arXiv subjects

Luca Salasnich

Publications and source records attributed to Luca Salasnich.

At least 37 records · Page 2Linked to original sources

arXiv2026

Variational treatment of the electromagnetic field in an optical cavity with a moving mirror

Optical cavities with moving mirrors provide a versatile platform for exploring radiation-matter interactions and optically mediated mechanical effects, whose control has wide technological implications. However, capturing the coupled dynamics of the electromagnetic field and of the mirror within a consistent theoretical framework remains challenging. We analyze the problem of the coupling between classical electromagnetic fields in a cavity and a movable mirror, considering both nonrelativistic and relativistic regimes of motion. Starting from the equations of motion for a mirror subject to a generic external potential, we provide a variational formulation of the mirror-radiation interaction. Within this framework, a single-mode variational approximation is introduced, which captures the essential dynamical features of the coupled system. In the special case of a mirror undergoing free motion, the variational method yields an exact solution. This unified treatment highlights the connection between different dynamical regimes and provides a basis for analyzing applications ranging from precision interferometry to relativistic radiation-pressure effects.

physics.optics↗

arXiv2025

Experimental Observation of Single- and Multisite Matter-Wave Solitons in an Optical Accordion Lattice

We report the experimental observation of discrete bright matter-wave solitons with attractive interaction in an optical lattice. Using an accordion lattice with adjustable spacing, we prepare a Bose-Einstein condensate of cesium atoms across a defined number of lattice sites. By quenching the interaction strength and the trapping potential, we generate both single-site and multisite solitons. Our results reveal the existence and characteristics of these solitons across a range of lattice depths and spacings. We identify stable regions of the solitons based on interaction strength and lattice properties, and compare these findings with theoretical predictions. The experimental results qualitatively agree with a Gaussian variational model and match quantitatively with numerical simulations of the three-dimensional Gross-Pitaevskii equation extended with a quintic term to account for the loss of atoms. Our results provide insights into the quench dynamics and collapse mechanisms, paving the way for further studies of transport and dynamical properties of matter-wave solitons in lattices.

cond-mat.quant-gas↗

arXiv2025

Effective action approach to quantum and thermal effects: from one particle to Bose-Einstein condensates

We present a detailed derivation of the quantum and quantum-thermal effective action for non-relativistic systems, starting from the single particle case and extending to the Gross-Pitaevskii (GP) field theory for weakly interacting bosons. In the single-particle framework, we introduce the one-particle irreducible 1PI effective action formalism taking explicitly into account the choice of the initial quantum state, its saddle-point plus Gaussian fluctuation approximation, and its finite temperature extension via Matsubara summation. This yields a clear physical interpretation in terms of zero-point and thermal contributions to the Helmholtz free energy. The formalism is then applied to the GP action producing the 1PI effective potential at zero and finite temperature including beyond-mean-field Lee-Huang-Yang and thermal corrections. We discuss the gapless and gapped Bogoliubov spectra, their relevance to equilibrium and nonequilibrium regimes, and the role of regularization. Applications include the inclusion of an external potential within the local density approximation, the derivation of finite-temperature Josephson equations, and the extension to D dimensional systems. This unified approach provides a transparent connection between microscopic quantum fluctuations and effective macroscopic equations of motion for Bose-Einstein condensates.

cond-mat.quant-gas↗

arXiv2025

Quantum action of the Josephson dynamics

We study the beyond-mean-field Josephson dynamics of the relative phase between two coupled macroscopic quantum systems. Using a covariant background field method, we derive the one-loop only-phase quantum effective action and the corresponding equation of motion for the quantum average of the phase. These analytical results are benchmarked against the exact quantum dynamics of the two-site Bose-Hubbard model, demonstrating a relevant improvement over the standard mean-field predictions across a wide range of interaction strengths.

cond-mat.quant-gas↗

arXiv2025

Entropy production and irreversibility in the linearized stochastic Amari neural model

