Màster Oficial - Ciència i Tecnologia Quàntiques / Quantum Science and Technology

URI permanent per a aquesta col·leccióhttps://hdl.handle.net/2445/188101

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Stabilizer codes and absolutely maximally entangled states for mixed-dimensional systems
(2025-07) Zhang, Raven; Ball, Simeon
A major difficulty in quantum computation is the ability to implement fault tolerant computations, protecting information against undesired interactions with the environment. The theory of stabiliser codes has been developed over recent years which protects information when storing or applying computations in Hilbert spaces where the local dimension is fixed, i.e. in Hilbert spaces of the form (CD)⊗n. If D is a prime power then one can consider stabiliser codes over finite fields [KKKS06], which allows a deeper mathematical structure to be used to develop stabiliser codes. However, there is no practical reason that the subsystems should be required to have the same local dimension and in this work, we introduce a stabiliser formalism for mixed dimension Hilbert spaces, i.e. of the form CD1 ⊗ · · · ⊗ CDn. We redefine entanglement measures for these Hilbert spaces and follow [HESG18] to define absolutely maximally entangled states as states which maximize this entanglement measure, and give an example of such a state on a mixed dimension Hilbert space.
Towards Efficient Spatial Variational 2-RDM via Measurement Constraints
(2025-08) Nel, Annika; Acín dal Maschio, Antonio; Zambrano, Leonardo
Reduced density matrices (RDMs) offer a more scalable alternative to full wavefunctions when performing chemical calculations. The variational twoelectron RDM (v2RDM) method exploits the efficiency of RDMs, employingsemidefinite programming (SDP) to enable polynomial scaling of ground state simulations. Recent work by Avdic & Mazziotti seeks to improve the performance of the v2RDM by incorporating classical shadow constraints, simultaneously reducing the number of measurements required for tomography. Drawing from this work, we introduce a spatial orbital variant of the v2RDM with measurement constraints (m-v2RDM). The proposed method achieves comparable accuracy for small to medium-sized molecules such as H2, H4, and HF, while substantially reducing memory and runtime costs. Its comparatively simple implementation also allows for the approximation of larger systems like N2, which are otherwise intractable on modest computational resources using standard v2RDM. As a pedagogical resource, the spatial variant more closely resembles the underlying theory, making it an accessible introduction to RDMs. The spatial m-v2RDM further highlights the complementary nature of measurement constraints and N-representability conditions, framing the RDM as a potential tool for noise mitigation in quantum information processing.
Entanglement Properties and Dynamics of Collectively Dissipating Multilevel Atom Arrays
(2025-09) Lancis Beneyto, Guillem; Moreno Cardoner, Maria; Sánchez Llorente, Eric
Achieving an efficient and controllable atom-light interface is essential for quantum technologies. In this context, subwavelegnth atomic arrays provide a promising platform, as collective radiance effects can be exploited to achieve an enhanced atom-light coupling and a higher fidelity in certain quantum optics protocols. In such systems, constructive (superradiance) and destructive (subrradiance) interference between the scattered photons enables to suppress spontaneous emission into undesired optical modes, while enhancing it into desired, detectable modes. In this work, we explore how these ideas, originally developed for two-level atoms, can be extended to multilevel structures with a focus on Λ-type atoms with one excited state and two degenerate ground states. To this end, we generalize the open quantum spin model to multilevel atoms and apply it to Λ systems. We study the collective radiative properties and the entanglement of Dicke states, using a mapping onto SU(3) algebra. Furthermore, we analyse how finite-size effects and coherent interactions modify collective radiance, leading to the emergence of darker states in the two excitation manifold of Λ-systems, compared to the case of two-level atoms, for an atom number N ≥ 10. We also study the dissipative Dicke dynamics for a fully inverted initial state, showing that the evolution is restricted to the symmetric sector. In the finite-size array case in presence of coherent interactions, we identify a peak in the dynamical evolution of entanglement, coinciding with the superradiant burst and find that the system reaches a non-trivial entangled steady state.
