arXiv subjects

Paolo Comaron

Publications and source records attributed to Paolo Comaron.

15 recordsLinked to original sources

arXiv2026

Dynamical stabilisation of a quantum fluid using a single topological defect

The size and shape of a quantum fluid in equilibrium is strongly influenced by the inter-particle interactions. In the attractive interaction regime, atomic quantum fluids ultimately collapse in a violent process that expels most of the particles from the macroscopically occupied state. Here, we use a quantum fluid of light in propagating geometry as an analogue to a two-dimensional Bose-Einstein condensate (BEC) with large attractive interactions to show that non-trivial topology significantly alters the dynamical behaviour of the collapse, enhancing the BEC stability and delaying the collapse time by an order of magnitude. We measure direct experimental signatures of topology affecting quantum hydrodynamics, unveiling the inherent competition between attractive nonlinearities, that lead to the collapse, and the preservation of topological charge from a multi-charged vortex. We fully characterise the collapse process in coordinate space and the eventual `solitonification' of the system and connect it to the mode structure of the excitation spectrum.

cond-mat.quant-gas↗

arXiv2026

Dynamical development of long-range spatial coherence in non-equilibrium bosonic condensation

Development of spontaneous coherence is one of the hallmarks of bosonic condensation in a variety of physical systems, such as cold atoms, confined photons, and hybrid light-matter quasiparticles like exciton polaritons in semiconductors. While spatial coherence is well understood once a steady-state condensate has been established, its temporal evolution as the condensate forms is largely unexplored. Here, we explore the dynamical formation of a non-equilibrium, driven-dissipative exciton-polariton condensate through both time-resolved experiments and numerical modeling. Our study reveals that the spatial coherence is established through two distinct stages. The early-time stage is interaction-driven and features transient oscillations in spatial coherence. This stage is followed by a steady-state regime characterized by a spatially-uniform high degree of coherence that extends over the entire size of the system and persists over time. These stages of spatial coherence development occur in both free flowing and confined exciton-polariton systems that undergo a quench - rapid growth of the condensate starting from two different initial settings. Our study offers a deep insight into the process by which long-range spatial coherence is established in a non-equilibrium bosonic condensate.

cond-mat.quant-gas↗

arXiv2026

Universality beyond the Kibble-Zurek mechanism in the condensation of coherently coupled Bose gases

We study the universal spatial statistics of point-like topological defects formed during the nonequilibrium condensation of a coherently coupled Bose gas using the stochastic projected Gross-Pitaevskii equation. The symmetry-breaking transition is driven by a linear quench of the chemical potential, leading to stochastic vortex nucleation in the individual condensate components. When the two components are considered together, these elementary defects may combine across components to emerge as composite topological defects known as full quantum vortices. Beyond the mean defect density predicted by the Kibble-Zurek mechanism (KZM), we investigate the spatial organization of both the elementary and composite defects and show that their positions are well described by a Poisson point process, revealing a universal stochastic geometry. This universality is further described through Voronoi tessellation, whose cell-area statistics follow Poisson-Voronoi predictions. We also introduce the spatial form factor for characterizing the vortex configurations and demonstrate the emergence of a characteristic dip-ramp-plateau structure. Our results establish universal stochastic geometry of topological defects beyond conventional Kibble-Zurek scaling and identify it as a fundamental feature of nonequilibrium condensation in coherently coupled Bose gases.

cond-mat.quant-gas↗

arXiv2026

Dynamical universality in a driven quantum fluid of light

Universal scaling near phase transitions is one of the central ideas of physics, linking the growth of spatial correlations to the slowing down of dynamics. So far, direct experimental access to this critical behavior has remained largely confined to equilibrium many-body systems, and especially to static critical behavior. Here we probe how universality emerges in a driven quantum fluid of light formed by exciton--polaritons in a semiconductor microcavity. By probing the fluctuation-dominated disordered phase below the condensation threshold, we directly measure both the static growth of the correlation length $\xi$ and the dynamical slowing down of the relaxation time $\tau$. We find that these quantities obey the universal relation $\tau \propto \xi^{z}$ with dynamical exponent $z \approx 2$, revealing diffusive dynamics of a non-conserved order parameter. Our results extend the physics of critical dynamics from equilibrium matter to driven optical systems, bridging quantum condensates and lasers.

