arXiv subjects

Iacopo Carusotto

Publications and source records attributed to Iacopo Carusotto.

At least 145 records · Page 8Linked to original sources

arXiv2016

The role of geometry in the superfluid flow of nonlocal photon fluids

Recent work has unveiled a new class of optical systems that can exhibit the characteristic features of superfluidity. One such system relies on the repulsive photon-photon interaction that is mediated by a thermal optical nonlinearity and is therefore inherently nonlocal due to thermal diffusion. Here we investigate how such a nonlocal interaction, which at a first inspection would not be expected to lead to superfluid behavior, may be tailored by acting upon the geometry of the photon fluid itself. Our models and measurements show that restricting the laser profile and hence the photon fluid to a strongly elliptical geometry modifies thermal diffusion along the major beam axis and reduces the effective nonlocal interaction length by two orders of magnitude. This in turn enables the system to display a characteristic trait of superfluid flow: the nucleation of quantized vortices in the flow past an extended physical obstacle. These results are general and apply to other nonlocal fluids, such as dipolar Bose-Einstein condensates, and show that "thermal" photon superfluids provide an exciting and novel experimental environment for probing the nature of superfluidity, with applications to the study of quantum turbulence and analogue gravity.

cond-mat.quant-gas↗

arXiv2016

Floquet topological system based on frequency-modulated classical coupled harmonic oscillators

We theoretically propose how to observe topological effects in a generic classical system of coupled harmonic oscillators, such as classical pendula or lumped-element electric circuits, whose oscillation frequency is modulated fast in time. Making use of Floquet theory in the high frequency limit, we identify a regime in which the system is accurately described by a Harper-Hofstadter model where the synthetic magnetic field can be externally tuned via the phase of the frequency-modulation of the different oscillators. We illustrate how the topologically-protected chiral edge states, as well as the Hofstadter butterfly of bulk bands, can be observed in the driven-dissipative steady state under a monochromatic drive. In analogy with the integer quantum Hall effect, we show how the topological Chern numbers of the bands can be extracted from the mean transverse shift of the steady-state oscillation amplitude distribution. Finally we discuss the regime where the analogy with the Harper-Hofstadter model breaks down.

cond-mat.mes-hall↗

arXiv2016

Four-Dimensional Quantum Hall Effect with Ultracold Atoms

We propose a realistic scheme to detect the 4D quantum Hall effect using ultracold atoms. Based on contemporary technology, motion along a synthetic fourth dimension can be accomplished through controlled transitions between internal states of atoms arranged in a 3D optical lattice. From a semi-classical analysis, we identify the linear and non-linear quantized current responses of our 4D model, relating these to the topology of the Bloch bands. We then propose experimental protocols, based on current or center-of-mass-drift measurements, to extract the topological 2nd Chern number. Our proposal sets the stage for the exploration of novel topological phases in higher dimensions.

cond-mat.quant-gas↗

arXiv2016

Momentum-space Landau levels in driven-dissipative cavity arrays

We theoretically study the driven-dissipative Harper-Hofstadter model on a 2D square lattice in the presence of a weak harmonic trap. Without pumping and loss, the eigenstates of this system can be understood, in certain limits, as momentum-space toroidal Landau levels, where the Berry curvature, a geometrical property of an energy band, acts like a momentum-space magnetic field. We show that key features of these eigenstates can be observed in the steady-state of the driven-dissipative system under a monochromatic coherent drive, and present a realistic proposal for an optical experiment using state-of-the-art coupled cavity arrays. We discuss how such spectroscopic measurements may be used to probe effects associated both with the off-diagonal elements of the matrix-valued Berry connection and with the synthetic magnetic gauge.

cond-mat.mes-hall↗

arXiv2015

How to directly observe Landau levels in driven-dissipative strained honeycomb lattices

We study the driven-dissipative steady-state of a coherently-driven Bose field in a honeycomb lattice geometry. In the presence of a suitable spatial modulation of the hopping amplitudes, a valley-dependent artificial magnetic field appears and the low-energy eigenmodes have the form of relativistic Landau levels. We show how the main properties of the Landau levels can be extracted by observing the peaks in the absorption spectrum of the system and the corresponding spatial intensity distribution. Finally, quantitative predictions for realistic lattices based on photonic or microwave technologies are discussed.