Measurement of Thermomechanical Motion in the Few-Phonon Regime Using Carbon Nanotube Charge Sensors
(2025-08) ElDik, Julie; Forstner, Stefan; Bachtold, Adrian
We report the detection of thermomechanical motion in suspended carbon nanotube (CNT) resonators operating in the few-phonon regime, using an integrated charge sensor at cryogenic temperatures. We fabricate ultra-clean single-walled CNTs using a chemical vapor deposition (CVD) method and suspend them across predefined gate and electrode structures. The devices allow confinement of single and double quantum dots electrostatically defined in a CNT and capacitively coupled to a nearby charge sensor quantum dot. A radiofrequency (RF) readout circuit enables sensitive detection of thermomechanical motion at mode temperatures as low as 50 mK, corresponding to an average phonon occupation number below 10. We observe Lorentzian power spectral densities of the mechanical resonance and track the evolution of displacement amplitude with temperature. Deviations from ideal thermal scaling suggest additional temperature-dependent effects not fully captured by charge sensor sensitivity alone. These results aim to improve quantum nanomechanical sensing.
Neural Quantum States: Fermions on D-Dimensions
(2025-09) Carrasco Arango, MIguel; Rios Huguet, Arnau; Rozalén Sarmiento, Javier
In this work, we explore the use of Neural Quantum States to approximate the ground-state wavefunctions of fully polarized fermionic systems confined in a D-dimensional harmonic trap. Building on the architecture introduced in [1], we generalize the input representation and network structure to handle arbitrary spatial dimensionality, extending the applicability of the method beyond one-dimensional systems. The antisymmetric nature of the fermionic wavefunction is preserved through the use of equivariant neural layers, and a generalized Slater determinant is constructed from learned single-particle orbitals modulated by a Gaussian envelope. Training is carried out in two stages: first, a supervised pretraining phase based on analytical solutions of the non-interacting system, which is then followed by variational Monte Carlo optimization of the network parameters using the energy as the loss function. We validate our approach on non-interacting systems with up to 4 particles in 2D and 3 particles in 3D, where analytical solutions are available for benchmarking. Results show excellent agreement in terms of mean energy, one-body density, and the one-body density matrix, with observed spatial symmetries and degeneracy patterns matching theoretical expectations. While the training protocol has been generalized to incorporate finite-range interactions, this study focuses on non-interacting systems to establish a solid baseline. The framework developed here provides a flexible and scalable foundation for future exploration of interacting quantum systems in higher dimensions using neural variational methods.
Cryocharacterization of an integrated superconducting cavity for suspended carbon nanotube quantum dot readout
(2025-09) Berasategui Miguéliz, Beñat; Román, Víctor; Bachtold, Adrian
Conventional transport measurements cannot detect charge transitions in carbon nanotube quantum dots when no net current flows, whereas existing dispersive readout approaches using separate chips suffer from parasitic capacitances and limited scalability. We developed the first fully integrated platform within our research group that capacitively couples a λ/4 niobium superconducting resonator (fr = 5.88 GHz, Qi = 1150, Qe = 1730) directly to a suspended carbon nanotube quantum dot, enabling cryogenic measurements from 10 mK to 6 K. We successfully demonstrated dispersive readout of interdot charge transitions that are invisible to transport techniques, while also observing clear Coulomb peaks, diamonds, and stability diagrams through reflectometry measurements. This integrated approach achieves higher signalto-noise ratios, precise control over resonance coupling, and eliminates wiring losses, establishing quantum non-demolition readout capabilities and opening new possibilities for charge qubit studies and electromechanical coupling experiments.
