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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Analog quantum simulation of high-harmonic generation in cavity QED
(2026-07) Zafra-Bono, Diego; Argüello-Luengo, Javier; Lewenstein, Maciej
High-harmonic generation (HHG) is the process where an intense laser field drives electron dynamics to emit high-frequency radiation and it stands as the cornerstone of attosecond science. However, because traditional HHG is driven by macroscopic, classical laser fields, exploring the underlying quantum properties of the emitted light—such as squeezing, entanglement, and non-classical photon statistics—remains a formidable experimental challenge. In this thesis, we propose an analog simulator for HHG in a cavity QED architecture, where an atom trapped in an optical tweezer directly mimics an electron bound to a nuclear potential. The driving electric field is simulated by the dispersive force of an optical cavity, which is proportional to the cavity’s oscillating photon number. We show analytically that by combining this optical force with a static bias, the strictly positive cavity photon number mimics the alternating, ultra-intense electric field characteristic of standard HHG. Crucially, the interaction force naturally inherits the quantum character of the cavity field and the external cavity drive, enabling the direct observation of quantum-optical HHG. We numerically verify the underlying classical-analog mapping by solving the Time-Dependent Schrödinger Equation (TDSE) for the atomic wavepacket, confirming that the dynamics exhibit the characteristic HHG emission spectrum. Finally, we detail the experimental feasibility of the platform using state-of-theart Rydberg systems. This work bridges the gap between strong-field physics and quantum optics, offering a fully controllable environment to explore the extreme non-linear dynamics of HHG in the quantum regime.
Quantum complexity in high-energy many-body systems
(2026-07) Picañol Narbona, Roger; Stornati, Paolo
The study of quantum many-body systems is a fundamental problem in modern physics, with direct connections to quantum simulation, condensed matter, high-energy physics, and quantum information. A central challenge is to understand which many-body states can be efficiently represented and simulated classically, and which ones require genuinely quantum computational resources. This question is naturally tied to the notion of quantum complexity, which can be probed through diagnostics such as entanglement entropy and non-Gaussianity. In this thesis, we study complexity markers in interacting fermionic quantum many-body systems. We first analyze a model interpolating between the chaotic Sachdev–Ye–Kitaev (SYK) model and the integrable transverse-field Ising model. Using the half-chain von Neumann entanglement entropy and the fermionic antiflatness, a recently introduced measure of fermionic non-Gaussianity, we characterize the transition between chaotic and integrable regimes in both ground states and excited states. We also discuss symmetryprotected effects in the SYK limit, where antiunitary symmetries can lead to the exact vanishing of the fermionic covariance matrix for specific system sizes. We then apply these ideas to the lattice Schwinger model, where tensornetwork methods are used to study the ground-state phase transition and realtime dynamics. In particular, we investigate how fermionic antiflatness and entanglement behave near criticality, and how their finite-size scaling can be used to extract universal features of the transition. Overall, this thesis shows that fermionic magic and entanglement provide complementary information about the structure and complexity of quantum many-body states.
Two-Color Quantum Light for Strong-Field Physics
(2027-07) Petrovic, Lidija; Lewenstein, Maciej; Rivera-Dean, Javier
High-harmonic generation (HHG) is a highly nonlinear process that enables the conversion of strong low-frequency driving fields in to high-frequency radiation.While conventionally driven by classical laser fields,the quantum properties of the driving field provide new possibilities for control ling the photon statistics and correlation soft he generated harmonics. In this work, we investigate HHG driven by bichromatic fields composed of a coherent state and bright squeezed light. By analyzing the photon statistics, we reveal distinct statistical properties of the generated harmonics depending on the quantum nature of the driving field.ForDSV-driven HHG, the emitted harmonics exhibit weakly super-Poissonian statistics, depending on the degree of squeezing and on the polarization configuration of the driving field.Incontrast,BSV-driven HHG leads to strongly enhanced photon correlations. Furthermore, by splitting the BSV field on a beam splitter and recombining the resulting modes with the coherent driver, we demonstrate the generation of entangled harmonic radiation between the two HHG out puts. Our results show that quantum fluctuations and correlations of the driving field can be transfer red to the high-harmonic regime, establishing HH Gasa platform for generating entangled quantum light at extreme frequencies.