cond-mat.quant-gas↗

arXiv2026

Temporal hopping dynamics in exciton-polariton condensation

Polariton condensates provide a versatile platform for exploring non-equilibrium phase transitions and collective phenomena in open quantum systems. Near the condensation threshold, these systems are particularly sensitive to fluctuations and instabilities, which can strongly influence the condensate formation. Using optical trapping and homodyne detection, we directly access the photon statistics and second-order correlation function $g^{(2)}(0)$ of the condensate. We show that polariton condensation near the threshold is not a purely static transition, but instead undergoes a dynamical regime characterized by stochastic hopping between condensed and non-condensed states. These intermittent dynamics are accompanied by a gradual reduction of $g^{(2)}(0)$ towards unity, revealing the progressive build-up of coherence even in the presence of strong temporal fluctuations. Numerical simulations, based on a stochastic Truncated Wigner description of the driven-dissipative polariton field, reproduce these dynamics and capture the essential role of noise and reservoir interactions. This work demonstrates that the observed temporal hopping is an intrinsic feature of polariton condensation, providing a dynamical perspective that goes beyond static descriptions of the condensation phase transition.

cond-mat.mes-hall↗

arXiv2025

Directional optical parametric amplification in a hyperbolic metamaterial

Optical parametric amplification (OPA) comprises essentially a nonlinear four-wave mixing process in which a "pump" and a "signal" field give rise to an "idler" field under certain phase-matching conditions. Here we use a photonic crystal waveguide strongly-coupled with an excitonic reservoir to generate this process between different guided modes at optical wavelengths. Differently from classical nonlinear optical crystals, where the pump and idler photons travel almost collinearly, our exciton-polaritons are naturally separated in the waveguide due to their opposite group velocities. Due to the high efficiency of the process we can generate the idler field of the parametric process by pumping with a continuous wave laser and choose its direction of propagation in the waveguide by adjusting the angle of incidence of the seed laser. We show the OPA process to be robust against surface defects of the waveguide and can lead to simple-to-fabricate devices compared to microcavities that take advantage of strong signal-idler correlations in a propagating geometry. Our results closely agree with mean-field numerical simulations.

physics.optics↗

arXiv2024

Quantum impurities in finite-temperature Bose gases: Detecting vortex proliferation across the BKT and BEC transitions

Detecting vortices in neutral superfluids represents an outstanding experimental challenge. Using stochastic classical-field methods, we theoretically show that a quantum impurity repulsively coupled to a weakly-interacting Bose gas at finite temperature carries direct spectroscopic signatures of vortex proliferation. In two dimensions, we find that a low-energy (attractive) branch in the excitation spectrum becomes prominent when the temperature is tuned across the Berezinskii-Kosterlitz-Thouless (BKT) transition. We explain this red-shifted resonance as originating from the binding of the impurity to vortices, where the bosons density (and hence, the repulsive Hartree energy) is reduced. This mechanism could be exploited to spectroscopically estimate the BKT transition in excitonic insulators. In contrast, in three dimensions, the impurity spectra reflect the presence of vortex rings well below the condensation temperature, and herald the presence of a thermal gas above the Bose-Einstein condensation transition. Importantly, we expect our results to have impact on the understanding of Bose-polaron formation at finite temperatures.

cond-mat.quant-gas↗

arXiv2024

Multicomponent Kardar-Parisi-Zhang Universality in Degenerate Coupled Condensates

We show that the multicomponent Kardar-Parisi-Zhang equation describes the low-energy theory for phase fluctuations in a $\mathbb{Z}_{2}$ degenerate non-equilibrium driven-dissipative condensate with global $U(1)\times U(1)$ symmetry. Using dynamical renormalisation group in spatial dimension $d=1$, we demonstrate that coupled stochastic complex Ginsburg-Landau equations exhibit an emergent stationary distribution, enforcing KPZ dynamical exponent $z=3/2$ and static roughness exponent $\chi=1/2$ for both components. By tuning intercomponent interactions, the system can access other regimes, including a fragmented condensate regime from a dynamical instability in the phase fluctuations, as well as a spacetime vortex regime driven by the non-linear terms in the coupled KPZ equations. In stable regimes, we show that in specific submanifolds relevant to polaritons, the RG fixed point offers a transformation to decoupled KPZ equations. Our findings have broad implications for understanding multicomponent KPZ systems in the long-wavelength limit.

cond-mat.quant-gas↗

arXiv2022

Critical fluctuations in a confined driven-dissipative quantum condensate

Phase fluctuations determine the low-energy properties of quantum condensates. However, at the condensation threshold, both density and phase fluctuations are relevant. While strong emphasis has been given to the investigation of phase fluctuations, which dominate the physics of the quantum system away from the critical point -- number fluctuations have been much less explored, even in thermal equilibrium. In this work, we report experimental observation and theoretical description of fluctuations in a circularly-confined non-equilibrium Bose-Einstein condensate of polaritons near the condensation threshold. We observe critical fluctuations, which combine the number fluctuations of a single-mode condensate state and competition between different states. The latter are analogous to mode hopping in photon lasers. Our theoretical analysis indicates that this phenomenon is of a quantum character, while classical noise of the pump is not sufficient to explain the experiments. The manifestation of a critical quantum state competition unlocks new possibilities for the study of condensate formation while linking to practical realizations in photonic lasers.