Solving the Quantum Many-Body Lindbladian Learning Problem With Neural Differential Equations
(2025-08) Aseguinolaza Gallo, Roman; Heightman, Timothy; Jiang, Edward
Learning open quantum many-body dynamics is challenging: full Liouvillian models grow exponentially with system size, and dissipation and dephasing force us to follow mixed states from noisy, limited data. These factors make routine characterisation and control difficult, so we need methods that are data-efficient, scalable, and easy to interpret. We present an interpretable, robust framework for learning Lindbladian dynamics from minimal, hardwarefriendly data. The method pairs a physics-first CPTP Lindblad model with a small Neural Differential Equation (NDE) residual and uses a two-stage curriculum (neural warm-up, then analytic-only refinement) to reliably recover coherent and dissipative parameters on challenging 1D benchmarks. There are two ways in which robustness emerges in Lindladian learning: modest physical dissipation that smoothens loss landscapes via steady-state attraction, and the NDE residual that resolves remaining nonconvexity when paired with an optimizer reset. A transient infidelity metric shows short-time power-law error and small steady-state plateaus. Extending beyond CPTP to a stochastic dissipative qubit shows failures in noise-induced or deep PT-unbroken phases that are information-limited, not optimization-limited
Quantifying entanglement in ℂ𝑁 ⊗ ℂ𝑁 by analyzing separability in ℂ2 ⊗ ℂ𝑁
(2025-08) Aguilera Ayuso, Júlia; Sanpera Trigueros, Anna; Romero-Pallejà, Jordi
A fundamental challenge in quantum entanglement is determining whether a given bipartite quantum state is separable or entangled, a problem known to be computationally intractable in general. This thesis focuses on bipartite systems of the form C2 ⊗ CN, consisting of a qubit and a qudit, which offer a rich yet tractable setting for studying entanglement. The central objective of this thesis is to investigate the maximal Schmidt number that an entangled quantum state can attain in CN ⊗CN systems. The Schmidt number is a bona fide measure of entanglement in bipartite systems. Here, we investigate how this measure of entanglement correlates with the structure of separable states in C2 ⊗ CN. To achieve this, the work combines analytical and numerical tools. Here, we examine structured quantum states, constructing families of states with computable or bounded Schmidt number, and apply criteria to assess their entanglement. In particular, we focus on the study of those entangled states that are positive under partial transposition (PPT), also denoted as bound entangled states. By integrating the above approaches, we provide a better characterization of entanglement in low-dimensional bipartite systems and we offer novel insights on how to classify quantum states according to their Schmidt number. Overall, this study advances the characterization of quantum correlations in CN ⊗ CN systems for a particular family of states, but offers a foundation for future investigations into entanglement quantification and separability criteria in generic states.
Quantum Generative Adversarial Networks: Improving Dynamics Simulation with an Ancilla Qubit
(2025-09) Abad López, Guillermo; Bhattacharya, S.S.; Usui, A.
Simulating complex quantum systems remains a critical challenge, as conventional quantum techniques– such as those based on the Suzuki–Trotter decomposition—often result in deep circuits that demand substantial computational resources. Quantum Generative Adversarial Networks (QGANs) offer a promising alternative by learning the time evolution of target Hamiltonian using significantly fewer gates. However, standard QGAN architectures commonly suffer from unstable convergence and learning plateaus in the loss landscape, which hinder training and prevent the generator from achieving high-fidelity solutions. To address these limitations, we propose augmenting the generator with an ancilla qubit, expanding the learning space, and providing additional degrees of freedom that enable training to progress when the model becomes trapped in certain regions of the loss landscape. In this work, we investigate the effect of incorporating an ancilla under various connectivity topologies and at different stages of training, in order to perturb the optimization landscape and aid the generator overcome problematic training cases Simulation results demonstrate that ancilla-assisted QGANs successfully escape learning plateaus and other non-convergent behaviours, particularly when the ancilla’s connectivity links distant regions of the ansatz. Notably, the optimized fidelity overall improves when the ancilla is introduced mid-way through the training.
