arXiv subjects

Luigi Amico

Publications and source records attributed to Luigi Amico.

At least 19 recordsLinked to original sources

arXiv2026

Solitary waves of attracting SU(N) fermions

We study the formation, dynamics, and disorder robustness of bound states in attractively interacting SU(N) fermions on a one-dimensional ring lattice. Using exact diagonalization in fixed-momentum sectors and Bethe ansatz exact results as a guide, we resolve the many-body spectrum into bands related to the possible partitions of the particles into bound composites, and characterize their internal structure also through density-density and N-body correlations. A pinning quench protocol reveals a transition from dispersive spreading to dynamical localization as the attractive interaction increases relative to the single-particle hopping. We find that the bound state dynamics for one-particle per component occurs as a many-body quantum walk similar to that of a single particle with a re-normalized effective mass. Such a property, that is the quantum version of the shape-preserving motion of classical solitons, can provide the dynamical signature of the fermionic solitary waves. We probe the robustness of the fermionic bound-state dynamics under on-site disorder.

cond-mat.quant-gas↗

arXiv2026

Quantum solitons and their quantum walks in transmon arrays

Superconducting qubits are artificial atoms whose spectra and interactions can be engineered through appropriate circuit design, a versatility that can be exploited for quantum simulation. We theoretically investigate a linear array of capacitively coupled transmons, effectively described by a Bose-Hubbard Hamiltonian with attractive interaction. We revisit the discrete-soliton nature of the lowest-energy band of the spectrum, and identify spatially localized quantum solitons. The solitonic character of these states is revealed through their time evolution, which displays a quantum interference pattern, or quantum walk, highlighting their composite nature. We discuss protocols for preparing spatially localized quantum solitons that are compatible with current state-of-the-art tunable-transmon circuits. Our results demonstrate that superconducting circuits provide a promising and experimentally accessible platform for the investigation of quantum soliton physics.

quant-ph↗

arXiv2026

Rydberg atoms for electric field gradiometry

We propose a quantum sensor for electric fields based on networks of Rydberg atoms. The sensing mechanism exploits the strong dependence of the Rydberg blockade on the applied electric field near a Förster resonance. In this regime, variations of the electric field across the array lead to local changes in the blockade radius. Therefore, owing to its spatially distributed architecture, the device can operate as a gradiometer. Our analysis shows that our scheme enables detection of spatial variations in the electric field with a resolution of a few $μ$m. We analyse the dynamics of Rydberg excitations for systems with different spatial geometries and electric field configurations to establish the relation between the applied field and the blockade response. For spatially inhomogeneous fields, we also provide another observable, density-density correlations, that can probe the field's spatial structure.

quant-ph↗

arXiv2026

Weak-link to tunneling crossover in an atomic Josephson junction

We present a unified, quantitative description of transport across the crossover between hydrodynamic weak-link flow and tunneling-dominated Josephson dynamics in a three-dimensional quantum fluid. Using an atomic Josephson junction realized in a Bose-Einstein condensate, we continuously tune the barrier strength to access both regimes within a single, well-controlled system. Measurements of the critical current and Josephson oscillations are in quantitative agreement with numerical simulations and analytical modeling, enabling a consistent inference of the microscopic mechanisms governing dissipation. In the weak-link regime, dissipative transport is consistent with vortex-ring-mediated phase slips, whereas in the tunneling regime it is consistent with rarefaction-pulse excitations. The crossover is further reflected in a transition from a multi-harmonic to a predominantly single-harmonic current-phase relation, signaling the emergence of tunneling-dominated transport. These results establish a general framework linking nonlinear excitations to coherent quantum transport across distinct dynamical regimes. More broadly, they provide insight into the microscopic origin of dissipation in driven quantum fluids, a problem that remains difficult to access in conventional solid-state systems.

cond-mat.quant-gas↗

arXiv2026

Fraxonium: Fractional fluxon states for qudit encoding

We propose a superconducting circuit hosting $d$ low-lying states, well separated from the rest of the spectrum, that naturally realizes a qudit system protected from leakage errors. The system represents a generalization of the fluxonium and the low-energy states are constituted by fractional fluxon states, that we call {\it fraxons}, localized in the minima of a suitably designed Josephson potential. The latter is tailored through a Fourier engineering approach, that employs multi-harmonic Josephson building block elements composed by a Josephson junction and an inductance connected in series. We present the spectrum of a $d=4$ and a $d=5$ qudit system and study in detail the qutrit case. We analyze the dipole matrix elements for coupling to radiation and propose a non-Abelian, stimulated Raman adiabatic passage (STIRAP) protocol for single-qutrit gates, that is particularly suited for the present system. The proposed platform opens novel perspectives in circuit engineering and quantum computing beyond the qubit paradigm.

