Bogoliubov's description of Bose gases relies on the linear dynamics of noninteracting quasiparticles on top of a homogeneous condensate. Here, we theoretically explore the weakly nonlinear regime of a one-dimensional photon superfluid in which phononlike elementary excitations interact via their backreaction on the background flow. The generalized dispersion relation extracted from spatiotemporal intensity spectra reveals additional branches that correspond to Bogoliubov quasiparticles—phase-locked collective excitations originating from nonresonant harmonic generation and wave-mixing processes. These mechanisms are inherent to fluctuation dynamics and highlight nontrivial scattering channels other than resonant interactions that could be relevant in the emergence of dissipative and turbulent phenomena in superfluids. Published by the American Physical Society 2024
Bogoliubov's description of Bose gases relies on the linear dynamics of noninteracting quasiparticles on top of a homogeneous condensate. Here, we theoretically explore the weakly nonlinear regime of a one-dimensional photon superfluid in which phononlike elementary excitations interact via their backreaction on the background flow. The generalized dispersion relation extracted from spatiotemporal intensity spectra reveals additional branches that correspond to Bogoliubov quasiparticles—phase-locked collective excitations originating from nonresonant harmonic generation and wave-mixing processes. These mechanisms are inherent to fluctuation dynamics and highlight nontrivial scattering channels other than resonant interactions that could be relevant in the emergence of dissipative and turbulent phenomena in superfluids. Published by the American Physical Society 2024
The Bogoliubov dispersion relation for the elementary excitations of the weakly interacting Bose gas is shown to hold for the case of the weakly interacting photon gas (the ``photon fluid'') in a nonlinear Fabry-Perot cavity. The chemical potential of a photon in the two-dimensional photon fluid does not vanish. The Bogoliubov relation, which is also derived by means of a linearized fluctuation analysis in classical nonlinear optics, implies the possibility of a new, superfluid state of light. The theory underlying an experiment in progress to observe sound waves in the photon fluid is described, and another experiment to measure the critical velocity of this superfluid is proposed.
The Bogoliubov dispersion relation for the elementary excitations of the weakly-interacting Bose gas is shown to hold for the case of the weakly-interacting photon gas (the ``photon fluid'') in a nonlinear Fabry-Perot cavity. This relation, which is also derived by means of a linearized fluctuation analysis in classical nonlinear optics, implies the possibility of a new superfluid state of light. The theory underlying an experiment in progress to measure this dispersion relation is described, and another experiment to test the prediction of the superfluidity of this state of light is proposed.
Motivated by rapid experimental progress in the fields of ultracold atoms and quantum optics, I present a series of theoretical studies which explore collective phenomena in quantum gases of atoms and photons. In Chapter 1, I highlight the major developments in the research field and identify the overarching themes and motivations. I also provide a roadmap for the rest of the thesis and summarize the main results. The remaining eight chapters contain original studies, organized along three broad motifs. In Chapters 2 through 5, I investigate how the nature of collective excitations and quasiparticles can be explored in modern experiments. More specifically, I model the dynamics of a spin impurity in a Bose lattice gas, develop a protocol for observing fractionalized excitations or anyons in an optical cavity, and characterize the collective dynamics of Bogoliubov quasiparticles and domain walls in a Fermi superfluid. In Chapters 6 and 7, I examine unconventional superfluid phases in spin-imbalanced Fermi gases. In particular, I propose a novel technique for engineering the long-sought-after Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase and study the relative stability of exotic phases across a dimensional crossover. Finally, Chapters 8 and 9 are devoted to studies of kinetics in out-of-equilibrium systems. I model the formation of a Bose-Einstein condensate in a dimple trap and characterize the approach to thermal equilibrium in quasi-one-dimensional geometries.
Excitons-polaritons of semiconductor microcavities are quasiparticles originating from the dressing of optical cavity photons by the interactions between quantum well electron-hole pairs. In planar geometry, their belonging to the family of quantum fluids has been many times demonstrated through the observation of macroscopic coherence phenomena, such as superfluidity and Bose-Einstein condensation. Like ultracold atoms, polaritons support on their surface a set of collective excitations, the Bogoliubov modes, whose spectrum encompasses the key information to describe the dynamics of quantum fluids. In this thesis, we present a wide variety of results concerning the detection and control of such modes in different configurations. Thanks to the development of a new high-resolution coherent spectroscopy method, we carried out a detailed study of the spectrum of polariton collective excitations. In particular, we highlighted the effects of the driven-dissipative nature of the system on the fundamental dynamics of the fluid, by investigating the interplay between the Bogoliubov modes and the polariton bistability under coherent pumping. This was the opportunity to reveal, in addition to the well-known phonon-type excitations, precursors of dynamical instability and the so-called ghost branch. Moreover, we studied the behavior of out-of-equilibrium phase transitions: first, via the effect of the system dimension on the bistability and, second, with the onset of a Goldstone mode, as a result of a spontaneous symmetry breaking in a parametric oscillation regime. Finally, we designed a pumping geometry that directly controls the instability modes between dark solitons of a pair guided by an intensity channel, leading to the formation of vortex-antivortex streets in the density.
The elementary excitations in weakly interacting quantum fluids have a nontrivial nature which is at the basis of defining quantum phenomena such as superfluidity.These excitations and the physics they lead to have been explored in closed quantum systems at thermal equilibrium both theoretically within the celebrated Bogoliubov framework and experimentally in quantum fluids of ultracold atoms.Over the past decade, the relevance of Bogoliubov excitations has become essential to understand quantum fluids of interacting photons.Their driven-dissipative character leads to distinct properties with respect to their equilibrium counterparts.For instance, the condensate coupling to the photonic vacuum environment leads to a nonzero generation rate of elementary excitations with many striking implications.In this work, considering that quantum fluids of light are often hosted in solid-state systems, we show within a joint theory-experiment analysis that the vibrations of the crystal constitute another environment that the condensate is fundamentally coupled to.This coupling leads to a unique heat transfer mechanism, resulting in a large generation rate of elementary excitations in typical experimental conditions, and to a fundamental nonzero contribution at vanishing temperatures.Our work provides a complete framework for solidembedded quantum fluids of light, which is invaluable in view of achieving a regime dominated by photonvacuum fluctuations.
Superfluidity—frictionless flow—has been observed in various physical systems such as liquid helium, cold atoms, and exciton polaritons. Superfluidity is usually realized by cooling and suppressing all dissipation. Here we challenge this paradigm by demonstrating signatures of superfluidity, enabled by dissipation, in the flow of light within a room-temperature oil-filled cavity. Dissipation in the oil mediates effective photon-photon interactions which are noninstantaneous and nonlocal. Such interactions were expected to severely limit the emergence of superfluidity in conservative photonic systems. Surprisingly, when launching a photon fluid with sufficiently high density and low velocity against an obstacle in our driven-dissipative cavity, we observe a record suppression of backscattering. Our experiments also reveal the reorganization dynamics of photons into a nonscattering steady state and a qualitatively changing behavior of the optical phase as light propagates around the obstacle. The phase is locked between the laser and the obstacle but evolves with the intensity in the wake of the obstacle where the density of the photon fluid and its mean-field interaction energy decrease. Using a generalized Gross–Pitaevskii equation for photons coupled to a thermal field, we model our experiments and elucidate how the noninstantaneous and nonlocal character of interactions influences the suppression of scattering associated with superfluidity. Beyond providing the first signatures of cavity photon superfluidity, and of any superfluid both at room temperature and in steady state, our results pave the way for probing photon hydrodynamics in arbitrary potential landscapes using structured mirrors. Published by the American Physical Society 2024