Spinor Bose–Einstein Condensate Magnetometry for Searches in Fundamental Physics
(2025-07) Zhou, Wenjing; Mitchell, Morgan; Blas Temiño, Diego
This thesis investigates the use of a Spinor Bose-Einstein Condensate as a micrometer-scale quantum sensor for probing new fundamental physics. The sensor’s sensitivity is limited by a complex interplay of noise sources. We develop a theoretical framework to identify, model, and quantify these limitations, using the truncated Wigner approximation to capture interactioninduced shearing of the quantum noise and project sensitivity beyond the standard quantum limit. This model is supported by experimental efforts, including finite-element method simulations of an apertured magnetic shield and the development of a shot-noise-limited Faraday polarimeter. Applying the full framework, we show that an SBEC comagnetometer can set improved laboratory constraints on axion-like particle–proton couplings. In contrast, we find that the sensor’s small interaction volume limits its competitiveness for detecting high-frequency gravitational waves via spin-gravity coupling
Towards Scalable Quantum Simulation: Distributed Circuit Cutting for Hybrid Quantum-HPC Systems
(2025-07) Tejedor Ninou, Mar; Badia Sala, Rosa M. (Rosa Maria); Cervera Lierta, Alba
As quantum computing advances, practical deployment of quantum algorithms remains hindered by hardware limitations such as restricted qubit counts and and limited connectivity. Circuit cutting has emerged as a promising strategy to extend quantum computations beyond these hardware constraints by decomposing large circuits into smaller subcircuits that can be executed individually and recombined through classical post-processing. This master thesis presents Qdislib, an open-source software library that integrates quantum circuit cutting with high-performance computing (HPC) infrastructure to enable scalable and hybrid quantum-classical workflows. Qdislib builds on PyCOMPSs, a task-based runtime system, to orchestrate the parallel execution of subcircuits across heterogeneous resources, including CPUs, GPUs, and quantum processors (QPUs). The library supports both wire cutting and gate cutting techniques and introduces an automated cut selection algorithm, FindCut, to optimize circuit partitioning based on user-defined constraints. Benchmarking is performed on Hardware-Efficient Ansatz (HEA) and Random Circuit (RC) workloads, evaluating execution on MareNostrum 5 and IBM Quantum Cloud. Results demonstrate strong scalability for classical simulations and hybrid execution, achieving near-linear speedups on up to 64 compute nodes and successfully integrating local and remote QPUs. Qdislib thus provides a practical and extensible framework for distributed quantum simulation, paving the way for scalable quantum computation in heterogeneous environments
Atomic Properties & Red Laser System for a Sr-Based Rydberg Quantum Simulator
(2025-07) Rochlitzer Puig, Derik; Redon, Quentin; Tarruell, Leticia
Quantum simulation seeks to study physical models that are beyond the reach of classical computation. Within this scope, ICFO’s Ultracold Quantum Gases group is developing a strontium Rydberg atom array platform to simulate high-dimensional lattice gauge theories with plaquette interactions, many-body couplings yet to be experimentally realised. This experiment requires a detailed theoretical understanding and a technically involved setup. This master’s thesis contributes to both fronts. On the theoretical side, we studied strontium’s clock state and its magnetic-field-induced excitation, characterised the properties and interactions of Rydberg states, and analysed a scheme for selective Rydberg excitation based on light shifts. Our results show that the clock transition can be broadened to the 0.1 mHz range to enable excitation, that Rydberg states with n ≈ 60 offer favourable interaction landscapes and coupling strengths, and that selective excitation should be feasible by scaling the intensity of optical tweezers. Experimentally, we implemented the core of the 689 nm laser system, including a slave diode for power amplification and an optical cavity for monitoring. We also verified the finesse of an ultrastable cavity for future frequency stabilisation and successfully tested the Pound-Drever-Hall locking technique on the monitoring cavity. Overall, these developments mark significant progress towards completing the experimental platform and provide a theoretical basis for future design choices. The future steps will focus on assembling the remaining experimental systems and testing our theoretical predictions.
