arXiv subjects

Iacopo Carusotto

Publications and source records attributed to Iacopo Carusotto.

At least 55 records · Page 3Linked to original sources

arXiv2023

Observation of the diffusive Nambu-Goldstone mode of a non-equilibrium phase transition

Second-order phase transitions are governed by spontaneous symmetry breaking, which yield collective excitations with a gapless spectrum called Nambu-Goldstone (NG) modes. While NG modes in conservative systems are propagating excitations, non-equilibrium phase transitions have been predicted to feature a diffusive NG mode. We present the first experimental evidence of a diffusive NG mode in a non-equilibrium Bose-Einstein condensate of microcavity polaritons. The NG mode is observed as a spectral narrowing in the spectroscopic response of the condensate. Additionally, explicitly breaking the symmetry causes the opening of a gap in the spectrum and the disappearance of the NG mode. Our observations confirm the diffusive dynamics of the NG mode of non-equilibrium phase transitions and establish a promising framework to investigate fundamental questions in statistical mechanics.

cond-mat.quant-gas↗

arXiv2023

Photonic lattices of coaxial cables: flat bands and artificial magnetic fields

We propose the use of networks of standard, commercially-available coaxial cables as a platform to realize photonic lattice models. As a specific example, we consider a brick wall lattice formed from coaxial cables and T-shaped connectors. We calculate the dispersion of photonic Bloch waves in the lattice: we find a repeated family of three bands, which include a flat band and two Dirac points. We then demonstrate a method to displace the Dirac points, leading to an induced artificial gauge field, and a method to energetically isolate the flat band. Our results readily suggest that the interplay of nonlinearities and non-trivial topology are a natural avenue to explore in order to unlock the full power of this proposed platform.

physics.optics↗

arXiv2023

Chiral quantum optics in the bulk of photonic quantum Hall systems

We study light-matter interactions in the bulk of a two-dimensional photonic lattice system, where photons are subject to the combined effect of a synthetic magnetic field and an orthogonal synthetic electric field. In this configuration, chiral waveguide modes appear in the bulk region of the lattice, in direct analogy to transverse Hall currents in electronic systems. By evaluating the non-Markovian dynamics of emitters that are coupled to those modes, we identify critical coupling conditions, under which the shape of the spontaneously emitted photons becomes almost fully symmetric. Combined with a directional, dispersionless propagation, this property enables a complete reabsorption of the photon by another distant emitter, without relying on any time-dependent control. We show that this mechanism can be generalized to arbitrary in-plane synthetic potentials, thereby enabling flexible realizations of re-configurable networks of quantum emitters with arbitrary chiral connectivity.

quant-ph↗

arXiv2023

Artificial gauge fields in the t-z mapping for optical pulses: spatio-temporal wavepacket control and quantum Hall physics

We extend the $t-z$ mapping formalism of time-dependent paraxial optics by identifying configurations displaying a synthetic magnetic vector potential, leading to a non-trivial band topology in propagating geometries. We consider an inhomogeneous 1D array of coupled optical waveguides beyond the standard monochromatic approximation, and show that the wave equation describing paraxial propagation of optical pulses can be recast in the form of a Schrödinger equation, including a synthetic magnetic field whose strength can be controlled via the transverse spatial gradient of the waveguide properties across the array. We use an experimentally-motivated model of a laser-written waveguide array to demonstrate that this synthetic magnetic field can be engineered in realistic setups and can produce interesting observable effects such as cyclotron motion, a controllable Hall drift of the wavepacket displacement in space or time, and unidirectional propagation in chiral edge states. These results significantly extend the variety of physics that can be explored within propagating geometries and pave the way for exploiting this platform for higher-dimensional topological physics and strongly correlated fluids of light.

physics.optics↗

arXiv2023

Observation of false vacuum decay via bubble formation in ferromagnetic superfluids

