arXiv subjects

Ivan Amelio

Publications and source records attributed to Ivan Amelio.

At least 19 recordsLinked to original sources

arXiv2026

Competition between pair and single-particle superfluidity in bosonic quasi-flat bands: A Gaussian state approach

The interplay between interactions and quantum geometry can drive weakly dispersive bosons into different exotic many-body phases. In this work we study a quasi flat-band model in one dimension that exhibits an extended pair-superfluid phase in the all-flat-band limit. Introducing single-particle hopping leads to an intriguing competition with a more conventional single-particle superfluid: we find that the pair superfluid remains stable for a finite range of the hopping strength until the system eventually transitions into the conventional superfluid phase. In our study, we make use of a variational Gaussian state approach that provides a unified description of the single-particle and pair superfluid phases, regarding both the ground state wavefunction and the collective excitation spectrum. In particular, we derive a general relation between the speed of sound and a ``quantum geometric kernel'', thereby extending earlier connections to the quantum metric, which relied on single-particle mean-field theory. This approach is combined with insights from the two-boson problem and exact diagonalization to map out the full phase diagram of the model. Our results show that the Gaussian approach is a versatile tool for studying a broad range of superfluid phases of interacting bosons in multi-orbital lattices.

cond-mat.quant-gas↗

arXiv2026

Quantum Simulation with Fluxonium Qutrit Arrays

Fluxonium superconducting circuits were originally proposed to realize highly coherent qubits. In this work, we explore how these circuits can be used to implement and harness qutrits, by tuning their energy levels and matrix elements via an external flux bias. In particular, we investigate the distinctive features of arrays of fluxonium qutrits, and their potential for the quantum simulation of exotic quantum matter. We identify four different operational regimes, classified according to the plasmon-like versus fluxon-like nature of the qutrit excitations. Highly tunable on-site interactions are complemented by correlated single-particle hopping, pair hopping and non-local interactions, which naturally emerge and have different weights in the four regimes. Dispersive corrections and decoherence are also analyzed. We investigate the rich ground-state phase diagram of qutrit arrays and propose practical dynamical experiments to probe the different regimes. Altogether, fluxonium qutrit arrays emerge as a versatile and experimentally accessible platform to explore strongly correlated bosonic matter beyond the Bose-Hubbard paradigm, and with a potential toward simulating lattice gauge theories and non-Abelian topological states.

quant-ph↗

arXiv2025

Fate of chiral order and impurity self-pinning in flat bands with local symmetry

Interacting bosons on a single plaquette threaded by a $π$-flux can spontaneously break time-reversal symmetry, resulting in a chiral loop current. Connecting such bosonic $π$-flux plaquettes in a dispersive configuration was recently shown to lead to long-range chiral order. Here, instead, we design a chain of $π$-flux plaquettes that exhibits an all-flat-bands single-particle energy spectrum and an extensive set of local symmetries. Using Elitzur's theorem, we show that these local symmetries prevent the emergence of long-range chiral order. Moreover, projecting the dynamics to a Creutz ladder model with an effective intra-rung interaction allows one to derive simple spin Hamiltonians capturing the ground state degeneracy and the low-energy excitations, and to confirm the absence of chiral order. Nevertheless, we show how to obtain gauge-invariant information from a mean-field approach, which explicitly break gauge-invaraince. Finally, we observe an ``impurity self-pinning'' phenomenon, when an extra boson is added on top of a ground state at integer filling, resulting in a non-dispersive density peak. Exact diagonalization benchmarks are also provided, and experimental perspectives are discussed.

cond-mat.quant-gas↗

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

First order Quantum Hall to Wigner crystal phase transition on a triangular lattice: an iDMRG study

In this work we study a system of interacting fermions on a triangular lattice in the presence of an external magnetic field. We neglect spin and fix a density of one third, with one unit of magnetic flux per particle. The infinite density matrix renormalization group algorithm is used to compute the ground state of this generalized Fermi-Hubbard model. Increasing the strength of the nearest-neighbor repulsion, we find a first order transition between an Integer Quantum Hall phase and a crystalline, generalized Wigner crystal state. The first-order nature of the phase transition is consistent with a Ginzburg-Landau argument. We expect our results to be relevant for moir\'e heterostructures of two-dimensional materials.