quant-ph↗

arXiv2026

Fraunhofer Patterns in Atomic Josephson Junctions

Driven atomic Josephson junctions allow one to monitor phase-coherent dynamics with unprecedented control and flexibility of the system's physical conditions. While cold-atom manifestations of the Josephson effect have been extensively studied in a wide variety of settings, atomic Josephson junctions in synthetic electromagnetic fields remain largely unexplored. Here, we show that synthetic magnetic fields can induce Fraunhofer-like modulations of the critical current in atomic Josephson junctions. Although this effect presents analogies to the Fraunhofer patterns found in superconducting devices, distinctive features emerge due to the neutral nature of the superfluid. We investigate the underlying spatial interference mechanisms and elucidate the role of Josephson vortices in the formation of spatially modulated current distributions based on numerical simulations. Our results open up new avenues for matter-wave circuits to deepen our understanding of spatial coherence in Josephson junctions, which are fundamental to the development of novel quantum technologies.

cond-mat.quant-gas↗

arXiv2026

Josephson Dynamics in 2D Ring-shaped Condensates

We investigate Josephson transport in a fully closed, two-dimensional superfluid circuit formed by a ring-shaped 87Rb Bose-Einstein condensate that contains two optical barriers acting as movable weak links. Translating these barriers at controlled speeds imposes a steady bias current, enabling direct mapping of the current-chemical-potential (I-Δμ) characteristics. For narrow junctions (w \approx 1μm) the circuit exhibits a pronounced dc branch that terminates at a critical current I_c = 9(1) x 10^3 s^{-1}; above this threshold the system switches to an ac, resistive regime. Classical-field simulations that include the moving barriers quantitatively reproduce both the nonlinear I-Δμ curve and the measured I_c, validating the underlying microscopic picture. Analysis of the ensuing phase dynamics shows that dissipation is mediated by the nucleation and traversal of vortex-antivortex pairs through the junctions, while the bulk condensate remains globally phase-locked \textemdash direct evidence of the ring's topological constraint enforcing quantized circulation. These results establish a cold-atom analogue of a SQUID in which Josephson dynamics can be resolved at the single-vortex level, providing a versatile platform for atomtronic circuit elements, non-reciprocal Josephson devices, and on-chip Sagnac interferometers for multi-axis rotation sensing.

cond-mat.quant-gas↗

arXiv2026

Observation of Shapiro steps in an ultracold atomic Josephson junction

The current-voltage characteristic of a driven superconducting Josephson junction displays discrete steps. This phenomenon, called the Shapiro steps, forms today's voltage standard! Here, we report the observation of Shapiro steps in a driven Josephson junction in a gas of ultracold atoms. We demonstrate that the steps exhibit universal features, and provide insight into the microscopic dissipative dynamics that we directly observe in the experiment. We find that the steps are directly connected to phonon emission and nucleation of solitonic excitations, whose dynamics we follow in space and time. The experimental results are underpinned by extensive numerical simulations based on classical-field dynamics and may enable metrological and fundamental advances.

cond-mat.quant-gas↗

arXiv2025

Interferometric probe for the zeros of the many-body wavefunction

The nodal surfaces of the many-body wavefunction are fundamental geometric features that encode critical information regarding particle statistics and their interaction. Directly probing these structures, particularly in correlated quantum systems, remains a significant experimental challenge. Here, we provide rigorous results on the structure of the many-body wavefunction and propose to use an interferometric technique to probe its zeros in ultra-cold atomic systems. Specifically, we refer to the so-called heterodyne interferometric reconstruction of the phase of the many-body wavefunction. We prove that the sought nodal surfaces show up as specific discontinuities in the interference fringes. Following Leggett, both `symmetry-dictated' nodal surfaces, due to particle statistics, and `non-symmetry dictated' nodal surfaces emerging from interaction effects, can be probed. We demonstrate how the spin degrees of freedom, effectively modifying the structure of the nodal surfaces of the many-body wavefunction, leave distinct fingerprints in the resulting interference pattern. Our work addresses important features of the structure of the many-body wavefunction that are broadly relevant for quantum science ranging from conceptual aspects to computational questions of extended systems and quantum simulation.