In quantum field theory, the decay of an extended metastable state into the real ground state is known as ``false vacuum decay'' and it takes place via the nucleation of spatially localized bubbles. Despite the large theoretical effort to estimate the nucleation rate, experimental observations were still missing. Here, we observe bubble nucleation in isolated and highly controllable superfluid atomic systems, and we find good agreement between our results, numerical simulations and instanton theory opening the way to the emulation of out-of-equilibrium quantum field phenomena in atomic systems.

hep-ph↗

arXiv2023

Ferromagnetism in an extended coherently-coupled atomic superfluid

Ferromagnetism is an iconic example of a first-order phase transition taking place in spatially extended systems and is characterized by hysteresis and the formation of domain walls. In this paper we demonstrate that an extended atomic superfluid in the presence of a coherent coupling between two internal states exhibits a quantum phase transition from a para- to a ferromagnetic state. The nature of the transition is experimentally assessed by looking at the phase diagram as a function of the control parameters, at hysteresis phenomena, at the magnetic susceptibility and the magnetization fluctuations around the critical point. We show that the observed features are in good agreement with mean-field calculations. Additionally, we develop experimental protocols to deterministically generate domain walls that separate spatial regions of opposite magnetization in the ferromagnetic state. Thanks to the enhanced coherence properties of our atomic superfluid system compared to standard condensed matter systems, our results open the way towards the study of different aspects of the relaxation dynamics in isolated coherent many-body quantum systems.

cond-mat.quant-gas↗

arXiv2023

Magnetic-field-induced cavity protection for intersubband polaritons

We analyse the effect of a strong perpendicular magnetic field on an intersubband transition in a disordered doped quantum well strongly coupled to an optical cavity. The magnetic field changes the lineshape of the intersubband optical transition due to the interface roughness of the quantum well from a Lorentzian to a Gaussian one. In this regime, a novel form of magnetic-field-induced cavity protection sets in, which strongly reduces the polariton linewidth to the cavity contribution only. Implications of our results for fundamental studies of nonlinear polariton dynamics and for technological applications to polariton lasers are finally highlighted.

cond-mat.mes-hall↗

arXiv2023

Kardar-Parisi-Zhang universality in the linewidth of non-equilibrium 1D quasi-condensates

We investigate the finite-size origin of the emission linewidth of a spatially-extended, one-dimensional non-equilibrium condensate. We show that the well-known Schawlow-Townes scaling of laser theory, possibly including the Henry broadening factor, only holds for small system sizes, while in larger systems the linewidth displays a novel scaling determined by Kardar-Parisi-Zhang physics. This is shown to lead to an opposite dependence of the linewidth on the optical nonlinearity in the two cases. We then study how sub-universal properties of the phase dynamics such as the higher moments of the phase-phase correlator are affected by the finite size and discuss the relation between the field coherence and the exponential of the phase-phase correlator. We finally identify a configuration with enhanced open boundary conditions, which supports a spatially uniform steady-state and facilitates experimental studies of the linewidth scaling.

cond-mat.stat-mech↗

arXiv2023

Quantum vacuum excitation of a quasi-normal mode in an analog model of black hole spacetime

Vacuum quantum fluctuations near horizons are known to yield correlated emission by the Hawking effect. We use a driven-dissipative quantum fluid of microcavity polaritons as an analog model of a quantum field theory on a black-hole spacetime and numerically calculate correlated emission. We show that, in addition to the Hawking effect at the sonic horizon, quantum fluctuations may result in a sizeable stationary excitation of a quasi-normal mode of the field theory. Observable signatures of the excitation of the quasi-normal mode are found in the spatial density fluctuations as well as in the spectrum of Hawking emission. This suggests an intrinsic fluctuation-driven mechanism leading to the quantum excitation of quasi-normal modes on black hole spacetimes.

