arXiv subjects

G. Di Bello

Publications and source records attributed to G. Di Bello.

10 recordsLinked to original sources

arXiv2026

Dynamical Crossover of the Quantum Fisher Information in the Spin-Boson Model

We investigate the dynamical quantum Fisher information of a two-level system coupled to a bosonic environment, focusing on the estimation of the qubit gap. We combine analytical calculations with numerically controlled matrix-product-state simulations. In the exactly solvable pure-dephasing Ohmic regime at zero temperature, the long-time quantum Fisher information displays a coupling-dependent algebraic behavior, leading to a dynamical crossover: it grows without bound at weak coupling, approaches a finite asymptotic value at the crossover coupling, and vanishes at strong coupling. At finite temperature, thermal fluctuations suppress the long-time growth and generate a finite-time maximum, whose dependence on the dephasing coupling retains a clear signa- ture of the zero-temperature crossover. We show that this crossover is absent at zero temperature in non-Ohmic baths: the long-time quantum Fisher information vanishes in the sub-Ohmic and diverges in the super-Ohmic regimes. At non-zero temperature instead, the crossover appears for a super-Ohmic quadratic bath and the crossover coupling becomes temperature-dependent. Moreover, in the zero-temperature Ohmic regime, we introduce an additional amplitude-damping system-bath coupling that induces energy relaxation. This relaxation channel replaces the unbounded long-time growth with a finite asymptotic quantum Fisher information associated with the reduced interacting ground state, while a signature of the pure-dephasing crossover persists in the early-time dynamics. These results establish a direct connection between the low-frequency structure of the bath and the asymptotic metrological behavior of dynamical gap sensing, and show how thermal fluctuations and energy relaxation regularize the ideal pure-dephasing dynamical crossover of the quantum Fisher information.

quant-ph↗

arXiv2026

Bloch sphere picture and restoration of the quantum Mpemba effect beyond the weak coupling regime in the spin boson model

Understanding relaxation dynamics in open quantum systems is a central problem in nonequilibrium quantum physics. Here we investigate the quantum Mpemba effect in the spin boson model. In the weak coupling Markovian regime we show that the occurrence of the effect strongly depends on the choice of distance measure at low temperature: while it appears in the trace distance, it can disappear in the quantum relative entropy. Going beyond the weak-coupling approximation, numerically exact simulations of the full system bath dynamics reveal that increasing coupling enhances the effect in the trace distance and restores it in the quantum relative entropy. We uncover a simple Bloch sphere picture of the effect: within the excited-state hemisphere, pairs of states related by rotations and having the same Bloch-vector modulus generically exhibit an inversion of their relaxation ordering. This behavior is robust across sub Ohmic, Ohmic, and super Ohmic bath spectra. In the Ohmic and sub Ohmic regimes, the crossing time becomes strongly suppressed upon approaching the critical region, whereas in the super Ohmic regime the effect remains enhanced despite the absence of a finite-coupling localization transition. These results highlight the interplay between geometry, distance measures, system--environment coupling, and bath spectral properties in anomalous quantum relaxation.

quant-ph↗

arXiv2026

Ballistic-to-Localized Dynamics as Signature of Quantum Phase Transition in Josephson Junction

Using state-of-the-art numerical techniques, we investigate how quantum phase fluctuations and quasiparticle tunneling shape the behavior of a small-capacitance Josephson junction across Ohmic, sub-Ohmic, and super-Ohmic dissipation regimes. We show that increasing the Ohmic dissipation strength drives a Berezinskii-Kosterlitz-Thouless quantum phase transition at thermodynamic equilibrium. Deviations from Ohmic behavior profoundly alter this scenario: the super-Ohmic regime exhibits no phase transition, whereas the sub-Ohmic regime displays a continuous second-order transition, consistent with the universality classes of the spin-boson model. Within the Ohmic regime, real-frequency linear-response calculations reveal that the phase particle does not undergo the commonly assumed diffusive-to-localized crossover. Instead, finite resistance progressively suppresses the singular zero-frequency response, producing a ballistic-to-localized change in the dynamics. At finite frequencies, coupling to the environment generates a long-lived excitation in the charge response, which evolves into a resonance as the subgap and shunt resistances are reduced.