cond-mat.quant-gas↗

arXiv2025

Diode effect in the Fraunhofer pattern of disordered planar Josephson junctions

The Josephson diode effect describes the property of a Josephson junction to have different values of the critical current for different direction of applied bias current and it is the focus of intense research thanks to the possible applications. The ubiquity of the effect experimentally reported calls for a study of the impact that disorder can have in the appearance of the effect. We study the Fraunhofer pattern of planar Josephson junctions in presence of different kinds of disorder and imperfections and we find that a junction that is {\it mirror} symmetric at zero-field forbids the diode effect and that the diode effect is typically magnified at the nodal points of the Fraunhofer pattern. The work presents a comprehensive treatment of the role of pure spatial inhomogeneity in the emergence of a diode effect in planar junctions, with an extension to the multi-terminal case and to systems of Josephson junctions connected in parallel.

cond-mat.supr-con↗

arXiv2025

Controlled generation of 3D vortices in driven atomic Josephson junctions

We propose an ac-driven atomic Josephson junction as a clean and tunable source of three dimensional (3D) solitary waves in quantum fluids. Depending on the height of the junction barrier, the emitted excitations appear as vortex rings at low velocity or vorticity-free rarefaction pulses near the sound velocity, thus spanning the complete Jones-Roberts family of solitons. The Shapiro-step phenomenon renders the emission deterministic: on the first, second, third Shapiro steps, the junction ejects one, two, and three solitary excitations per drive cycle. This enables controlled generation of single- and multi-excitation configurations, allowing detailed studies of the full crossover between vortex rings and rarefaction pulses and their interaction dynamics. In particular, deterministic multi-ring emission provides insights into leapfrogging dynamics of two and three coaxial rings and their decay via boundary-assisted, sound-mediated processes. This ac-driven protocol establishes a compact and reproducible platform for generating, classifying, and controlling 3D solitonic excitations, paving the way for precision studies of nonlinear vortex dynamics, dissipation, and quantum turbulence in trapped superfluids.

cond-mat.quant-gas↗

arXiv2025

Atomic Josephson Parametric Amplifier

We study the dynamics of a driven atomic Josephson junction that we propose as a parametric amplifier. By periodically modulating the position of the barrier, we induce a small current across the junction, serving as our input signal. The pump field is implemented by modulating the barrier height at twice the Josephson plasma frequency. The resulting dynamics exhibit parametric amplification of the signal through nonlinear mixing between the signal and pump fields, which is encoded in a specific microscopic pattern of density waves and phase excitations that can be addressed within the experimental cold atoms capabilities. This work paves the way for tunable amplifiers in atomtronic circuits, with potential applications in several fields including precision measurements and quantum information processing. At the same time, our analysis provides the microscopic explanation of the general notion of parametric amplification occurring in nonlinear coherent devices.

cond-mat.quant-gas↗

arXiv2025

Static impurity in a mesoscopic system of SU($N$) fermionic matter-waves

We investigate the effects of a static impurity, modeled by a localized barrier, in a one-dimensional mesoscopic system comprised of strongly correlated repulsive SU($N$)-symmetric fermions. For a mesoscopic sized ring under the effect of an artificial gauge field, we analyze the energy spectrum, the particle density and the current flowing through the impurity at varying interaction strengths, barrier heights, and number of components. We find that the physics of the system is governed by the competition between effective single-particle process and the formation of a high-stiffness spin-correlated state associated to the phenomenon of fractionalization of the flux quantum characterizing the $N$-component fermionic system. Our findings provide a route to probe the response of SU($N$) fermions to effective magnetic fields; at the same time, they hold significance for fundamental understanding of localized impurity problems.