cond-mat.quant-gas↗

arXiv2021

Crossover in the dynamical critical exponent of a quenched two-dimensional Bose gas

We study the phase ordering dynamics of a uniform Bose gas in two dimensions following a quench into the ordered phase. We explore the crossover between dissipative and conservative evolution by performing numerical simulations within the classical field methodology. Regardless of the dissipation strength, we find clear evidence for universal scaling, with dynamical critical exponent $z$ characterising the growth of the correlation length. In the dissipative limit we find growth consistent with the logarithmically corrected law $[t/\log(t/t_0)]^{1/z}$, and exponent $z=2$, in agreement with previous studies. Decreasing the dissipation towards the conservative limit, we find strong numerical evidence for the expected growth law $t^{1/z}$. However, we observe a smooth crossover in $z$ that converges to an anomalous value distinctly lower than $2$ at a small finite dissipation strength. We show that this lower exponent may be attributable to a power-law vortex mobility arising from vortex--sound interactions.

cond-mat.quant-gas↗

arXiv2020

Periodic quenches across the Berezinskii-Kosterlitz-Thouless phase transition

The quenched dynamics of an ultracold homogeneous atomic two-dimensional Bose gas subjected to periodic quenches across the Berezinskii-Kosterlitz-Thouless (BKT) phase transition are discussed. Specifically, we address the effect of periodic cycling of the effective atomic interaction strength between a thermal disordered state above, and a highly ordered state below the critical BKT interaction strength, by means of numerical simulations of the stochastic projected Gross-Pitaevskii equation. Probing the emerging dynamics as a function of the frequency of sinusoidal driving from low to high frequencies reveals diverse dynamical features, including phase-lagged quasi adiabatic reversible condensate formation, resonant excitation consistent with an intrinsic system relaxation timescale, and gradual establishment of dynamically-recurring or time-averaged non-equilibrium states with enhanced coherence which are neither condensed, nor thermal. Our study paves the way for experimental observation of such driven non-equilibrium ultracold superfluid states.

cond-mat.quant-gas↗

arXiv2020

Non-Hermitian Topological End-Mode Lasing in Polariton Systems

We predict the existence of non-Hermitian topologically protected end states in a one-dimensional exciton-polariton condensate lattice, where topological transitions are driven by the laser pump pattern. We show that the number of end states can be described by a Chern number and a topological invariant based on the Wilson loop. We find that such transitions arise due to {\it enforced exceptional points} which can be predicted directly from the bulk Bloch wave functions. This allows us to establish a new type of bulk-boundary correspondence for non-Hermitian systems and to compute the phase diagram of an open chain analytically. Finally, we demonstrate topological lasing of a single end-mode in a realistic model of a microcavity lattice.

cond-mat.mes-hall↗

arXiv2020

Coherent transfer of topological domain walls

We demonstrate the controlled coherent transfer of topological domain walls in a one-dimensional non-Hermitian chain of interacting Bose-Einstein condensates. The topological protection stems from a spatially patterned pump in an open-dissipative system. As a testbed setup of the proposed phenomenon, we consider a chain of coupled micropillars with embedded quantum wells, possessing exciton-polariton resonances. The transfer of a domain wall is driven by spatially localised, adiabatic pump modulation in the vicinity of the domain wall. The stochastic calculations prove the coherent nature of the domain wall transfer. For appropriate system parameters the coherence degree is preserved after multiple transitions, paving the way towards long-range transfer of a coherent quantum state.

cond-mat.mes-hall↗

arXiv2020

Persistent current formation in double-ring geometries

Quenching an ultracold bosonic gas in a ring across the Bose-Einstein condensation phase transition is known, and has been experimentally observed, to lead to the spontaneous emergence of persistent currents. The present work examines how these phenomena generalize to a system of two experimentally accessible explicitly two-dimensional co-planar rings with a common interface, or to the related lemniscate geometry, and demonstrates an emerging independence of winding numbers across the rings, which can exhibit flow both in the same and in opposite directions. The observed persistence of such findings in the presence of dissipative coupled evolution due to the local character of the domain formation across the phase transition and topological protection of the randomly emerging winding numbers should be within current experimental reach.

cond-mat.quant-gas↗

arXiv2019

Quench dynamics of an ultracold two-dimensional Bose gas

We study the dynamics of a two-dimensional Bose gas after an instantaneous quench of an initially ultracold thermal atomic gas across the Berezinskii-Kosterlitz-Thouless phase transition, confirming via stochastic simulations that the system undergoes phase ordering kinetics and fulfills dynamical scaling hypothesis at late-time dynamics. Specifically, we find in that regime the vortex number decaying in time as $\langle N_v \propto t^{-1}\rangle$, consistent with a dynamical critical exponent $z \approx 2$ for both temperature and interaction quenches. Focusing on finite-size box-like geometries, we demonstrate that such an observation is within current experimental reach.

cond-mat.quant-gas↗