gr-qc↗

arXiv2022

Probing many-body correlations using quantum-cascade correlation spectroscopy

The radiative quantum cascade, i.e. the consecutive emission of photons from a ladder of energy levels, is of fundamental importance in quantum optics. For example, the two-photon cascaded emission from calcium atoms was used in pioneering experiments to test Bell inequalities. In solid-state quantum optics, the radiative biexciton-exciton cascade has proven useful to generate entangled-photon pairs. More recently, correlations and entanglement of microwave photons emitted from a two-photon cascaded process were measured using superconducting circuits. All these experiments rely on the highly non-linear nature of the underlying energy ladder, enabling direct excitation and probing of specific single-photon transitions. Here, we use exciton polaritons to explore the cascaded emission of photons in the regime where individual transitions of the ladder are not resolved, a regime that has not been addressed so far. We excite a polariton quantum cascade by off-resonant laser excitation and probe the emitted luminescence using a combination of spectral filtering and correlation spectroscopy. Remarkably, the measured photon-photon correlations exhibit a strong dependence on the polariton energy, and therefore on the underlying polaritonic interaction strength, with clear signatures from two- and three-body Feshbach resonances. Our experiment establishes photon-cascade correlation spectroscopy as a highly sensitive tool to provide valuable information about the underlying quantum properties of novel semiconductor materials and we predict its usefulness in view of studying many-body quantum phenomena.

quant-ph↗

arXiv2022

Superradiant phononic emission from the analog spin ergoregion in a two-component Bose-Einstein condensate

We make use of an analog gravity perspective to obtain a physical understanding of hydrodynamic instabilities stemming from the presence of quantized vortices in two-component atomic condensates and of their relation to ergoregion instabilities of rotating massive objects in gravitation. In addition to the localized instabilities related to vortex splitting, configurations displaying dynamically unstable modes that extend well outside the vortex core are found. In this case, the superradiant scattering process involves phonon emission into the much wider ergoregion of spin modes, so the physics most closely resembles the one of rotating massive objects. Our results confirm the potential of two-component condensates as analog models of rotating space-times in different regimes of gravitational interest.

cond-mat.quant-gas↗

arXiv2022

Spectrum of collective excitations of a quantum fluid of polaritons

We use a recently developed high-resolution coherent probe spectroscopy method to investigate the dispersion of collective excitations of a polaritonic quantum fluid. We measure the dispersion relation with high energy and wavenumber resolution, which allows us to determine the speed of sound in the fluid and to evidence the contribution of an excitonic reservoir. We report on the generation of collective excitations at negative energies, on the ghost branch of the dispersion curve. Precursors of dynamical instabilities are also identified. Our methods open the way to the precise study of quantum hydrodynamics of quantum fluids of light.

cond-mat.quant-gas↗

arXiv2022

Quantum Fluids of Light

In this Chapter, we give a brief review of the state of the art of theoretical and experimental studies of quantum fluids of light. Such systems consist of ensembles of photons that acquire a finite mass from spatial confinement or diffraction and finite binary interactions from the optical nonlinearity of the optical medium. The peculiar properties of these fluids are highlighted in comparison with standard condensed matter systems, with a special emphasis on the novel possibilities that they offer for the generation, the manipulation and the diagnostics of the fluid, as well as on their intrinsically non-equilibrium and/or dynamical nature. Perspectives towards a new generation of experiments on strongly correlated fluids of light and towards opto-electronic applications are finally sketched.

quant-ph↗

arXiv2022

Collective excitations of a strongly-correlated non-equilibrium photon fluid across the Mott/superfluid phase transition

We develop a Gutzwiller theory for the non-equilibrium steady states of a strongly-interacting photon fluid driven by a non-Markovian incoherent pump. In particular, we explore the collective excitation modes across the out-of-equilibrium Mott/superfluid transition, characterizing the diffusive Goldstone mode in the superfluid phase and the particle/hole excitations in the insulating one. Observable features in the pump-and-probe optical response of the system are highlighted. Our results appear as experimentally accessible to state-of-the-art circuit-QED devices and open the way for driven-dissipative fluids of light as quantum simulators of novel many-body scenarios.