cond-mat.supr-con↗

arXiv2026

Kibble-Zurek Mechanism in the Open Quantum Rabi Model

The Kibble-Zurek mechanism provides a universal framework for predicting defect formation in non-equilibrium phase transitions. While Markovian dissipation typically degrades universal scaling, the impact of non-Markovian memory remains largely unexplored. We demonstrate that an Ohmic bath induces a Berezinskii-Kosterlitz-Thouless transition in the open quantum Rabi model. Using simulations based on Matrix Product States, we show that the excitation energy strictly follows universal Kibble-Zurek power-law scaling when evaluated at the freeze-out time. Crucially, we find that since the environment defines the universality class, dissipation does not inherently compete with adiabatic dynamics, in stark contrast to Markovian regimes. Our results establish the Kibble- Zurek mechanism as a robust witness of universality in open quantum systems, revealing that non-Markovian memory preserves the integrity of non-equilibrium scaling.

quant-ph↗

arXiv2024

Local ergotropy and its fluctuations across a dissipative quantum phase transition

We investigate a two-qubit open Rabi model, focusing on local ergotropy--the maximum extractable work by acting solely on the two qubits--within a parameter regime where a Berezinskii-Kosterlitz-Thouless dissipative phase transition occurs. First, we aim to define a protocol for charging, storing, and discharging the two-qubit system, interpreted as the working principle of an open quantum battery. Second, we examine the impact of the phase transition on ergotropy and identify potential markers. To achieve these goals, we construct an ad-hoc charging unitary operator, leveraging our knowledge of the ground state near the transition to bring it into a decoherence-free state during storage. Using state-of-the-art numerics based on matrix product state representation, we reveal that high couplings to an external bath approximately double the local ergotropy immediately post-charging. Over time we observe oscillatory behaviors in ergotropy and its fluctuations, which undergo significant changes near the transition, signaling its occurrence. Furthermore, we optimize local ergotropy over time using a physically inspired ansatz, enabling work extraction at a generic time (local ergotropy never reaches zero). Our work proposes a tunable, experimentally realizable protocol for work extraction, leveraging decoherence-free states and phase transitions. Additionally, it sheds light on the complex interaction between local ergotropy and quantum phase transitions.

quant-ph↗

arXiv2024

Environment induced dynamical quantum phase transitions in two-qubit Rabi model

The physics of quantum states beyond thermodynamic equilibrium represents a fascinating and cutting-edge research. Using numerical state-of-the-art approaches, we observe dynamical quantum phase transitions in the dissipative two-qubit Rabi model. By quenching the qubits-oscillator coupling, the system (Rabi + Environment) exhibits dynamical quantum phase transitions signalled by kinks of Loschmidt echo's rate function at parameter values close to thermodynamic transition. Notably, these transitions also manifest in two-qubit entanglement. While at equilibrium one class of Beretzinski-Kosterlitz-Thouless-type transitions occurs, non-equilibrium conditions reveal two classes of dynamical critical phenomena, depending on qubits' interactions and entanglement. When qubits directly interact, the kink critical exponent describes a linear behavior, reminiscent of nearest neighbors Ising chains, with short-range interactions dominating at short times. Conversely, non-interacting qubits exhibit critical exponents much smaller than unity due to bath-induced long-range interactions. These findings shed light on the complex behavior of dynamical quantum phase transitions in non-integrable models, showing unusual entanglement features and the environment's significant role.

quant-ph↗

arXiv2024

Optimal encoding of two dissipative interacting qubits

We investigate a system of two coupled qubits interacting with an Ohmic bath as a physical model for the implementation of one logical qubit. In this model, the interaction with the other qubit represents unitary noise while the Ohmic bath is responsible for finite temperature. In the presence of a one-dimensional decoherence-free subspace (DFS), we show that, while this is not sufficient to protect a qubit from decoherence, it can be exploited to encode one logical qubit with greater performance than the physical one. We show different possible strategies for the optimal encoding of a logical qubit through a numerical analysis based on matrix product states. This method reproduces faithfully the results of perturbative calculations, but it can be extended to cases of crucial interest for physical implementations, e.g., in the case of strong coupling with the bath. As a result, a logical qubit encoded in the subspace which is the direct sum of the antiferromagnetic states in Bell basis, the DFS and the one in the triplet, is the optimally robust one, as it takes advantage of both the anchoring to the DFS and the protection from the antiferromagnetic interaction. These authors contributed equally to this work, and their names are listed in alphabetical order.

