Dipòsit Digital de la Universitat de Barcelona

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The Dipòsit Digital de la Universitat de Barcelona is the institutional repository that contains, in digital format, materials resulting from teaching, research and institutional activities of members of the university community.

Recent Submissions

Sorting atomic positions in an optical tweezer atom arrays
(2026-09) Volante-Abovich, Matias; Redon, Quentin ; Tarruell Pellegrin, Leticia
In this work we report on the development of a dynamical rearrangement system for optical tweezer atom arrays. This lays the foundation for the Rydberg atom-array experiment at ICFO which aims to simulate lattice gauge theories with strontium-88. The developed system uses acousto-optic deflectors (AODs) to move individually trapped atoms into defect-free target geometries. We focus on the laser source built to feed the AODs, a tapered amplifier at the 813.4nm clock-magic wavelength. Therein, the beam shaping and fiber-coupling optics have been assembled and characterized as part of this thesis project, reaching 58% fiber coupling efficiency. We explore the practical alignment and control of movable tweezers through an independent test setup, specifically characterizing the frequency-dependent diffraction efficiency of the crossed AODs and their single- and multi-tone performance. In addition, we address the need for efficient atom rearrangement by implementing and benchmarking three sorting algorithms of varying computational cost. Finally, we tackle the required scaling analysis by estimating the maximum array size rearrangeable within the atomic trap lifetime, ensuring thereby the suitability of the system for defectfree array assembly and the limits for the current experiment
Bounding Spectral Properties of Many-Body Quantum Systems
(2026-09) Tudurache, Gerard-Constantin; Frías Pérez, Miguel; Farina, Donato; Acín dal Maschio, Antonio
Simulating quantum many-body systems with large size is a considerable challenge in physics. Often, understanding complex many-body phenomena requires information about the spectrum of their Hamiltonian. Apart from performing the exact diagonalisation, which is not possible for systems with tens of particles, there are some scalable methods for obtaining information about specific energy regions (e.g., ground state properties, spectral gap). The main methods that address the problem of characterising properties of the spectrum are variational methods and quantum Monte Carlo methods. These methods were mostly used for low energy states, performing poorly for excited states. On the other hand, semidefinite programming is emerging as a complementary tool, able to yield certified results for quantities of interest. In this thesis, we explore the capabilities of semidefinite programming in the context of quantum many-body systems. Recent works regarding the ground state [WSF+24], [WJF+26] showcased the potential of these programs and we proceed to focus on their performance in the context of highly entangled states. The proposed semidefinite program can determine parts of the spectrum with no eigenstate and provide bounds for the expectation values of a target eigenstate
Creating Non-Equilibrium States with Quantum Generative Models
(2026-09) Ruiz Galindo, Pedro; De Chiara, Gabriele; Domingo Colomer, Laia
Quantum denoising diffusion models (QDDMs) have recently been proposed as generative models for quantum-state distributions, with existing approaches primarily designed for pure or nearly pure-state ensembles. In this work, we develop design strategies to extend QDDMs to complex mixed-state distributions. The main modification to the denoising architecture introduces a Haar-random auxiliary qubit and replaces post-measurement extraction with a partial trace over the auxiliary qubits, naturally enabling mixed-state generation. We investigate different auxiliary-state constructions and two forward diffusion processes based on depolarizing noise and scrambling circuits. The proposed framework is benchmarked on single-qubit distributions and mixed-state ensembles arising from open quantum-system dynamics, including thermal and non-equilibrium steady-state distributions, extending the evaluation to physically motivated ensembles. Across the benchmarks considered, the partial-trace approach generally achieves better reconstruction for single-qubit mixed-state distributions and accurately reproduces the considered steady-state distributions, while post-measurement approach remains advantageous for pure-state ensembles. We further find that the choice of auxiliary states strongly influences the geometry of the generated distributions, whereas no systematic advantage is observed between depolarizing and scrambling diffusion. Finally, we demonstrate the feasibility of implementing the proposed framework on a real quantum processor through an adaptation for IBM Quantum hardware.
Towards a compact and transportable solid-state quantum memory
(2026-09) Ramos Sánchez, José Ernesto; De Riedmatten, Hugues ; Teller, Markus
The distribution of entanglement over long-distance quantum networks requires quantum repeaters placed along the network, since otherwise it is impeded by losses in the quantum channels. A possible implementation of quantum repeaters is based on quantum memories paired with sources of entangled photons. In this thesis, we present the design, construction, and characterization of compact solid-state quantum memory hardware based on a Pr3+:Y2SiO5 crystal. The hardware encompasses an AOM rack for precise optical pulse shaping, a laser system stabilized via the Pound-Drever-Hall (PDH) technique, and a cryogenically cooled Pr3+:Y2SiO5 sample on an optical table setup with small footprint. We measure the efficiency, temporal response, and thermal stability of the AOM rack, and we estimate a stabilized laser linewidth of 2.7(3) kHz. Additionally, we characterize the quantum memory crystal within our setup, measuring an inhomogeneous broadening of 11.2(4) GHz and a coherence time (T2) of 58(11) μs, which corresponds to an homogeneous linewidth of 5.5(10) kHz. This confirms the suitability of the hardware for the implementation of the AFC protocol as a quantum memory protocol. The advancements present in this compact system constitute a key step towards the deployment of rackintegrated quantum repeater nodes for long-distance quantum communication networks.
Optical Engineering of Quantum Materials: Floquet Theory and Beyond
(2026-09) Pérez Checa, Joan Ernest; Martínez Romeral, Jorge; Canonico Armas, Luis M.
In this thesis, the relationship between Floquet theory and a non-equilibrium quantum dynamics simulation in the particular case of graphene irradiated with a laser is studied. First, Floquet theory is applied to identify the quasienergy spectrum of the laser-material system. Next, the electron dynamics of the system are studied in real-time using a non-equilibrium quantum dynamics simulation. Then, the results of both approaches are compared and used to understand the behaviour of electrons in the system. Finally, by using effective models, an analytical expression relating the size of the Floquet dynamical gap with the rate of absorption and emission of photons given by the results of the quantum dynamics simulation is obtained.