quant-ph↗

arXiv2022

Intertwining of lasing and superradiance under spintronic pumping

We introduce a quantum optics platform featuring the minimal ingredients for the description of a spintronically pumped magnon condensate, which we use to promote driven-dissipative phase transitions in the context of spintronics. We consider a Dicke model weakly coupled to an out-of-equilibrium bath with a tunable spin accumulation. The latter is pumped incoherently in a fashion reminiscent of experiments with magnet-metal heterostructures. The core of our analysis is the emergence of a hybrid lasing-superradiant regime that does not take place in an ordinary pumped Dicke spin ensemble, and which can be traced back to the spintronics pumping scheme. We interpret the resultant non-equilibrium phase diagram from both a quantum optics and a spintronics standpoint, supplying a conceptual bridge between the two fields. The outreach of our results concern dynamical control in magnon condensates and frequency-dependent gain media in quantum optics.

cond-mat.stat-mech↗

arXiv2022

Quantum Hall and Synthetic Magnetic-Field Effects in Ultra-Cold Atomic Systems

In this Chapter, we give a brief review of the state of the art of theoretical and experimental studies of synthetic magnetic fields and quantum Hall effects in ultracold atomic gases. We focus on integer, spin, and fractional Hall effects, indicate connections to topological matter, and discuss prospects for the realization of full-fledged gauge field theories where the synthetic magnetic field has its own dynamics. The advantages of these systems over traditional electronic systems are highlighted. Finally, interdisciplinary comparisons with other synthetic matter platforms based on photonic and trapped-ion systems are drawn. We hope this chapter to illustrate the exciting progress that the field has experienced in recent years.

cond-mat.quant-gas↗

arXiv2022

Interplay of Kelvin-Helmholtz and superradiant instabilities of an array of quantized vortices in a two-dimensional Bose--Einstein condensate

We investigate the various physical mechanisms that underlie the dynamical instability of a quantized vortex array at the interface between two counter-propagating superflows in a two-dimensional Bose--Einstein condensate. Instabilities of markedly different nature are found to dominate in different flow velocity regimes. For moderate velocities where the two flows are subsonic, the vortex lattice displays a quantized version of the hydrodynamic Kelvin--Helmholtz instability (KHI), with the vortices rolling up and co-rotating. For supersonic flow velocities, the oscillation involved in the KHI can resonantly couple to acoustic excitations propagating away in the bulk fluid on both sides. This makes the KHI rate to be effectively suppressed and other mechanisms to dominate: For finite and relatively small systems along the transverse direction, the instability involves a repeated superradiant scattering of sound waves off the vortex lattice; for transversally unbound systems, a radiative instability dominates, leading to the simultaneous growth of a localized wave along the vortex lattice and of acoustic excitations propagating away in the bulk. Finally, for slow velocities, where the KHI rate is intrinsically slow, another instability associated to the rigid lateral displacement of the vortex lattice due to the vicinity of the system's boundary is found to dominate.

cond-mat.quant-gas↗

arXiv2022

Numerical studies of back-reaction effects in an analog model of cosmological pre-heating

We theoretically propose an atomic Bose-Einstein condensate as an analog model of back-reaction effects during the pre-heating stage of early Universe. In particular, we address the out-of-equilibrium dynamics where the initially excited inflaton field decays by parametrically exciting the matter fields. We consider a two-dimensional, ring-shaped BEC under a tight transverse confinement whose transverse breathing mode and the Goldstone and dipole excitation branches simulate the inflaton and quantum matter fields, respectively. A strong excitation of the breathing mode leads to an exponentially-growing emission of dipole and Goldstone excitations via parametric pair creation: our numerical simulations of the BEC dynamics show how the associated back-reaction effect not only results in an effective friction of the breathing mode but also in a quick loss of longitudinal spatial coherence of the initially in-phase excitations. Implications of this result on the validity of the usual semiclassical description of back-reaction are finally discussed.

quant-ph↗