Development of a Receiver System for Passive Polarization-Encoded BB84 Protocol
(2025-07) Öztürk, Berkant Özgür; Gómez Cama, José María
As the development of quantum computing threatens classical cryptographic methods, Quantum Key Distribution (QKD) provides a pathway to physically secure communication. The practical implementation of robust and efficient receiver modules, however, remains a key challenge for real world QKD systems. This thesis presents the complete design, implementation, and characterization of a high fidelity optical receiver (Bob) for the BB84 QKD protocol. The architecture is distinguished by its use of a passive random basis selection mechanism, employing a non-polarizing beam splitter to enhance system simplicity, robustness, and cost-effectiveness. Furthermore, the modular design features a fiber coupled input, ensuring flexibility for easy integration into both free-space and fiberbased quantum communication links. For polarization analysis, Wollaston prisms were selected over other alternatives due to their high extinction ratio, which is critical for minimizing measurement errors. The system was designed and optimized for an operational wavelength of 810nm for compatibility with a single photon source based on a beta barium borate (BBO) crystal. The receiver’s performance was experimentally validated by measuring its response to a full range of input polarization states. The successful realization of this project provides a robust, flexible, and high performance receiver module suitable for deployment in further QKD experiments
Quantum State Transfer with Ising Hamiltonians
(2025-07) Michel González, Oscar; Werner, Matthias; Riera, Arnau
Quantum state transfer is a fundamental requirement for scalable quantum computation, where fast, reliable communication between distant qubits is essential. In this work, we present a protocol for quantum state transfer in linear spin chains tailored to superconducting flux qubits. Starting from a perfect state transfer scheme via a Heisenberg Hamiltonian with inhomogeneous couplings [CDEL04], we adapt it to superconducting architectures by encoding the information in domain walls. The resulting Hamiltonian only contains ZZ interactions, allowing us to produce quantum transport in superconducting devices constrained to Ising-like couplings. We test the protocol for 1-, 2-, and 3- qubit states, obtaining high transfer fidelities of up to 0.99, and study the accuracy dependence on the domain wall approximation. Additionally, we analyze the protocol’s robustness to hardware errors, and determine tolerances to 7% variations in the transverse X fields, 0.9% in the coupling strengths, and up to 3MHz in local Z perturbations. Finally, we estimate the parameters required for a fluxonium qubit [MKGD09] to effectively run our algorithm, paving the way for an experimental implementation of the protocol.
Observer: An Information-Theoretic Perspective
(2025-07) Khan, Aatif Kaisar; Dourdent, Hippolyte; Leitherer, Andreas
The boundary between quantum and classical domains remains one of the most profound puzzles in physics, intimately tied to the nature of observation itself. This thesis advances a principled framework wherein observers are recast as System Identification Algorithms (SIAs), finite informational agents1 whose capacity to observe and track external systems is governed by their Kolmogorov complexity. Grinbaum’s hypothesis formalizes observerness2 as an algorithmic resource and gives a relational3 criterion for quantum-classical transitions: a system appears quantum to an observer only when its Kolmogorov complexity lies below that of the observer. Within this framework, classicality emerges as a thermodynamic necessity once memory saturation of the observer forces irreversible erasure, as dictated by Landauer’s principle. We further integrate this perspective into the Local Friendliness experiment, revealing that violations of Local Friendliness inequalities are computationally constrained: they persist only within regimes where complexity gaps between agents remain open. The undecidability of Kolmogorov complexity implies that the precise location of the quantum-classical cut is itself algorithmically inaccessible. We finally interpret the notion of an epistemic horizon discussed in Claim 1 of Restriction A [JM25] through complexity constraints.
On the Contextuality of Multi-Agent Quantum Paradoxes via Anomalous Weak Values
(2025-07) Izquierdo García, Víctor; Dourdent, Hippolyte; Leitherer, Andreas
In this thesis we investigate the role of quantum contextuality in multi-agent paradoxes by tracing the emergence of anomalous weak values (AWVs). Starting from the logical pre- and post-selection (LPPS) formulation of the Hardy paradox, we construct an explicit one-way LOCC protocol that reproduces its statistics and provides a proof of Kochen–Specker (KS) contextuality. We then embed this construction into a particular extended Wigner’s friend argument known as Local Friendliness (LF). In a coarse-grained model where two friends perform joint two-qubit measurements, we show that the LF assumptions lead to a logical contradiction identical to that of the LPPS Hardy paradox, thereby providing a proof of the LF no-go theorem. We further develop two extensions of this model in which the intermediate measurements are implemented as weak interactions. In both cases, the anomalous weak value of –1 is preserved, confirming its robustness as a witness of contextuality in this setting. A complementary fine-grained decomposition resolves each joint measurement into our LOCC-based construction. Although the same logical contradiction is recovered under the LF assumption, the weak measurement schemes in this decomposition no longer exhibit AWVs. This reveals an apparent tension between two seemingly equivalent descriptions, suggesting that the presence of AWVs may depend sensitively on how multi-agent scenarios are modeled.
