arXiv subjects

Olivier Bleu

Publications and source records attributed to Olivier Bleu.

At least 19 recordsLinked to original sources

arXiv2026

Quantum Droplets of Light in Semiconductor Microcavities

Quantum droplets are dilute self-bound configurations of bosons that result from the balance between a mean-field attraction and a repulsion induced by quantum fluctuations. Such droplets have been successfully realized in cold atomic gases and represent a signature of their quantum nature. Here, we predict the existence of a similar droplet phase in a solid-state system, involving polaritons formed from the strong coupling between excitons (bound electron-hole pairs) and photons in a semiconductor microcavity. We consider a spin mixture of exciton-polaritons near a biexciton Feshbach resonance, which allows one to tune the interspecies interactions to be attractive and comparable in magnitude to the intraspecies repulsion. We find that self-bound quantum droplets are achievable for realistic parameters in atomically thin semiconductors, and that they can be detected via their excitation spectrum and spatial profile. This exotic phase could potentially lead to polariton condensation at lower thresholds and it opens an alternative avenue to achieve the long-sought quantum polaritonic regime.

cond-mat.mes-hall↗

arXiv2026

Anderson orthogonality scaling in the Rabi-driven heavy Fermi polaron

The Anderson orthogonality catastrophe (AOC) is a paradigmatic many-body phenomenon in which a local perturbation induces a macroscopic response of a Fermi sea. We probe signatures of the AOC by coherently driving heavy Fermi polarons in an ultracold $^6$Li-$^{133}$Cs mixture. We observe a power-law dependence of the measured Rabi frequency on the drive strength, with exponents consistent with AOC predictions. Finite-temperature simulations quantitatively reproduce the observed scaling, indicating that AOC signatures persist beyond the idealized zero-temperature, infinite-mass limit. The damping of the Rabi oscillations provides access to polaron dephasing and reveals a nonmonotonic drive dependence, qualitatively consistent with current theories. Our results establish coherently driven impurities as a versatile probe of quantum many-body dynamics through local coherent control.

cond-mat.quant-gas↗

arXiv2026

Role of impurity statistics and medium constraints in polaron-polaron interactions

We consider the behavior of a small density of mobile impurities (polarons) immersed in a quantum gas, a generic scenario that can be realized in cold atomic gases, liquid helium mixtures, and doped semiconductors. We present a unified theoretical framework for understanding polaron quasiparticles beyond the single-impurity limit, and we identify two key factors that control the polaron-polaron interactions: (i) the statistics of the impurities, including whether or not they are degenerate, and (ii) the constraints on the medium response, i.e., whether the medium density or chemical potential is held fixed. By constructing wave functions for two bosonic, fermionic, or distinguishable impurities immersed in a Bose or Fermi gas, we derive rigorous results for the polaron interactions in the limit of weak impurity-medium coupling. We furthermore obtain an exact relationship between the polaron interactions at fixed medium density and at fixed chemical potential, a result which is valid for arbitrary interaction strength. Our work provides an important guide for understanding experiments, and it acts as a starting point for future strong-coupling theories of polaron interactions that capture all of the effects identified in this work.

cond-mat.quant-gas↗

arXiv2026

Light-enhanced dipolar interactions between exciton polaritons

We consider the scenario of excitons in a semiconductor bilayer that are strongly coupled to cavity photons, leading to the formation of dipolar exciton polaritons (dipolaritons). Using a realistic pseudopotential for the dipolar interactions, we exactly determine the scattering between dipolaritons, accounting for the hybridization between interlayer and intralayer excitons. Similar to conventional non-dipolar polaritons, we find that the light-matter coupling enhances the interactions between dipolaritons by forcing excitons to scatter at energies that would otherwise be forbidden in ordinary exciton-exciton collisions. However, we show that this light enhancement is larger for long-range dipolar interactions than for short-range intralayer interactions, and is sensitive to the (non-uniform) dielectric environment of the bilayer. Crucially, we find that the largest dipolariton interactions are achieved for transition metal dichalcogenide bilayers in vacuum. Our results thus reveal the optimal dipolariton setup for realizing strong photon correlations.

cond-mat.mes-hall↗

arXiv2026

Quantum droplets in a resonant Bose-Fermi mixture

We study the canonical problem of a Fermi gas interacting with a weakly repulsive Bose-Einstein condensate at zero temperature. To explore the quantum phases across the full range of boson-fermion interactions, we construct a versatile variational ansatz that incorporates pair correlations and correctly captures the different polaron limits. Remarkably, we find that self-bound quantum droplets can exist in the strongly interacting regime, preempting the formation of boson-fermion dimers, when the Fermi pressure is balanced by the resonant boson-fermion attraction. This scenario can be achieved in experimentally available Bose-Fermi mixtures for a range of boson-fermion mass ratios in the vicinity of equal masses. We furthermore show that a larger fermion density instead yields phase separation between a Bose-Fermi mixture and excess fermions, as well as behavior reminiscent of a liquid-gas critical point. Our results suggest that first-order quantum phase transitions play a crucial role in the phase diagram of Bose-Fermi mixtures.