One among the most intriguing results coming from the application of statistical mechanics to the study of brain is the understanding that it, as a dynamical system, is inherently out of equilibrium. In the realm of non-equilibrium statistical mechanics and stochastic processes the standard observable computed to discriminate whether a system is at equilibrium or not is the entropy produced along the dynamics. For this reason we present here a detailed calculation of the entropy production in the Amari model, a coarse-grained model of the brain neural network, consisting in an integro-differential equation for the neural activity field, when stochasticity is added to the original dynamics. Since the way to add stochasticity is always to some extent arbitrary, i.e., in particular for coarse-grained models, there is no general prescription to do it, we precisely investigate the interplay between the noise properties and the original model features, discussing in which cases the stationary state is of thermal equilibrium and which cases is out of equilibrium, providing explicit and simple formulas. We also show how, following for the derivation the particular case considered, how the entropy production rate is related to the variation in time of the Shannon entropy of the system.

cond-mat.dis-nn↗

arXiv2025

Optical Cavity in Relativistic Regime for Laser Propulsion

Laser propulsion has been proposed for relativistic interstellar flights, but it faces the significant challenge of requiring extremely powerful laser radiation due to the inherently low momentum transfer between the beam and the sail. The photon-recycling technique enhances thrust by transferring momentum through multiple reflections within a cavity setup, formed by the lightsail and a ground-based mirror in a laser system array. In this work, a delay differential model is developed to describe the evolution of the beam and thrust, incorporating both the Doppler effect and the round-trip time delay experienced by each beam component. With optimized multilayer reflectors, the thrust performance gain is shown to be significant for interstellar flight, though limited by diffraction and the necessity of removing harmful redshifted radiation that could overheat the lightsail. By balancing thrust performance with thermal stability, we derive a simple condition for determining the spectral requirements of the mirrors. Given a selected laser wavelength, this condition fully specifies the necessary properties of the cavity mirrors, enabling the same system to effectively support a range of launch protocols.

physics.optics↗

arXiv2025

Low-energy atomic scattering: s-wave relation between the interaction potential and the phase shift

We investigate the on-shell approximation in the context of s-wave scattering for ultracold two-body collisions. Our analysis systematically covers spatial dimensions D=1,2,3 , with the aim of identifying the regimes in which the approximation remains valid when applied to commonly used model interaction potentials. Specifically, we focus on the square well and delta shell potentials, both of which admit analytical solutions for the s-wave scattering problem in all dimensions considered. By employing the exact analytical expressions for the s-wave scattering phase shift, we perform a direct comparison between the exact on-shell matrix element of the interaction potential and their corresponding approximations across a range of collision momenta. Particular attention is given to the low-energy regime. Our findings indicate that, although the on-shell approximation generally improves with increasing momentum, its accuracy also improves for weaker potentials. Remarkably, in the limit of weak interactions, we demonstrate that the on-shell approximation becomes exact at leading order. In this regime, the approximation offers a controlled means of deriving the low-momentum expansion of the potential and may serve as a useful tool in constructing effective interactions for quantum field theories.

cond-mat.quant-gas↗

arXiv2025

Collective Excitations of Self-Gravitating Ultralight Dark Matter Cores

A distinctive feature of ultralight bosonic dark matter is its ability to form a Bose-Einstein condensate with a dense, stationary configuration at the center of galactic halos. In this work, we investigate the internal dynamics of such cores by numerically solving the Bogoliubov-de Gennes equations within a fully self-consistent gravitational framework, accounting for both gravitational potential perturbations and local self-interactions. We demonstrate that the solitonic core supports a discrete spectrum of well-defined collective modes. These oscillations characterize the linear response of the core to perturbations and may influence various dynamical processes. We also discuss potential astrophysical implications of these excitations on galactic scales.

astro-ph.CO↗

arXiv2025

Solitons in Quasiperiodic Lattices with Fractional Diffraction

We study the dynamics of solitons under the action of one-dimensional quasiperiodic lattice potentials, fractional diffraction, and nonlinearity. The formation and stability of the solitons is investigated in the framework of the fractional nonlinear Schrödinger equation. By means of variational and numerical methods, we identify conditions under which stable solitons emerge, stressing the effect of the fractional diffraction on soliton properties. The reported findings contribute to the understanding of the soliton behavior in complex media, with implications for topological photonics and matter-wave dynamics in lattice potentials.

nlin.PS↗

arXiv2025

Variational approach to multimode nonlinear optical fibers

We analyze the spatiotemporal solitary waves of a graded-index multimode optical fiber with a parabolic transverse index profile. Using the nonpolynomial Schrödinger equation approach, we derive an effective one-dimensional Lagrangian associated with the Laguerre-Gauss modes with a generic radial mode number p and azimuthal index m. We show that the form of the equations of motion for any Laguerre-Gauss mode is particularly simple, and we derive the critical power for the collapse for every mode. By solving the nonpolynomial Schrödinger equation, we provide a comparison of the stationary mode profiles in the radial and temporal coordinates.