Properties of 2D Bose gases at non-zero temperatures
(2025-07) Hernández Toledo, Adrià; Massignan, Pietro; Richaud, Andrea
In this work, we study the Berezinskii–Kosterlitz–Thouless (BKT) transition in two-dimensional ultracold Bose gases using the stochastic Gross–Pitaevskii equation at finite temperature. Through numerical simulations, we analyze several physical observables across the critical region, including the quasi-condensate density, vortex population, first-order correlation function g(1)(r), and superfluid density. Our results show clear signatures of the BKT transition: the onset of algebraic order in g(1)(r), the proliferation of free vortices above the critical temperature, and a universal jump in the superfluid density. We also examine the extent to which the energy distribution obeys the classical equipartition theorem. These findings demonstrate the effectiveness of stochastic Gross–Pitaevskii dynamics in capturing the essential features of 2D Bose gases across the BKT transition and provide insight into the interplay between coherence, topological defects, and superfluidity in low-dimensional systems.
Prototype of a Free Space Optical Link for Quantum Communications
(2025-07) Guerrero Muñoz, Lucía; Gómez Cama, José María; Bosch Estrada, José
This Master’s thesis presents the development of a free-space optical link for quantum communications, followed by the design and characterization of the single-photon source used to power it. A laboratory-scale link was established using Ritchey–Chrétien telescopes. To overcome the central obstruction of the telescopes, two different methods were tested: the use of axicon lenses to shape the beam into a toroidal beam with a Bessel-Gaussian profile and the lateral displacement of the input beam. The latter enabled successful photon transmission, revealing a bunching measurement in the second order cross-correlation function at the expected arrival time difference between the heralding photons and the signal ones measured at the receiver of 44 coincident counts per minute. To provide the required quantum light source, an entangled-photon source based on spontaneous parametric down-conversion in beta barium borate crystals was developed. Pairs of polarization-entangled photons at 810 nm were produced and verified using a CHSH inequality test, which resulted in a Bell parameter of S = 2.55 ± 0.15, which confirms the presence of quantum entanglement
Spin dynamics in heterostructures composed of ferroelectric van der Waals semiconductors and graphene
(2025-07) De Araújo Masanti, Gabriel; Valenzuela, Sergio O.; Sierra, Juan F.
Spintronics is a field that harnesses the spin degree of freedom of electrons to store, process, and transmit information. Van der Waals heterostructures combining graphene and high spin orbit coupling 2D materials are a promising platform to control spin transport, where proximity-induced effects change the behaviour of spin relaxation in graphene giving rise to spin lifetime anisotropy. In this work we study devices made of graphene and CuInP2S6 (CIPS), demonstrated to be a ferroelectric material in the few layers limit, with the goal of controlling spin transport properties in graphene by means of electric fields. We demonstrate that charge transport in graphene can be tuned by altering the ferroelectric state of CIPS, and we observe spin-lifetime anisotropy in graphene. Furthermore, we explore the potential of controlling the strength of this anisotropy through ferroelectric modulation
Numerical simulations of p-wave fermions in a one-dimensional harmonic trap
(2025-07) Camus Sais, Marc; Juliá-Díaz, Bruno; Rojo Francàs, Abel
We consider a system of N spin-aligned p-wave fermions confined within a one-dimensional harmonic trap. We study the energy spectrum and ground state properties across different regimes of interaction strength by performing numerical calculations. We compute the particle density and the eigenvalues of the one-body density matrix. Additionally, we study two-particle properties by calculating the pair correlation matrix. In the infinitely interacting limit, our results coincide with those of a fermionic Tonks-Girardeau gas. We analyze the discontinuity behavior near the non-interacting limit and provide an explanation. We propose a novel square well representation of the p-wave interaction in discrete space. We demonstrate the efficiency of this representation by comparing the results obtained with it to those obtained from analytical solutions and other numerical methods. We simulate the dynamics of the system after altering a parameter of the system, such as the trap depth or the interaction strength. We compute spectral properties of the system by analyzing the Fourier transform of the time-dependent spatial spread of the wave function.

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