cond-mat.quant-gas↗

arXiv2025

Scattering resonances and pairing in a Rabi-coupled Fermi gas

We investigate the possibility of using a Rabi drive to tune the interactions in an atomic Fermi gas. Specifically, we consider the scenario where two fermion species (spins) are Rabi coupled and interacting with a third uncoupled species. Using an exact calculation within a minimal low-energy model, we derive analytical expressions for the effective scattering length and effective range that characterize the collisions between a Rabi-dressed atom and an atom from the third species. In particular, we find that new scattering resonances emerge in the Rabi-coupled system, which we demonstrate are linked to the existence of hybrid two-body bound states. Furthermore, we show via a generalized Thouless criterion that the scattering properties have a direct impact on the superfluid transitions in the Rabi-coupled Fermi gas. The presence of Rabi-induced resonances thus has implications for the investigation of many-body physics with driven atomic gases.

cond-mat.quant-gas↗

arXiv2025

Hybrid-pair superfluidity in a strongly driven Fermi gas

We explore the paired superfluid phases of a Fermi gas in the presence of a continuous Rabi drive. We focus on the case where two components are strongly coupled by the drive, forming hybrid superpositions, and interacting with an uncoupled third component. Using a generalized Bardeen- Cooper-Schrieffer (BCS) ansatz, we show that there are two coupled superfluid order parameters, and we obtain the associated free energy and quasiparticle excitation spectrum. We find that we can drive BCS-BCS, BCS-Bose-Einstein condensate (BEC) and BEC-BEC crossovers purely by varying the detuning of the Rabi drive from the bare transition, with the precise crossover depending on the sign of the underlying interactions between the coupled and uncoupled components. We furthermore identify an exotic excited branch which features both normal to BCS superfluid transitions, as well as a BCS-BEC-BCS crossover. Introducing a generalized Thouless criterion, we show that this behavior is reflected in the critical temperature for superfluidity. Our Rabi-coupled scenario also gives possesses additional thermodynamic properties related to the pseudospin of the coupled components, which provide novel signatures of the state of the many-body system. The Rabi-driven Fermi gas thus emerges as a unique platform for engineering and probing a rich array of multi-band superfluid phases.

cond-mat.quant-gas↗

arXiv2025

Medium-enhanced polaron repulsion in a dilute Bose mixture

We investigate the fundamental problem of a small density of bosonic impurities immersed in a dilute Bose gas at zero temperature. Using a rigorous perturbative expansion, we show that the presence of the surrounding medium enhances the repulsion between dressed bosonic impurities (polarons) in the regime of weak interactions. Crucially, this differs from prevailing theories based on Landau quasiparticles, which neglect the possibility of quantum degenerate impurities and predict an exchange-induced attraction. We furthermore show that the polaron-polaron interactions are strongly modified if the medium chemical potential rather than the density is held fixed, such that the medium-induced attraction between thermal impurities becomes twice the expected Landau effective interaction. Our work provides a possible explanation for the differing signs of the polaron-polaron interactions observed in experiments across cold atomic gases and two-dimensional semiconductors, and it has important implications for theories of quasiparticles and quantum mixtures in general.

cond-mat.quant-gas↗

arXiv2025

Resonantly enhanced polariton-mediated superconductivity in a doped transition metal dichalcogenide monolayer

We present a proposal for achieving light-induced superconductivity using exciton polaritons - hybrid light-matter particles of excitons (bound electron-hole pairs) and microcavity photons. In contrast to previous theories of polariton-mediated superconductivity, which typically require multiple semiconductor layers, we show that superconductivity can be induced within a single semiconductor monolayer with inverted conduction bands, such as in the tungsten-based transition metal dichalcogenides. The key ingredient is that we can resonantly excite exciton polaritons into bands that are different from those occupied by the doped electrons, thus avoiding any Pauli blocking effects. Crucially, we can exploit the trion fine structure (i.e., multiple exciton-electron bound states) and tune the electron-polariton interactions via Feshbach resonances. Our theory of polariton-mediated superconductivity includes the energy dependence of the polariton-mediated interactions between electrons, as well as the polariton-induced changes to the electron quasiparticles. We find that superconductivity at elevated temperatures is within reach of current experiments.