cond-mat.quant-gas↗

arXiv2015

Momentum-space Harper-Hofstadter model

We show how the weakly trapped Harper-Hofstadter model can be mapped onto a Harper-Hofstadter model in momentum space. In this momentum-space model, the band dispersion plays the role of the periodic potential, the Berry curvature plays the role of an effective magnetic field, the real-space harmonic trap provides the momentum-space kinetic energy responsible for the hopping, and the trap position sets the boundary conditions around the magnetic Brillouin zone. Spatially local interactions translate into nonlocal interactions in momentum space: within a mean-field approximation, we show that increasing interparticle interactions leads to a structural change of the ground state, from a single rotationally symmetric ground state to degenerate ground states that spontaneously break rotational symmetry.

cond-mat.quant-gas↗

arXiv2015

Artificial Magnetic Fields in Momentum Space in Spin-Orbit Coupled Systems

The Berry curvature is a geometrical property of an energy band which can act as a momentum space magnetic field in the effective Hamiltonian of a wide range of systems. We apply the effective Hamiltonian to a spin-1/2 particle in two dimensions with spin-orbit coupling, a Zeeman field and an additional harmonic trap. Depending on the parameter regime, we show how this system can be described in momentum space as either a Fock-Darwin Hamiltonian or a one-dimensional ring pierced by a magnetic flux. With this perspective, we interpret important single-particle properties, and identify analogue magnetic phenomena in momentum space. Finally we discuss the extension of this work to higher spin systems, as well as experimental applications in ultracold atomic gases and photonic systems.

cond-mat.quant-gas↗

arXiv2015

Experimental characterisation of nonlocal photon superfluids

Quantum gases of atoms and exciton-polaritons are nowadays a well established theoretical and experimental tool for fundamental studies of quantum many-body physics and suggest promising applications to quantum computing. Given their technological complexity, it is of paramount interest to devise other systems where such quantum many-body physics can be investigated at a lesser technological expense. Here we examine a relatively well-known system of laser light propagating through thermo-optical defocusing media: based on a hydrodynamical description of light as a quantum fluid of interacting photons, we investigate such systems as a valid, room temperature alternative to atomic or exciton-polariton condensates for studies of many-body physics. First, we show that by using a technique traditionally used in oceanography, it is possible to perform a direct measurement of the single-particle part of the dispersion relation of the elementary excitations on top of the photon fluid and to detect its global flow. Then, using a pump-and-probe set-up, we investigate the collective nature of low-wavevector sound modes of the fluid and observe signatures of superfluid behaviour.

physics.optics↗

arXiv2015

Laser operation and Bose-Einstein condensation: analogies and differences

After reviewing the interpretation of laser operation as a non-equilibrium Bose-Einstein condensation phase transition, we illustrate the novel features arising from the non-equilibrium nature of photon and polariton Bose-Einstein condensates recently observed in experiments. We then proposea quantitative criterion to experimentally assess the equilibrium vs. non-equilibrium nature of a specific condensation process, based on fluctuation-dissipation relations. The power of this criterion is illustrated on two models which shows very different behaviours.

cond-mat.quant-gas↗

arXiv2015

Photon transport in a dissipative chain of nonlinear cavities

We analyze a chain of coupled nonlinear optical cavities driven by a coherent source of light localized at one end and subject to uniform dissipation. We characterize photon transport by studying the populations and the photon correlations as a function of position. When complemented with input-output theory, these quantities provide direct information about photon transmission through the system. The position of single- and multi-photon resonances directly reflect the structure of the many-body energy levels. This shows how a study of transport along a coupled cavity array can provide rich information about the strongly correlated (many-body) states of light even in presence of dissipation. By means of a numerical algorithm based on the time-evolving block decimation scheme adapted to mixed states, we are able to simulate arrays up to sixty cavities.

cond-mat.other↗

arXiv2014

A quantum Langevin model for non-equilibrium condensation

We develop a quantum model for non-equilibrium Bose-Einstein condensation of photons and polaritons in planar microcavity devices. The model builds upon laser theory and includes the spatial dynamics of the cavity field, a saturation mechanism and some frequency-dependence of the gain: quantum Langevin equations are written for a cavity field coupled to a continuous distribution of externally pumped two-level emitters with a well-defined frequency. As a an example of application, the method is used to study the linearised quantum fluctuations around a steady-state condensed state. In the good-cavity regime, an effective equation for the cavity field only is proposed in terms of a stochastic Gross-Pitaevskii equation. Perspectives in view of a full quantum simulation of the non-equilibrium condensation process are finally sketched.

