Speaker
Description
We investigate the dynamical and thermodynamic properties of the open Rabi model, a two-level system coupled to an oscillator beyond the rotating-wave approximation, and its two-qubit extension, focusing on equilibrium quantum phase transitions (QPTs) and dynamical quantum phase transitions (DQPTs) induced by environmental interactions. To address these problems, we employ the Density-Matrix Renormalization Group (DMRG) algorithm for equilibrium states and the Time-Dependent Variational Principle (TDVP) for non-equilibrium dynamics, both based on a tensor-network variational ansatz, representing open quantum systems as Matrix Product States (MPSs). These approaches are complemented by worldline Monte Carlo simulations at equilibrium and methods inspired by Feynman and Mori for dynamical analysis. On the thermodynamic side, we show that increasing the coupling to the oscillator mediating environmental interactions triggers a Berezinskii-Kosterlitz-Thouless (BKT) transition in both single- and two-qubit models within the deep-strong coupling regime. Dynamically, we observe a coherent-to-incoherent crossover and the BKT transition: weak coupling leads to Rabi oscillations, whereas stronger coupling induces exponential relaxation, with relaxation functions flattening at the BKT point [1]. As an application, we compute local ergotropy in a work-extraction protocol, finding that both its static and dynamical behavior witness the BKT transition [2]. Furthermore, quenches of the qubit–oscillator coupling reveal two distinct classes of DQPTs, depending on qubit interactions and entanglement, each with unique critical exponents. These are signaled by non-analyticities in the Loschmidt echo, occurring in the same parameter regime as the thermodynamic BKT transition [3]. Finally, we employ DMRG and TDVP to analyze the Quantum Fisher Information Matrix (QFIM) as a metrological probe in a paradigmatic two-level open quantum system: the spin-boson model. Its static component exhibits peaks near the BKT transition, allowing the critical coupling to be extrapolated, and these sensitivity enhancements correlate with the monotonic growth of von Neumann entropy, linking parameter estimation precision to entanglement. Dynamically, QFIM displays oscillations in the coherent regime, and their suppression across the coherent-to-incoherent crossover, providing a quantitative signature of non-Markovian effects [4]. These results clarify how criticality and non-Markovian effects manifest in open quantum systems and suggest possible strategies for exploiting these features in quantum sensing protocols.
References
[1] G. De Filippis, A. de Candia, G. Di Bello, C. A. Perroni, L. M. Cangemi, A. Nocera, M. Sassetti, R. Fazio, and V.
Cataudella, “Signatures of dissipation driven quantum phase transition in Rabi model”, Phys. Rev. Lett. 130,
210404 (2023).
[2] G. Di Bello, D. Farina, D. Jansen, C. A. Perroni, V. Cataudella, and G. De Filippis, “Local ergotropy and its
fluctuations across a dissipative quantum phase transition”, Quantum Sci. Technol. 10, 015049 (2025).
[3] G. Di Bello, A. Ponticelli, F. Pavan, V. Cataudella, G. De Filippis, A. de Candia, and C. A. Perroni, “Environment
induced dynamical quantum phase transitions in two-qubit Rabi model”, Commun. Phys. 7, 364 (2024).
[4] D. Parlato, G. Di Bello, F. Pavan, G. De Filippis, and C. A. Perroni, “Quantum Fisher information as a witness of non-Markovianity and criticality in the spin-boson model”, arXiv:2508.16413 (2025).
