Non-Hermitian dynamics and nonreciprocity of optically coupled nanoparticles

Non-Hermitian dynamics, as observed in photonic, atomic, electrical, and optomechanical platforms, holds great potential for sensing applications and signal processing. Recently, fully tunable nonreciprocal optical interaction has been demonstrated between levitated nanoparticles. Here, we use this tunability to investigate the collective non-Hermitian dynamics of two nonreciprocally and nonlinearly interacting nanoparticles. We observe parity-time symmetry breaking and, for sufficiently strong coupling, a collective mechanical lasing transition, where the particles move along stable limit cycles. This work opens up a research avenue of nonequilibrium multi-particle collective effects, tailored by the dynamic control of individual sites in a tweezer array

La dynamique hors équilibre dans un fluide de lumière paraxial

Quantum fluids produced with Bose-Einstein Condensates of ultracold atoms are commonly used for exploring the out-of-equilibrium evolution of many-body quantum systems. On the other hand, paraxial propagation of light in a non-linear Kerr medium confers to photons interactions and an effective mass, and transforms the light into an analogue fluid, whose time coordinate is the beam's propagation direction. In this work, hot Rb vapors were used to produce such a fluid of light. Three different aspects of its out-of-equilibrium dynamics were evidenced. First, disturbing the fluid with a strong density perturbation led to the observation of its non-linear hydrodynamics in form of blast waves. Second, disturbing a Gaussian initial state with small random fluctuations the fluid's pre-thermalization was observed with its spatial coherence. Finally, the fluid's response to interaction quenches, occurring at the vapor cell's interfaces, was probed with its intrinsic fluctuations stemming from the shot noise of the laser beam. The measurement of the fluid's spatial density power spectrum revealed the suppression of density fluctuations at low momenta and emergence of the acoustic peaks at later times.Les fluides quantiques produits avec des condensats de Bose-Einstein des gaz d'atomes ultra-froids sont utilisés pour l’exploration de l'évolution hors équilibre des systèmes quantiques à plusieurs corps. D'un autre coté, la propagation paraxiale de la lumière dans un milieu non-linéaire Kerr confère des interactions et une masse effective aux photons, et transforme la lumière en un fluide analogue dont la coordonnée temporelle est la direction de propagation du faisceau. Dans ce travail un tel fluide de lumière est produit dans les vapeurs atomiques de Rubidium. Trois aspects de sa dynamique hors équilibre ont été mis en évidence. D'abord, la perturbation du fluide avec une forte surdensité a permis d'observer son comportement hydrodynamique non-linéaire avec les ondes de choc. Ensuite, en perturbant un état initial Gaussien avec des faibles fluctuations, la pré-thermalisation du fluide a été observée à l'aide de sa cohérence spatiale. Enfin, la réponse du fluide à des trempes d’interactions photoniques survenant à des interfaces de la cellule de vapeur atomique, a été sondée avec ses fluctuations intrinsèques venant du bruit de grenaille du faisceau laser. La mesure du spectre de bruit de densité du fluide a révélé la suppression des fluctuations de densité à faibles impulsions et l’émergence des pics acoustiques à des temps ultérieurs

La dynamique hors équilibre dans un fluide de lumière paraxial

Les fluides quantiques produits avec des condensats de Bose-Einstein des gaz d'atomes ultra-froids sont utilisés pour l’exploration de l'évolution hors équilibre des systèmes quantiques à plusieurs corps. D'un autre coté, la propagation paraxiale de la lumière dans un milieu non-linéaire Kerr confère des interactions et une masse effective aux photons, et transforme la lumière en un fluide analogue dont la coordonnée temporelle est la direction de propagation du faisceau. Dans ce travail un tel fluide de lumière est produit dans les vapeurs atomiques de Rubidium. Trois aspects de sa dynamique hors équilibre ont été mis en évidence. D'abord, la perturbation du fluide avec une forte surdensité a permis d'observer son comportement hydrodynamique non-linéaire avec les ondes de choc. Ensuite, en perturbant un état initial Gaussien avec des faibles fluctuations, la pré-thermalisation du fluide a été observée à l'aide de sa cohérence spatiale. Enfin, la réponse du fluide à des trempes d’interactions photoniques survenant à des interfaces de la cellule de vapeur atomique, a été sondée avec ses fluctuations intrinsèques venant du bruit de grenaille du faisceau laser. La mesure du spectre de bruit de densité du fluide a révélé la suppression des fluctuations de densité à faibles impulsions et l’émergence des pics acoustiques à des temps ultérieurs.Quantum fluids produced with Bose-Einstein Condensates of ultracold atoms are commonly used for exploring the out-of-equilibrium evolution of many-body quantum systems. On the other hand, paraxial propagation of light in a non-linear Kerr medium confers to photons interactions and an effective mass, and transforms the light into an analogue fluid, whose time coordinate is the beam's propagation direction. In this work, hot Rb vapors were used to produce such a fluid of light. Three different aspects of its out-of-equilibrium dynamics were evidenced. First, disturbing the fluid with a strong density perturbation led to the observation of its non-linear hydrodynamics in form of blast waves. Second, disturbing a Gaussian initial state with small random fluctuations the fluid's pre-thermalization was observed with its spatial coherence. Finally, the fluid's response to interaction quenches, occurring at the vapor cell's interfaces, was probed with its intrinsic fluctuations stemming from the shot noise of the laser beam. The measurement of the fluid's spatial density power spectrum revealed the suppression of density fluctuations at low momenta and emergence of the acoustic peaks at later times