cond-mat.quant-gas↗

arXiv2014

Quantum Mechanics with a Momentum-Space Artificial Magnetic Field

The Berry curvature is a geometrical property of an energy band which acts as a momentum space magnetic field in the effective Hamiltonian describing single-particle quantum dynamics. We show how this perspective may be exploited to study systems directly relevant to ultracold gases and photonics. Given the exchanged roles of momentum and position, we demonstrate that the global topology of momentum space is crucially important. We propose an experiment to study the Harper-Hofstadter Hamiltonian with a harmonic trap that will illustrate the advantages of this approach and that will also constitute the first realization of magnetism on a torus.

cond-mat.quant-gas↗

arXiv2014

Spontaneous quantum emission from analog white holes in a nonlinear optical medium

We use a microscopic quantum optical model to compute the spectrum of quantum vacuum emission from strong laser pulses propagating in nonlinear optical media. Similarities and differences with respect to the emission of analog white holes as predicted by quantum field theory in curved spacetime are highlighted. Conceptual issues related to the role played by the material dispersion and to the presence or absence of the horizon are clarified. Critical comparison with available experimental data is made.

physics.optics↗

arXiv2014

Inter-mode reactive coupling induced by waveguide-resonator interaction

We report on a joint theoretical and experimental study of an integrated photonic device consisting of a single mode waveguide vertically coupled to a disk-shaped microresonator. Starting from the general theory of open systems, we show how the presence of a neighboring waveguide induces reactive inter-mode coupling in the resonator, analogous to an off-diagonal Lamb shift from atomic physics. Observable consequences of this coupling manifest as peculiar Fano lineshapes in the waveguide transmission spectra. The theoretical predictions are validated by full vectorial 3D finite element numerical simulations and are confirmed by the experiments.

physics.optics↗

arXiv2014

Spectrum and entanglement of phonons in quantum fluids of light

We study the quantum state of phonons propagating on top of a fluid of light coherently generated in a planar microcavity device by a quasi-resonant incident laser beam. In the steady-state under a monochromatic pump, because of the finite radiative lifetime of photons, a sizable incoherent population of low frequency phonons is predicted to appear. Their mean occupation number differs from a Planck distribution and is independent on the photon lifetime. When the photon fluid is subjected to a sudden change of its parameters, additional phonon pairs are created in the fluid with remarkable two-mode squeezing and entanglement properties. Schemes to assess non-separability of the phonon state from measurements of the correlation functions of the emitted light are discussed.

cond-mat.quant-gas↗

arXiv2014

Dynamical decoupling and dynamical isolation in temporally modulated coupled pendulums

We theoretically study the dynamics of a pair of coupled pendulums subject to a periodic temporal modulation of their oscillation frequency. Inspired from analogous developments in quantum mechanics, we anticipate dynamical localization and dynamical isolation effects, as well as the occurrence of non-trivial coupling phases. Perspectives in the direction of studying synthetic gauge fields in a classical mechanics context are outlined.

physics.class-ph↗

arXiv2014

Anomalous and Quantum Hall Effects in Lossy Photonic Lattices

We theoretically discuss analogues of the anomalous and the integer quantum Hall effect in driven-dissipative two-dimensional photonic lattices in the presence of a synthetic gauge field. Photons are coherently injected by a spatially localized pump, and the transverse shift of the in-plane light distribution under the effect of an additional uniform force is considered. Depending on pumping parameters, the transverse shift turns out to be proportional either to the global Chern number (integer quantum Hall effect) or to the local Berry curvature (anomalous Hall effect). This suggests a viable route to experimentally measure these quantities in photonic lattices.

cond-mat.quant-gas↗

arXiv2014

Direct observation of Dirac cones and a flatband in a honeycomb lattice for polaritons

Two-dimensional lattices of coupled micropillars etched in a planar semiconductor microcavity offer a workbench to engineer the band structure of polaritons. We report experimental studies of honeycomb lattices where the polariton low-energy dispersion is analogous to that of electrons in graphene. Using energy-resolved photoluminescence we directly observe Dirac cones, around which the dynamics of polaritons is described by the Dirac equation for massless particles. At higher energies, we observe p orbital bands, one of them with the nondispersive character of a flatband. The realization of this structure which holds massless, massive and infinitely massive particles opens the route towards studies of the interplay of dispersion, interactions, and frustration in a novel and controlled environment.

cond-mat.quant-gas↗