Towards a carbon nanotube-based hybrid quantum platform
(2026-07) Martín Pérez, Ekaitz; Bachtold, Adrian; Tormo-Queralt, Roger
Hybrid quantum systems that integrate electronic, mechanical, and optical degrees of freedom hold promise as a versatile platform for quantum state preparation and manipulation. Yet, combining all three within a single device is notoriously difficult, and very few platforms in the world are able to do so. Among these, suspended carbon nanotubes (CNTs) stand out as ideal candidates for performing experiments in the quantum regime. The device we study consists of a pristine, suspended CNT, which is electrostatically controlled by an underlying array of five independent gates. We present an exhaustive study of the electronic degree of freedom of this device, fabricated using a stamping protocol. By precisely tuning the voltage on the gates, we can define high-quality single and double quantum dots in the nanotube. From their electronic transport characteristics, all electrostatic parameters of the device can be extracted. The double quantum dot is shown to be highly tunable, and it allows us to attain a regime where an electron delocalized over the two quantum dots can be used as an electronic two-level system. We will successfully demonstrate how previous double dot operation levels in the field can be matched, and a promising qubit outlook will finally be presented
Quasi-adiabatic preparation of squeezed antiferromagnetic states
(2026-07) López i Iglesias, Mar; Chang, Darrick; Jaworowski, Błażej
Cavity-mediated interactions have recently been proposed as a route toward strongly correlated many-body regimes in atomic ensembles. The underlying mechanism is that an antiferromagnetic cavity-mediated interaction penalises total transverse spin fluctuations, pushing the system toward states of small total spin, where the physics is determined by the cavity fluctuations. Once the system reaches a degenerate singlet-like sector, additional spatially structured spin couplings lift the degeneracy and select specific many-body phases, such as quantum spin liquids. This thesis addresses the first of these steps: the quasi-adiabatic preparation of squeezed antiferromagnetic states in a collective cavity-QED setting. We consider two atomic sublattices initialised in a classical antiferromagnetic product state pinned by a staggered field, and ramp up a cavity-mediated interaction that generates correlations between the sublattices, suppressing their collective transverse fluctuations. Since the cavity is intrinsically lossy, we describe the dynamics with a Lindblad master equation for collective photon loss, which introduces a competition between diabatic excitations from fast ramps and dissipative degradation from slow ones. We solve this trade-off in the minimal two-atom case, where the target is the antisymmetric Bell state, and then extend the analysis to larger ensembles using linear spin-wave theory, where the target is a two-mode squeezed antiferromagnetic state. In both regimes, the optimal protocol is not the most adiabatic one, but the one that best balances diabatic and dissipative errors. As the atom number increases, the achievable squeezing improves monotonically despite collective dissipation, approaching Heisenberg-limited scaling (∝ 1/N) in the weakdissipation regime. These results show that squeezed antiferromagnetic states remain accessible under realistic cavity-QED conditions, supporting their role as a collective precursor toward richer, spatially structured many-body phases.
Exploring Planckian Thermalization Time in Strongly Coupled Open Quantum Systems
(2026-07) Jiménez Rodríguez, Jesús; Perarnau-Llobet, Martí; Calsamiglia Costa, John
Thermalization times in open quantum systems are often constrained using quantum speed limits formulated for a fixed Hamiltonian or a prescribed dynamical trajectory. Such bounds can become trivial at strong system–environment coupling and do not directly capture thermalization as a robust process that must adapt to different system Hamiltonians. In this thesis, we study an alternative information-theoretic approach based on the distinguishability of equilibrium states associated with a family of nearby Hamiltonians. Within this framework, thermalization is viewed as a Hamiltonian-estimation task, leading to Planckian lower bounds on thermalization times that depend only on equilibrium-state sensitivity and not on the microscopic relaxation path. We extend this formulation to strongly coupled open quantum systems, where the relevant equilibrium state is the mean-force Gibbs state rather than the Gibbs state of the bare Hamiltonian. We investigate the resulting bounds in exactly solvable fermionic models and in spin–boson models across different coupling regimes. We show that strong coupling does not generically remove Planckian constraints: when the reduced equilibrium state retains information about the system Hamiltonian, nontrivial lower bounds on thermalization times persist.