cond-mat.str-el↗

arXiv2024

Polaron formation in insulators and the key role of hole scattering processes: Band insulators, charge density waves and Mott transition

A mobile impurity immersed in a non-interacting Fermi sea is dressed by the gapless particle-hole excitations of the fermionic medium. This conventional Fermi-polaron setting is well described by the so-called ladder approximation, which consists in neglecting impurity-hole scattering processes. In this work, we analyze polaron formation in the context of insulating states of matter, considering increasing levels of correlation in the medium:~band insulators originating from external periodic potentials, spontaneously-formed charge density waves, and a Fermi-Hubbard system undergoing a metal-Mott insulator transition. The polaron spectral function is shown to exhibit striking signatures of the underlying fermionic background, such as the single-particle band gap, particle-hole symmetry and the transition to the Mott state. These signatures are identified within the framework of the Chevy ansatz, i.e. upon restricting the Hilbert space to single particle-hole excitations. Interestingly, we find that the ladder approximation is inaccurate in these band systems, due to the fact that the particle and hole scattering phase spaces are comparable. Our results provide a step forward in the understanding of polaron formation in correlated many-body media, which are relevant to both cold-atom and semiconductor experiments.

cond-mat.str-el↗

arXiv2024

Chiral polaron formation on the edge of topological quantum matter

Immersing a mobile impurity in a quantum many-body environment can reveal fundamental properties of the background medium, hence providing a powerful probe of quantum matter. This approach is particularly intriguing when considering media with exotic properties, such as strongly-correlated phases and topological states of matter. In this work, we study the dressing of a mobile impurity interacting with a chiral mode, as provided by the edge of topological quantum matter. The resulting ''chiral polaron'' is characterized by an asymmetric spectral function, which reflects the chirality and group velocity of the topological edge mode and the drag experienced by the mobile impurity. We first build our theoretical understanding from an effective one-dimensional chiral model, which captures the hallmark signatures of the chiral polaron. We then demonstrate how this simple picture extends to realistic models of integer and fractional Chern insulator states, by adapting tensor-network methods to polaron spectroscopy. Injecting mobile impurities on the edge of topological quantum matter is shown to be a powerful tool to probe exotic edge properties, particularly suitable for cold-atom experiments.

cond-mat.quant-gas↗

arXiv2024

Fermionization and collective excitations of 1D polariton lattices

We theoretically demonstrate that the hallmarks of correlation and fermionization in a one-dimensional exciton-polaritons gas can be observed with state-of-the-art technology. Our system consists of a chain of excitonic quantum dots coupled to a photonic waveguide, with a low filling of polaritons. We analytically identify the Tonks-Girardeau, Tavis-Cummings and mean-field limits and relate them to different regimes of the excitonic anharmonicity and photonic bandwidth. Using matrix-product states, we numerically calculate the ground-state energies, correlation functions and dynamic structure factor of the system. In particular, the latter has a finite weight in the Lieb-Liniger hole branch, and the density-density correlator displays Friedel-like oscillations for realistic parameters, which reveal the onset of fermionization close to the Tonks-Girardeau regime. Our work encourages future experiments aimed at observing, for the first time and in spite of the moderate excitonic anharmonicity, strongly correlated exciton-polariton physics.

quant-ph↗

arXiv2024

Polaron spectroscopy of interacting Fermi systems: insights from exact diagonalization

Immersing a mobile impurity into a many-body quantum system represents a theoretically intriguing and experimentally effective way of probing its properties.In this work, we study the polaron spectral function in various environments, within the framework of Fermi-Hubbard models. Inspired by possible realizations in cold atoms and semiconductor heterostructures, we consider different configurations for the background Fermi gas, including charge density waves, multiple Fermi seas and pair superfluids. While our calculations are performed using an exact-diagonalization approach, hence limiting our analysis to systems of few interacting Fermi particles, we identify robust spectral features supported by theoretical results. Our work provides a benchmark for computations based on mean-field approaches and reveal surprising features of polaron spectra, inspiring new theoretical investigations.