nlin.PS↗

arXiv2024

Interaction-induced dissipative quantum phase transition in a head-to-tail atomic Josephson junction

We propose a dissipative phase transition in a head-to-tail Bose Josephson junction. The quantum phase transition has the same origin as the one in a resistively shunted Josephson junction, but the intrinsic momentum coupling between the Josephson mode and the bath modes enables us to observe the dissipative phase transition without any synthetic dissipation. We show that the interatomic interaction strength plays the role of the damping parameter. Consequently, in contrast to a resistively shunted Josephson circuit, the Bose Josephson junction can exhibit an insulating phase in a wider parameter region by increasing the repulsive interaction strength, which is robust against nonperturbative effects. We argue that tight transverse confinement of the quasi-one-dimensional atomic gas allows us to reach the insulating phase.

cond-mat.quant-gas↗

arXiv2024

Strongly interacting bosons in 1D disordered lattice: phase coherence of distorted Mott phases

We explore the consequences of disorder on phase coherence in the Mott insulator phases in an optical lattice. Few bosons with contact interaction in small optical lattice can feature varieties of insulating phases: weakly interacting Mott in deep lattice, maximally fragmented and strongly interacting Mott in intermediate lattice, weak Mott with double filling and intra-well coherence, fermionized Mott with strong intra-well coherence. Utilization of the multiconfigurational time dependent Hartree method for bosons (MCTDHB) to solve the many-boson Schroedinger equation, facilitates to understand the microscopic effect of disorder on the different kinds of Mott phases in the primary lattice. The many-body properties are analyzed by distinct measures of the reduced one-body density in real and momentum space, fragmentation, order parameter, variance of spatial single-shot measurements, compressibility and the Glauber normalized correlation functions. We find very complex competition of localization due to disorder and Mott correlation. We observe distinct response of four different Mott phases in the disordered lattice. When the weakly interacting Mott exhibits the Bose Glass phase, the strongly correlated and fully fragmented Mott exhibits leakage, melting and central localization. In contrast, weak Mott with double filling which is a spatially separated dimer in each site, exhibits only some dissociation of intra-well coherence and assist in the development of inter-well coherence in the presence of strong disorder. For the fermionized Mott, when the density in each well is fragmented, strong disorder interfere with the intra-well correlation and the characteristic dip in each site starts to disappear leading to simple Mott localization with a pair of bosons.

cond-mat.quant-gas↗

arXiv2024

Amplitude, phase, and topological fluctuations shaping the complex phase diagram of two-dimensional superconductors

We study the amplitude and phase fluctuations of the Ginzburg-Landau quasiorder parameter for superconductors in two spatial dimensions. Starting from the mean-field critical temperature $T_{\mathrm{c}0}$, we calculate the beyond-mean-field critical temperature $T_{\rm c}$ by including thermal fluctuations of the quasiorder parameter within the Gaussian level. Moreover, from our beyond-mean-field results, we derive the Berezinskii-Kosterlitz-Thouless critical temperature $T_{\rm BKT}$, which takes into account topological vortex-antivortex excitations in the phase fluctuations as well as the amplitude fluctuations, to obtain the shifts of transition temperatures. We elucidate how the Gaussian thermal fluctuations and phase fluctuations associated with vortex excitations affect thermodynamic properties by determining the $H$-$T$ phase diagram for a type-II superconductor and computing the critical behaviors of the heat capacity, which are experimentally accessible, allowing the characterization of the cascade of different kinds of fluctuations in 2D superconductors.

cond-mat.supr-con↗

arXiv2024

Only-phase Popov action: thermodynamic derivation and superconducting electrodynamics

We provide a thermodynamic derivation of the only-phase Popov action functional, which is often adopted to study the low-energy effective hydrodynamics of a generic nonrelativistic superfluid. It is shown that the crucial assumption is the use of the saddle point approximation after neglecting the quantum-pressure term. As an application, we analyze charged superfluid (superconductors) coupled to the electromagnetic field at zero temperature. Our only-phase and minimally-coupled theory predicts the decay of the electrostatic field inside a superconductor with a characteristic length much smaller than the London penetration depth of the static magnetic field. This result is confirmed also by a relativistic only-phase Popov action we obtain from the Klein-Gordon Lagrangian.

