Dipòsit Digital de la Universitat de Barcelona

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El Dipòsit Digital de la Universitat de Barcelona és el repositori institucional que conté en format digital els materials derivats de l'activitat docent, investigadora i institucional de la comunitat universitària.

Enviaments recents

Universal properties from quantum many-body dynamics
(2026-09) Márquez Olguín, Joaquín G.; Carignano, Stefano
The Loschmidt echo—the amplitude for a quantum state to return to itself—provides a route to the universal properties of a critical many-body system. Continued from real to imaginary time, it becomes the partition function of a conformal field theory on a strip, and its universal data—the central charge and the operator content—are encoded in the spectrum of a transfer matrix, obtained by contracting the space–time tensor network along the space direction. We apply this framework to a transverse-field Ising model extended by a nextnearest-neighbour coupling: a non-integrable chain that remains critical and in the Ising universality class over a wide range of that coupling. After a quench to the critical point, the temporal entanglement that governs the cost of the transverse contraction grows only logarithmically with the evolution time, so the method remains efficient at times where conventional evolution algorithms are limited by the entanglement barrier. We develop a translation-invariant matrix product operator, accurate to second order in the time step, that represents the time evolution of the model, and use two power methods to compute the leading eigenvectors and eigenvalues of the resulting non-Hermitian transfer matrix. Within the accessible time windows, our results are consistent with the central charge and operator content predicted by the Ising universality class.
Entropic Spectroscopy of Spatiotemporal Replica Defects in (1 + 1)D Quantum Ising Dynamics
(2026-09) Majumdar, Avinaba; Bou Comas, Aleix; Tagliacozzo, Luca
Generalised temporal entropies extend entanglement-based diagnostics from spatial subsystems to cuts through real-time quantum evolution. This thesis develops a spatiotemporal Rényi-2 construction that combines a crossed temporal replica geometry with spatial SWAP readouts, and applies it to the finite open one-dimensional quantum Ising chain. A general Boolean/Möbius framework organises replica-defect geometries, while the selected protocol is formulated as a crossed-replica spacetime tensor network. On the integrable transverse-field line h = 0, an exact Gaussian–Pfaffian evaluator is constructed and benchmarked against DMRG/TEBD; the same protocol is then studied at h = 0.5 with TEBD. At g = 0.5, 1.0, 1.5, the integrable entropy dynamics show phase- and geometry-dependent real-space structure within an exactly known ballistic envelope, and site-resolved Fourier analysis resolves the freefermion energy-difference and two-quasiparticle continua. Breaking integrability reorganises these spectra qualitatively: g = 0.5 exhibits confinement-scale localisation and mesonic structure, g = 1.0 develops a finite lowest branch consistent with the lightest magnetic-Ising E8 scale, and g = 1.5 reveals a mobile interacting branch with two-excitation kinematic structure. Complementary symmetric two-region Rényi-2 mutual-information diagnostics are also evaluated for disconnected spatial supports. In the mobile mixed-field cases, fixed local entropies remain bounded and oscillatory over the accessible T = 8 window while their spatial support continues to expand. These results establish spatiotemporal entropy and entropic spectroscopy as controlled probes of locality, integrability breaking, interacting spectral organisation, and dynamical complexity
Calibration of a Double-Loop Galvanic Coupler between Tunable Fluxoniums
(2026-09) Laguna Rueda, Álvaro; Scarpelli, Lorenzo
Analog quantum computing requires independent, in-situ control over every term of the implemented Hamiltonian, including the qubit-qubit couplings. One issue of commonly used galvanic couplers between fluxoniums is the couplerinduced nonlinear crosstalk. The presence of a current circulating in the coupler main loop (the z-loop) generates a flux that shifts, nonlinearly, the degeneracy point of the fluxonium. For precise qubit control, such a crosstalk needs to be compensated. While for a single qubit this can be done accurately, this becomes a highly non-trivial task when multiple qubits are involved. Here we perform preliminary measurements on a double-loop galvanic coupler. Besides the z-loop, we include a DC SQUID as an x-loop. The DC SQUID enables us to tune the coupler susceptibility while keeping the net circulating current in the coupler at zero, therefore eliminating the nonlinear crosstalk. Furthermore, we show that the coupler susceptibility can be tuned through the ferromagnetic, zero and antiferromagnetic coupling regimes using only the SQUID flux, keeping the coupler at its sweet spot and therefore leaving the qubit sweet spots unperturbed. Finally, we measure and fit the spectrum of each qubit with the coupling switched off in order to extract the qubits’ energy parameters, and report a preliminary measurement of the two-qubit spectrum with the coupling turned on
Quantum-to-classical transition of non-Gaussian bosonic systems in non-markovian dynamics
(2026-09) Julián Abós, Iris; Centrone, Federico; Acín dal Maschio, Antonio
Non-Gaussian bosonic states offer resources that are inaccessible to Gaussian states and essential for quantum applications such as quantum computing and error correction. However, their fragility raises the question of whether their key features can survive realistic environments. Reservoirs with memory are promising candidates for state protection, since they can induce revivals of those features. This thesis investigates that mechanism within quantum Brownian motion (QBM), the minimal archetype of a structured bosonic environment and the standard description of a broad range of experimental platforms. QBM has the further advantage of remaining exactly tractable despite describing dynamics that are simultaneously non-Gaussian and non-Markovian. Our main methodological result is a set of closed expressions for the purity, the coherence, the non-Gaussianity and the Wigner function of the cat state and its two squeezed generalisations, valid for any environment of this class. Applying them to a bath with a band gap shows that the structure of the environment, rather than the strength of its coupling, decides what survives: part of the dynamics is trapped in an undamped bound mode–a mechanism already observed experimentally–which generates revivals that keep recurring at arbitrarily long times. However, the protection is only partial, since the gap shields the system from dissipation but not from thermal noise, which still degrades the superposition. Finally, we show that squeezing redistributes robustness among these features, so that no single squeezing is optimal for all of them at once.
The Robustness and Breakdown of Collective Physics in Cavity-coupled Rydberg Arrays
(2026-09) Jiménez Isábal, Albert; Chang, Darrick; Jaworowski, Błażej
Quantized optical cavities naturally mediate long-range and even all-toall interactions between atoms, offering a route towards quantum many-body regimes that are difficult to access in conventional condensed matter systems. Their initial response is typically collective, with the atoms coherently behaving as a macroscopic spin and exhibiting phenomena such as spin squeezing. However, recent work has shown that sufficiently strong cavity interactions can nevertheless drive these systems into highly entangled states with non-trivial local structure, including quantum spin liquids. How a state that initially responds collectively to an all-to-all interaction can evolve into one dominated by local quantum correlations, remains an important question. Focusing on Rydberg atom arrays coupled to a single-mode cavity, we first show that the collective regime is remarkably robust. Even with strong superextensive cavity couplings, finite-momentum correlations vanish in the thermodynamic limit leading to a purely collective description. We develop a nonlinear spin-wave theory that predicts how this collective regime can break down and local correlations build up, but only under an unusual rescaling of the cavity interaction. More broadly, this thesis also highlights the challenges of characterizing long-range interacting quantum systems with standard many-body approaches, such as perturbation theory. Developing new theoretical descriptions for these intrinsically non-local systems may open the way to discovering quantum phases and dynamics beyond those accessible in short-range systems.