cond-mat.quant-gas↗

arXiv2023

Lasing, quantum geometry and coherence in non-Hermitian flat bands

We show that lasing in flat band lattices can be stabilized by means of the geometrical properties of the Bloch states, in settings where the single-particle dispersion is flat in both its real and imaginary parts. We illustrate a general projection method and compute the collective excitations, which are shown to display a diffusive behavior ruled by quantum geometry through a peculiar coefficient involving gain, losses and interactions. Then, we analytically show that the phase dynamics display a surprising cancellation of the Kardar-Parisi-Zhang nonlinearity at the leading order. Because of the relevance of Kardar-Parisi-Zhang universality in one-dimensional geometries, we focus our study on the diamond chain and provide confirmation of these results through full numerical simulations.

cond-mat.quant-gas↗

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↗

arXiv2022

Polaron spectroscopy of a bilayer excitonic insulator

Recent advances in fabrication of two dimensional materials and their moiré heterostructures have opened up new avenues for realization of ground-state excitonic insulators, where the structure spontaneously develops a finite interlayer electronic polarization. We propose and analyze a scheme where an optically generated intralayer exciton is screened by excitations out of the excitonic insulator to form interlayer polarons. Using Quantum Monte-Carlo calculations we first determine the binding energy of the biexciton state composed of inter- and intralayer excitons, which plays a central role in understanding polaron formation. We describe the excitations out of the ground-state condensate using BCS theory and use a single interacting-quasiparticle-pair excitation Ansatz to describe dynamical screening of optical excitations. Our predictions carry the hallmarks of the excitonic insulator excitation spectrum and show how changing the interlayer exciton binding energy by increasing the layer separation modifies the optical spectra.

cond-mat.mes-hall↗

arXiv2022

Two-dimensional polaron spectroscopy of Fermi superfluids

Multidimensional spectroscopy is becoming an increasingly popular tool and there is an ongoing effort to access electronic transitions and many-body dynamics in correlated materials. We apply the protocol recently proposed by Wang to extract two-dimensional polaron spectra in a Fermi superfluid with an impurity. The bath is descibed by a BCS ansatz and it assumed that the impurity can scatter at most one quasiparticle pair. The spectral response contains a symmetric contribution, which carries the same information as Ramsey spectra, and an asymmetric one. While {\it a priori} it may seem promising to probe the quasiparticle gap from the asymmetric contribution, we show explicitly that this is not the case and, in the absence of incoherent processes, multidimensional spectroscopy does not bring much additional information. Our calculation is suitable for 3D ultracold gases, but we discuss implications for exciton-polarons in 2D materials.

cond-mat.quant-gas↗

arXiv2022

Observation of KPZ universal scaling in a one-dimensional polariton condensate

Revealing universal behaviors is a hallmark of statistical physics. Phenomena such as the stochastic growth of crystalline surfaces, of interfaces in bacterial colonies, and spin transport in quantum magnets all belong to the same universality class, despite the great plurality of physical mechanisms they involve at the microscopic level. This universality stems from a common underlying effective dynamics governed by the non-linear stochastic Kardar-Parisi-Zhang (KPZ) equation. Recent theoretical works suggest that this dynamics also emerges in the phase of out-of-equilibrium systems displaying macroscopic spontaneous coherence. Here, we experimentally demonstrate that the evolution of the phase in a driven-dissipative one-dimensional polariton condensate falls in the KPZ universality class. Our demonstration relies on a direct measurement of KPZ space-time scaling laws, combined with a theoretical microscopic analysis that consistently reveals the other key signatures of this universality class, together with the possible resilience of KPZ dynamics to the presence of space-time vortices. Our results highlight fundamental physical differences between out-of-equilibrium condensates and their equilibrium counterparts, and open a new paradigm for exploring universal behaviors in open systems.

cond-mat.mes-hall↗

arXiv2021

Optical dressing of the electronic response of two-dimensional semiconductors in quantum and classical descriptions of cavity electrodynamics