quant-ph↗

arXiv2023

Witnessing Environment Induced Topological Phase Transitions via Quantum Monte Carlo and Cluster Perturbation Theory Studies

Many-body interactions play a crucial role in quantum topological systems, being able to impact or alter the topological classifications of non-interacting fermion systems. In open quantum systems, where interactions with the environment cause dissipation and decoherence of the fermionic dynamics, the absence of hermiticity in the subsystem Hamiltonian drastically reduces the stability of the topological phases of the corresponding closed systems. Here we investigate the non-perturbative effects induced by the environment on the prototype Su-Schrieffer-Heeger chain coupled to local harmonic oscillator baths through either intra-cell or inter-cell transfer integrals. Despite the common view, this type of coupling, if suitably engineered, can even induce a transition to topological phases. By using a world-line Quantum Monte Carlo technique we determine the phase diagram of the model proving that the bimodality of the probability distribution of the polarization signals the emergence of the topological phase. We show that a qualitative description can be obtained in terms of an approach based on the Cluster Perturbation Theory providing, in particular, a non-Hermitian Hamiltonian for the fermionic subsystem and insights on the dissipative dynamics.

cond-mat.str-el↗

arXiv2023

Signatures of Dissipation Driven Quantum Phase Transition in Rabi Model

By using worldline Monte Carlo technique, matrix product state and a variational approach à la Feynman, we investigate the equilibrium properties and relaxation features of the dissipative quantum Rabi model, where a two level system is coupled to a linear harmonic oscillator embedded in a viscous fluid. We show that, in the Ohmic regime, a Beretzinski-Kosterlitz-Thouless quantum phase transition occurs by varying the coupling strength between the two level system and the oscillator. This is a non perturbative result, occurring even for extremely low dissipation magnitude. By using state-of-the-art theoretical methods, we unveil the features of the relaxation towards the thermodynamic equilibrium, pointing out the signatures of quantum phase transition both in the time and frequency domains. We prove that, for low and moderate values of the dissipation, the quantum phase transition occurs in the deep strong coupling regime. We propose to realize this model by coupling a flux qubit and a damped LC oscillator.

quant-ph↗

arXiv2023

Qubit-oscillator relationships in the open quantum Rabi model: the role of dissipation

Using a dissipative quantum Rabi model, we study the dynamics of a slow qubit coupled to a fast quantum harmonic oscillator interacting with a bosonic bath from weak to strong and ultra-strong coupling regimes. Solving the quantum Heisenberg equations of motion, perturbative in the internal coupling between qubit and oscillator, we derive functional relationships directly linking the qubit coordinates in the Bloch sphere to oscillator observables. We then perform accurate time-dependent Matrix Product State simulations, and compare our results both with the analytical solutions of the Heisenberg equations of motion, and with numerical solutions of a Lindblad master equation, perturbative in the external coupling between oscillator and environment. Indeed, we show that, up to the strong coupling regime, the qubit state accurately fulfills the derived functional relationships. We analyse in detail the case of a qubit starting with generic coordinates on the Bloch sphere of which we evaluate the three components of the Bloch vector through the averages of oscillator observables. Interestingly, a weak to intermediate oscillator coupling to the bath is able to simplify the Bloch vector evaluation since qubit-oscillator relationships are more immediate. Moreover, by monitoring the qubit fidelity with respect to free limit, we find the parameter regime where the combined effect of internal and external couplings is able to hinder the reliable evaluation of the qubit Bloch vector. Finally, in the ultra-strong coupling regime, non-Markovian effects become robust and the dynamics of qubit and oscillator are inextricably entangled making the qubit Bloch vector evaluation difficult.

quant-ph↗