Nonequilibrium Prethermal States in a Two-Dimensional Photon Fluid

International audienceWe report on the observation of a prethermal state in a nonequilibrium, two-dimensional fluid of light. Direct measurements of the first order coherence function of the fluid reveal the dynamical emergence of algebraic correlations, a quasi-steady-state with properties close to those of thermal superfluids. By a controlled increase of the fluctuations, we observe a crossover from algebraic to short-range (exponential) correlations. We interpret this phenomenon as a nonequilibrium precursor of the Kosterlitz-Thouless transition

Paraxial quantum fluids light in hot atomic vapors

Hot atomic vapors are widely used in non-linear and quantum optics due to their large Kerr non-linearity. This non-linearity induces effective photon-photon interactions allowing light to behave as a fluid displaying quantum properties such as superfluidity. In this presentation, I will show that we have full control over the Hamiltonian that drives the system and that we can engineer an analogue simulator with light

Non-equilibrium pre-thermal states in a two-dimensional photon fluid

Thermalization is the dynamical process by which a many-body system evolves toward a thermal equilibrium state that maximizes its entropy. In certain cases, however, the establishment of thermal equilibrium is significantly slowed down and a phenomenon of pre-thermalization can emerge. It describes the initial relaxation toward a quasi-steady state after a perturbation. While having similar properties to their thermal counterparts, pre-thermal states exhibit a partial memory of initial conditions. Here, we observe the dynamical formation of a pre-thermal state in a non-equilibrium, two-dimensional (2D) fluid of light after an interaction quench. Direct measurements of the fluid's first-order correlation function reveal the spontaneous emergence of long-range algebraic correlations spreading within a light-cone, providing a clear signature of a quasi steady-state strongly similar to a 2D thermal superfluid. Detailed experimental characterization of the algebraic order is presented and a partial memory of the initial conditions is demonstrated, in agreement with recent theoretical predictions. Furthermore, by a controlled increase of the fluid fluctuations, we unveil a cross-over from algebraic to short-range (exponential) correlations, analogous to the celebrated Kosterlitz-Thouless transition observed at thermal equilibrium. These results suggest the existence of non-equilibrium precursors for thermodynamic phase transitions

Blast waves in a paraxial fluid of light

We study experimentally blast wave dynamics on a weakly interacting fluid of light. The fluid density and velocity are measured in 1D and 2D geometries. Using a state equation arising from the analogy between optical propagation in the paraxial approximation and the hydrodynamic Euler's equation, we access the fluid hydrostatic and dynamic pressure. In the 2D configuration, we observe a negative differential hydrostatic pressure after the fast expansion of a localized over-density, which is a typical signature of a blast wave for compressible gases. Our experimental results are compared to the Friedlander waveform hydrodynamical model. Velocity measurements are presented in 1D and 2D configurations and compared to the local speed of sound, to identify supersonic region of the fluid. Our findings show an unprecedented control over hydrodynamic quantities in a paraxial fluid of light

Analogue cosmological particle creation in an ultracold quantum fluid of light

In inflationary cosmology, the rapid expansion of the early universe resulted in the spontaneous production of cosmological particles from vacuum fluctuations, observable today in the cosmic microwave background anisotropies. The analogue of cosmological particle creation in a quantum fluid could provide insight, but an observation has not yet been achieved. Here we report the spontaneous creation of analogue cosmological particles in the laboratory, using a quenched 3-dimensional quantum fluid of light. We observe acoustic peaks in the density power spectrum, in close quantitative agreement with the quantum-field theoretical prediction. We find that the long-wavelength particles provide a window to early times, and we apply this principle to the cosmic microwave background. This work introduces a new quantum fluid, as cold as an atomic Bose-Einstein condensate.Comment: 7 pages for the main text and 7 pages of supplementary materia