cond-mat.mes-hall↗

arXiv2024

Dissipative light-matter coupling and anomalous dispersion in nonideal cavities

We consider the scenario of an emitter embedded in a nonideal cavity. Using an input-output approach to describe the open system, we show that an effective dissipative coupling between the emitter and the cavity can emerge because of their interaction with a common photonic environment. The underlying mechanism is independent of the nature of the emitter and exists even at zero temperature; hence our results provide a pathway for accessing a range of non-Hermitian phenomena in a variety of light-matter coupled systems. In particular, we show that the effective dissipative coupling can lead to the phenomenon of level attraction between the emitter and cavity mode when the radiative decay rates exceed the conventional Rabi coupling. Our model thus provides a possible explanation for the anomalous dispersions and negative mass observed in recent photoluminescence measurements in semiconductor microcavities. Finally, we show that our effective non-Hermitian system can produce hybrid light-matter exceptional points and bound states in the continuum.

quant-ph↗

arXiv2023

Polaronic polariton quasiparticles in a dark excitonic medium

Exciton polaritons are hybrid particles of excitons (bound electron-hole pairs) and cavity photons, which are renowned for displaying Bose Einstein condensation and other coherent phenomena at elevated temperatures. However, their formation in semiconductor microcavities is often accompanied by the appearance of an incoherent bath of optically dark excitonic states that can interact with polaritons via their matter component. Here we show that the presence of such a dark excitonic medium can "dress" polaritons with density fluctuations to form coherent polaron-like quasiparticles, thus fundamentally modifying their character. We employ a many-body Green's function approach that naturally incorporates correlations beyond the standard mean-field theories applied to this system. With increasing exciton density, we find a reduction in the light-matter coupling that arises from the polaronic dressing cloud rather than any saturation induced by the fermionic constituents of the exciton. In particular, we observe the strongest effects when the spin of the polaritons is opposite that of the excitonic medium. In this case, the coupling to light generates an additional polaron quasiparticle - the biexciton polariton - which emerges due to the dark-exciton counterpart of a polariton Feshbach resonance. Our results can explain recent experiments on polariton interactions in two-dimensional semiconductors and potentially provide a route to tailoring the properties of exciton polaritons and their correlations.

cond-mat.mes-hall↗

arXiv2022

Bogoliubov excitations of a polariton condensate in dynamical equilibrium with an incoherent reservoir

The classic Bogoliubov theory of weakly interacting Bose gases rests upon the assumption that nearly all the bosons condense into the lowest quantum state at sufficiently low temperatures. Here we develop a generalized version of Bogoliubov theory for the case of a driven-dissipative exciton-polariton condensate with a large incoherent uncondensed component, or excitonic reservoir. We argue that such a reservoir can consist of both excitonic high-momentum polaritons and optically dark superpositions of excitons across different optically active layers, such as multiple quantum wells in a microcavity. In particular, we predict interconversion between the dark and bright (light-coupled) excitonic states that can lead to a dynamical equilibrium between the condensate and reservoir populations. We show that the presence of the reservoir fundamentally modifies both the energy and the amplitudes of the Bogoliubov quasiparticle excitations due to the non-Galilean-invariant nature of polaritons. Our theoretical findings are supported by our experiment, where we directly detect the Bogoliubov excitation branches of an optically trapped polariton condensate in the high-density regime. By analyzing the measured occupations of the excitation branches, we extract the Bogoliubov amplitudes across a range of momenta and show that they agree with our generalized theory.

cond-mat.quant-gas↗

arXiv2021

Interplay between polarization and quantum correlations of confined polaritons

We investigate polariton quantum correlations in a coherently driven box cavity in the low driving regime, with a particular focus on accounting for the polarization degree of freedom. The possibility of having different interaction strengths between co- and cross-circularly polarized polaritons as well as a realistic linear-polarization splitting allows one to model the system as two coupled nonlinear resonators with both self- and cross-Kerr-like nonlinearities, thus making our results potentially relevant for other experimental platforms. Within an effective wave-function approach, we obtain analytical expressions for the steady-state polarization-resolved polariton populations and second-order correlation functions, which agree very well with our numerical results obtained from a Lindblad master equation. Notably, we highlight that depending on the excitation polarization (circular or linear), both the unconventional (interference-mediated) and conventional (mediated by nonlinearities) antibunchings can be investigated in a single cavity. Moreover, using our results, we argue that recent experiments on confined fiber-cavity polaritons are likely to have probed a regime where the dominant interaction is between cross-polarized polaritons, which is characteristic of the polariton Feshbach resonance. We furthermore investigate the regime close to resonance using a two-channel model, and we show that systems with large biexciton binding energies, such as atomically thin semiconductors, are promising platforms for realizing strong polariton antibunching.

