucleation of phase slips is due to thermal effect to a regime of quantum phase slips and provides the first experimental evidence of quantum phase slips in a 1D atomic superfluid. In the second part of my work, I employed a different method to study the dissipation, by performing transport measurements at constant velocity, also for velocities lower than vC/5, both in the regime of shallow lattices and deep one. When the system is in the superfluid phase, the system dissipation is related to the presence of phase slips, both thermal and quantum depending on the interaction value. Surprisingly, I observed a finite dissipation also when the system is in the insulating phase and it should not dissipate. This dissipation may be due to two different phenomena. The First one is related to the coexistence of a superfluid and a Mott insulating phase, due to the inhomogeneity of our system, whereas the second phenomenon is related to the excitation of the gapped Mott phase. In the presence of a weak optical lattice, it is difficult to discriminate which one of the two effects dominates the observed dissipation. As a consequence, I have repeated the same measurements in the presence of a deep lattice and also in this case I observed a dissipation both in the superfluid phase and in the insulating phase. As in the case of a weak optical lattice, when the system is in the superfluid phase, the system dissipation is related to the presence of phase slips. By comparing the gap of the Mott insulator and the energy acquired during the harmonic potential trap displacement, I found that the Mott insulator gap was an order of magnitude larger than the energy due to the trap shift. As a consequence, I excluded that the finite damping was due to the excitation of the Mott insulator; it seems due to the dissipation of the superfluid phase coexistent with the insulating one. These results open the way for a deeper understanding of the intriguing phenomenon of phase slips, which is still an open topic. There are, in fact, a lot of open questions regarding this kind of excitation, and Bose Einstein condensates may be the ideal system to investigate this phenomenon thanks to their ample tunability and to the relative ease of modelling. For example, with the technique used in the first part of my work, it would be interesting to excite the dipole oscillations in the system in the presence of individual defects or controlled disorder. Moreover, with the technique used in the second part of my work, further studies of the system dissipation during a shift of the harmonic trap at constant velocity, in the absence of the Mott insulator, may give information about the phase slips phenomenon in the very strongly interacting regime.