cond-mat.quant-gas↗

arXiv2025

Synthetic fractional flux quanta in a ring of superconducting qubits

A ring of capacitively coupled transmons threaded by a synthetic magnetic field is studied as a realization of a strongly interacting bosonic system. The synthetic flux is imparted through a specific Floquet modulation scheme based on a suitable periodic sequence of Lorentzian pulses that are known as 'Levitons'. Such scheme has the advantage to preserve the translation invariance of the system and to work at the qubit sweet spots. We employ this system to demonstrate the concept of fractional values of flux quanta. Although such fractionalization phenomenon was originally predicted for bright solitons in cold atoms, it may be in fact challenging to access with that platform. Here, we show how fractional flux quanta can be read out in the absorption spectrum of a suitable 'scattering experiment' in which the qubit ring is driven by microwaves.

cond-mat.mes-hall↗

arXiv2025

Shapiro steps in strongly-interacting Fermi gases

We report the observation of Shapiro steps in a periodically driven Josephson junction between strongly-interacting Fermi superfluids of ultracold atoms. We observe quantized plateaus in the current-potential characteristics, the height and width of which mirror the external drive frequency and the junction nonlinear response. Direct measurements of the current-phase relationship showcase how Shapiro steps arise from the synchronization between the relative phase of the two reservoirs and the external drive. Such mechanism is further supported by the detection of periodic phase-slippage processes, in the form of vortex-antivortex pairs. Our results are corroborated by a circuital model and numerical simulations, overall providing a clear understanding of Shapiro dynamics in atomic Fermi superfluids. Our work demonstrates phase-coherent and synchronization effects in driven strongly-interacting superfluids, opening prospects for studying emergent non-equilibrium dynamics in quantum many-body systems under external drives.

cond-mat.quant-gas↗

arXiv2025

Suppressing chaos with mixed superconducting qubit devices

In quantum information processing, a tension between two different tasks occurs: while qubits' states can be preserved by isolating them, quantum gates can be realized only through qubit-qubit interactions. In arrays of qubits, weak coupling leads to states being spatially localized and strong coupling to delocalized states. Here, we study the average energy level spacing and the relative entropy of the distribution of the level spacings (Kullback-Leibler divergence from Poisson and Gaussian Orthogonal Ensemble) to analyze the crossover between localized and delocalized (chaotic) regimes in linear arrays of superconducting qubits. We consider both transmons as well as capacitively shunted flux qubits, which enables us to tune the qubit anharmonicity. Arrays with uniform anharmonicity, comprising only transmons or flux qubits, display remarkably similar dependencies of level statistics on the coupling strength. In systems with alternating anharmonicity, for typical disorder in the qubit frequencies the localized regime is found to be more resilient to the increase in qubit-qubit coupling strength in comparison to arrays with a single qubit type. Our results, which we also confirm using generalized Bose-Hubbard models, support designing devices that incorporate different qubit types to achieve higher performances.

quant-ph↗

arXiv2025

Persistent currents in ultracold gases

Persistent currents flowing in spatially closed tracks define one of the most iconic concepts in mesoscopic physics. They have been studied in solid-state platforms such as superfluids, superconductors and metals. Cold atoms trapped in magneto-optical toroidal circuits and driven by suitable artificial gauge fields allow us to study persistent currents with unprecedented control and flexibility of the system's physical conditions. Here, we review persistent currents of ultracold matter. Capitalizing on the remarkable progress in driving different atomic species to quantum degeneracy, persistent currents of single or multicomponent bosons/fermions, and their mixtures can be addressed within the present experimental know-how. This way, fundamental concepts of quantum science and many-body physics, like macroscopic quantum coherence, solitons, vortex dynamics, fermionic pairing and BEC-BCS crossover can be studied from a novel perspective. Finally, we discuss how persistent currents can form the basis of new technological applications like matter-wave gyroscopes and interferometers.

cond-mat.quant-gas↗

arXiv2024

Extractable energy from quantum superposition of current states

We explore the energy content of superpositions of current states. Specifically, we focus on the maximum energy that can be extracted from them through local unitary transformations. The figure of merit we employ is the local ergotropy. We perform a complete analysis in the whole range of the system's parameters. This way, we prove that superpositions of two current states in spatially closed spin networks are characterized by specific peaks in extractable energy, generally overcoming the ergotropy of each of the two separate current states characterized by a single winding number. The many-body state dynamics entails to ergotropy evolving in a controlled fashion. The implementation we suggest is based on a Rydberg-atom platform. Optimal transformations able to extract locally the maximum possible amount of energy are sorted out.

quant-ph↗