cond-mat.mes-hall↗

arXiv2021

Theory of polariton-electron interactions in semiconductor microcavities

We develop a microscopic description of an electron-doped two-dimensional semiconductor embedded in a microcavity. Specifically, we investigate the interactions between exciton-polaritons and electrons for the case where the interactions between charges are strongly screened and the system is spin polarized. As a starting point, we obtain an analytic expression for the exciton-polariton wave function, and we relate the microscopic parameters of the light-matter system to experimentally measurable quantities, such as the Rabi coupling and the cavity photon frequency. We then derive the polariton-electron interaction within the standard Born approximation and compare it with the exact polariton-electron scattering $T$ matrix that we obtain from a diagrammatic approach that has proven highly successful in the context of nuclear physics and ultracold atomic gases. In particular, we show that the Born approximation provides an upper bound on the polariton-electron coupling strength at vanishing momentum. Using our exact microscopic calculation, we demonstrate that polariton-electron scattering can be strongly enhanced compared to the exciton-electron case, which is the opposite of that expected from the Born approximation. We furthermore expose a resonance-like peak at scattering momenta near the polariton inflection point, whose size is set by the strength of the light-matter coupling. Our results arise from the non-Galilean nature of the polariton system and should thus be applicable to a range of semiconductor microcavities such as GaAs quantum wells and atomically thin materials.

cond-mat.mes-hall↗

arXiv2021

Enhanced scattering between electrons and exciton-polaritons in a microcavity

The interplay between strong light-matter interactions and charge doping represents an important frontier in the pursuit of exotic many-body physics and optoelectronics. Here, we consider a simplified model of a two-dimensional semiconductor embedded in a microcavity, where the interactions between electrons and holes are strongly screened, allowing us to develop a diagrammatic formalism for this system with an analytic expression for the exciton-polariton propagator. We apply this to the scattering of spin-polarized polaritons and electrons, and show that this is strongly enhanced compared with exciton-electron interactions. As we argue, this counter-intuitive result is a consequence of the shift of the collision energy due to the strong light-matter coupling, and hence this is a generic feature that applies also for more realistic electron-hole and electron-electron interactions. We furthermore demonstrate that the lack of Galilean invariance inherent in the light-matter coupled system can lead to a narrow resonance-like feature for polariton-electron interactions close to the polariton inflection point. Our results are potentially important for realizing tunable light-mediated interactions between charged particles.

cond-mat.mes-hall↗

arXiv2019

Nontrivial band geometry in an optically active system

Optical activity (OA), also called circular birefringence, is known for two hundred years, but its applications for topological photonics remain unexplored. Unlike the Faraday effect, OA provokes rotation of the linear polarization of light without magnetic effects, thus preserving the time-reversal symmetry. Here, we report a direct measurement of the Berry curvature and quantum metric of the photonic modes of a planar cavity containing an optically active organic microcrystal (perylene). Photonic spin-orbit-coupling induced by the cavity results in the action of a non-Abelian gauge field on photons. The addition of high OA makes emerge geometrically non-trivial bands containing two gapped Dirac cones with opposite topological charges. This experiment performed at room temperature and at visible wavelength establishes the potential of optically active organic materials for implementing non-magnetic and low-cost topological photonic devices.

cond-mat.mes-hall↗

arXiv2019

Observation of quantum depletion in a nonequilibrium exciton-polariton condensate

The property of superfluidity, first discovered in liquid 4He, is closely related to Bose-Einstein condensation (BEC) of interacting bosons. However, even at zero temperature, when one would expect the whole bosonic quantum liquid to become condensed, a fraction of it is excited into higher momentum states via interparticle interactions and quantum fluctuations -- the phenomenon of quantum depletion. Quantum depletion of weakly interacting atomic BECs in thermal equilibrium is well understood theoretically but is difficult to measure. This is even more challenging in driven-dissipative systems such as exciton-polariton condensates(photons coupled to electron-hole pairs in a semiconductor), since their nonequilibrium nature is predicted to suppress quantum depletion. Here, we observe quantum depletion of an optically trapped high-density exciton-polariton condensate by directly detecting the spectral branch of elementary excitations populated by this process. Analysis of the population of this branch in momentum space shows that quantum depletion of an exciton-polariton condensate can closely follow or strongly deviate from the equilibrium Bogoliubov theory, depending on the fraction of matter (exciton) in an exciton-polariton. Our results reveal the effects of exciton-polariton interactions beyond the mean-field description and call for a deeper understanding of the relationship between equilibrium and nonequilibrium BECs.

cond-mat.quant-gas↗

arXiv2018

Particle-like behavior of topological defects in linear wave packets in photonic graphene

Topological defects, such as quantum vortices, determine the properties of quantum fluids. The study of their properties has been at the center of activity in solid state and BEC communities. On the other hand, the non-trivial behavior of wavepackets, such as the self-accelerating Airy beams, has also been intriguing physicists. Here, we study the formation, evolution, and interaction of optical vortices in wavepackets at the Dirac point in photonic graphene. We show that even in a non-interacting two-component system the topological excitations appearing in a wavepacket can still be considered as "particles", capable of mutual interaction, with their trajectory obeying the laws of dynamics.

physics.optics↗