cond-mat.quant-gas↗

arXiv2024

Symplectic quantization III: Non-relativistic limit

First of all we shortly illustrate how the symplectic quantization scheme [Gradenigo and Livi, 2021] can be applied to a relativistic field theory with self-interaction. Taking inspiration from the stochastic quantization method by Parisi and Wu, this procedure is based on considering explicitly the role of an intrinsic time variable, associated with quantum fluctuations. The major part of this paper is devoted to showing how the symplectic quantization scheme can be extended to the non-relativistic limit for a Schrödinger-like field. Then we also discuss how one can obtain from this non-relativistic theory a linear Schrödinger equation for the single-particle wavefunction. This further passage is based on a suitable coarse-graining procedure, when self-interaction terms can be neglected, with respect to interactions with any external field. In the Appendix we complete our survey on symplectic quantization by discussing how this scheme applies to a non-relativistic particle under the action of a generic external potential.

cond-mat.stat-mech↗

arXiv2024

Electrodynamics of superconductors: from Lorentz to Galilei at zero temperature

We discuss the derivation of the electrodynamics of superconductors coupled to the electromagnetic field from a Lorentz-invariant bosonic model of Cooper pairs. Our results are obtained at zero temperature where, according to the third law of thermodynamics, the entropy of the system is zero. In the nonrelativistic limit we obtain a Galilei-invariant superconducting system which differs with respect to the familiar Schrödinger-like one. From this point of view, there are similarities with the Pauli equation of fermions which is derived from the Dirac equation in the nonrelativistic limit and has a spin-magnetic field term in contrast with the Schrödinger equation. One of the peculiar effects of our model is the decay of a static electric field inside a superconductor exactly with the London penetration length. In addition, our theory predicts a modified D'Alembert equation for the massive electromagnetic field also in the case of nonrelativistic superconducting matter. We emphasize the role of the Nambu-Goldstone phase field which is crucial to obtain the collective modes of the superconducting matter field. In the special case of a nonrelativistic neutral superfluid we find a gapless Bogoliubov-like spectrum, while for the charged superfluid we obtain a dispersion relation that is gapped by the plasma frequency.

cond-mat.supr-con↗

arXiv2024

Canonical vs. Grand Canonical Ensemble for Bosonic Gases under Harmonic Confinement

We analyze the general relation between canonical and grand canonical ensembles in the thermodynamic limit. We begin our discussion by deriving, with an alternative approach, some standard results first obtained by Kac and coworkers in the late 1970s. Then, motivated by the Bose-Einstein condensation (BEC) of trapped gases with a fixed number of atoms, which is well described by the canonical ensemble and by the recent groundbreaking experimental realization of BEC with photons in a dye-filled optical micro-cavity under genuine grand canonical conditions, we apply our formalism to a system of non-interacting Bose particles confined in a two-dimensional harmonic trap. We discuss in detail the mathematical origin of the inequivalence of ensembles observed in the condensed phase, giving place to the so-called grand canonical catastrophe of density fluctuations. We also provide explicit analytical expressions for the internal energy and specific heat and compare them with available experimental data. For these quantities, we show the equivalence of ensembles in the thermodynamic limit.

cond-mat.stat-mech↗

arXiv2024

Breathing mode of a quantum droplet in a quasi-one-dimensional dipolar Bose gas

We investigate the breathing mode and the stability of a quantum droplet in a tightly trapped one-dimensional dipolar gas of bosonic atoms. When the droplet with a flat-top density profile is formed, the breathing mode frequency scales as the inverse of the number of atoms in the cloud. This is straightforwardly derived within a phenomenological hydrodynamical approach and confirmed using both a variational method based on a generalized Gross-Pitaevskii action functional and the sum-rule approach. We extend our analysis also to the presence of axial confinement showing the effect of the trap on the density profile and therefore on the breathing mode frequency scaling. Our analysis confirms the stability of the quantum droplet against the particles emission when the flat-top density profile is observed. Our results can be used as a guide to the experimental investigations of collective modes to detect the formation of quantum droplets in quasi-one-dimensional dipolar gases.

cond-mat.quant-gas↗