We study quantum effects of the vacuum light-matter interaction in materials embedded in optical cavities. We focus on the electronic response of a two-dimensional semiconductor placed inside a planar cavity. By using a diagrammatic expansion of the electron-photon interaction, we describe signatures of light-matter hybridization characterized by large asymmetric shifts of the spectral weight at resonant frequencies. We follow the evolution of the light-dressing from the cavity to the free-space limit. In the cavity limit, light-matter hybridization results in a modification of the optical gap with sizeable spectral weight appearing below the bare gap edge. In the limit of large cavities, we find a residual redistribution of spectral weight which becomes independent of the distance between the two mirrors. We show that the photon dressing of the electronic response can be fully explained by using a classical description of light. The classical description is found to hold up to a strong coupling regime of the light-matter interaction highlighted by the large modification of the photon spectra with respect to the empty cavity. We show that, despite the strong coupling, quantum corrections are negligibly small and weakly dependent on the cavity confinement. As a consequence, in contrast to the optical gap, the single particle electronic band gap is not sensibly modified by the strong-coupling. Our results show that quantum corrections are dominated by off-resonant photon modes at high energy. As such, cavity confinement can hardly be seen as a knob to control the quantum effects of the light-matter interaction in vacuum.

cond-mat.mes-hall↗

arXiv2021

Bogoliubov theory of the laser linewidth and application to polariton condensates

For a generic semi-classical laser dynamics in the complex Ginzburg-Landau form, we develop a Bogoliubov approach for the computation of the laser emission linewidth. Our method provides a unifying perspective of the treatments by Henry and Petermann: both broadening mechanisms are ascribed to the non-orthogonality of the Bogoliubov modes, which live in a space with doubled degrees of freedom. As an example of application, the method allows to study the interplay of driven-dissipation, interactions and spatial inhomogeneity typical of polariton condensates. The traditional theory of the Henry and Petermann factors is found to fail dramatically in the presence of sizable polariton-polariton interactions. In particular, also in a strong confining potential, the intrinsically multi-mode nature of the density fluctuations has to be considered in order to describe quantitatively phase diffusion {\em à la} Henry.

physics.optics↗

arXiv2021

Linearized theory of the fluctuation dynamics in 2D topological lasers

We theoretically study the collective excitation modes of a topological laser device operating in a single-mode steady-state with monochromatic emission. We consider a model device based on a two-dimensional photonic Harper-Hofstadter lattice including a broadband gain medium localized on the system edge. Different regimes are considered as a function of the value of the optical nonlinearity and of the gain relaxation time. The dispersion of the excitation modes is calculated via a full two-dimensional Bogoliubov approach and physically interpreted in terms of an effective one-dimensional theory. Depending on the system parameters, various possible physical processes leading to dynamical instabilities are identified and characterized. On this basis, strategies to enforce a stable single-mode topological laser operation are finally pointed out.

physics.optics↗

arXiv2020

Theory of the coherence of topological lasers

We present a theoretical study of the temporal and spatial coherence properties of a topological laser device built by including saturable gain on the edge sites of a Harper--Hofstadter lattice for photons. For small enough lattices the Bogoliubov analysis applies, the emission is nearly a single-mode one and the coherence time is almost determined by the total number of photons in the device in agereement with the standard Schawlow-Townes phase diffusion. In larger lattices, looking at the lasing edge mode in the comoving frame of its chiral motion, the spatio-temporal correlations of long-wavelength fluctuations display a Kardar-Parisi-Zhang (KPZ) scaling. Still, at very long times, when the finite size of the device starts to matter, the functional form of the temporal decay of coherence changes from the KPZ stretched exponential to a Schawlow-Townes-like exponential, while the nonlinear many-mode dynamics of KPZ fluctuations remains visible as an enhanced linewidth as compared to the single-mode Schawlow-Townes prediction. While we have established the above behaviors also for non-topological laser arrays, the crucial role of topology in protecting the coherence from static disorder is finally highlighted: our ground-breaking numerical calculations suggest the dramatically reinforced coherence properties of topological lasers compared to corresponding non-topological devices. These results open exciting possibilities for both fundamental studies of non-equilibrium statistical mechanics and concrete applications to laser devices.

cond-mat.mes-hall↗