Benchmarking the DC-OPF Problem on Quantum Annealers
(2026-07) García-Fernández-Santaella, Lucía; Riu, Jordi; Bosch, Josep
Combinatorial optimization problems can be naturally formulated as Quadratic Unconstrained Binary Optimization (QUBO) models and subsequently mapped onto Ising Hamiltonians, making them suitable for Quantum Annealing (QA) techniques. However, the practical implementation of these approaches is hindered by the limited connectivity of current quantum hardware and by the difficulty of accurately simulating the annealing dynamics for large-scale instances. In this work, a Direct Current Optimal Power Flow (DC-OPF) problem is formulated as a QUBO model and studied from both the embedding and annealing perspectives. Two embedding strategies, namely minorminer and the Triangular Architecture, are analyzed in terms of their physical qubit requirements. In addition, two simulation approaches, QiliSim and a variational method, are employed to investigate the annealing dynamics. To improve the annealing performance, catalyst terms previously developed for systems without local fields are extended to account for the local field contributions naturally present in the DC-OPF formulation. These additional interactions, inspired by diagonal augmentations of the Quantum Approximate Optimization Algorithm (QAOA), act as catalysts by enhancing the ground state fidelity with respect to standard quantum annealing. Different catalyst configurations are studied and their effect on the probability of obtaining low energy states is analyzed. Furthermore, qubit reduction strategies based on thresholding and spectral decomposition are investigated in order to decrease the physical resources required for implementation. The results show that the inclusion of local field contributions in the catalyst term, leads to a significant improvement in the annealing performance and that suitable thresholding techniques allow the number of physical qubits to be reduced by approximately a factor of two while preserving the quality of the obtained solutions. These findings provide further support for the catalyst framework and contribute to bringing realistic optimization problems closer to current quantum hardware
Time-Dependent Neural Quantum States for Quantum Dynamics
(2026-07) Fernández Suárez, Miguel; Rios Huguet, Arnau; Rozalén Sarmiento, Javier
Neural Quantum States have emerged as powerful variational ansätze for modeling complex quantum systems, although their extension to real-time dynamics remains challenging. In this work, the wavefunctions of the threedimensional Quantum Harmonic Oscillator and the deuteron are parametrized through two real-valued feed-forward neural networks, representing separately the amplitude and the phase. Expectation values are estimated using Markov chain Monte Carlo sampling, ground states are optimized through Stochastic Reconfiguration, and real-time evolution is performed using the McLachlan time-dependent variational principle. After accurately reproducing the ground states of the considered systems, the optimized Neural Quantum States are used as initial conditions for time evolution. Successful dynamics are obtained for the Quantum Harmonic Oscillator, a mass-rescaled deuteron and the physical deuteron in a periodic box, with fidelities against independent benchmarks remaining above 0.9986 in all cases considered. Ultimately, these results show that Neural Quantum States provide flexible variational ansätze capable of describing accurate real-time dynamics
Effects of Atmospheric Turbulence on Spatially Entangled Photon Pairs Generated by SPDC
(2026-07) Fernández González, Jose Javier; Gil López, Jano; Juliá-Díaz, Bruno
Spatially entangled photon pairs are promising resources for free-space quantum communication but, unlike polarization encoding, spatial encoding is directly affected by turbulenceinduced wavefront distortions that reduce coherence between the spatial alternatives defining the quantum state. This thesis studies an effective two-qubit spatial state generated by degenerate Type-I spontaneous parametric down-conversion, in which signal and idler photons occupy two correlated transverse-path alternatives. Entanglement is quantified through concurrence, determined by the off-diagonal coherence of the two-photon density matrix. The atmosphere is modeled as an stochastic phase screen, and the change in concurrence is expressed as an ensemble average of phase differences evaluated at the four positions defining the spatial qubits. Theoretical predictions are derived from the phase structure functions of the Kolmogorov and modified von Kármán models, using the Fried parameter r0 to characterize turbulence strength. Numerical phase screens are generated through a finite Zernike-polynomial expansion with correlated coefficients obtained from model-dependent covariance matrices. Simulations use 500 Zernike modes and 1,000 statistically independent phase screens for each sampled value of r0. The numerical results agree closely with theory for both turbulence models. Concurrence approaches zero for strong turbulence and unity when the turbulent wavefront is coherent across the biphoton state. Entanglement degradation is governed mainly by the relation between r0 and the effective transverse size Δρ′, while the separation d between spatial alternatives has a weaker influence. Both turbulence models give similar predictions at the scalesconsidered, with differences expected near the inner turbulence scale. The framework directly connects atmospheric phase statistics with spatial-entanglement degradation and provides a basis for more robust spatial encodings and future laboratory validation using spatial light modulators.
Entanglement in Top Quark Pair Production Experiments
(2026-07) Belmonte Giménez, Blanca; Cervera Lierta, Alba; Blas, Diego
Collider events are the sets of particles produced in high energy collisions. They are usually analysed through kinematic distributions, which describe how quantities such as energies, momenta and angles are distributed, and through production rates, which measure how often a given process occurs. This thesis uses a complementary description based on the spin state of the particles produced in the final state. The spin information is reconstructed as a density matrix, which encodes the quantum state of the selected particles and allows the use of tools from quantum information. The study starts from the bipartite spin system pp → t¯t, where two protons collide and produce a top quark and an antitop quark. In this case, the concurrence quantifies the entanglement between the two particles. The analysis is then extended to three-particle final states, including the Standard Model processes pp → t¯tγ, with an additional photon, and pp → t¯tZ, with an additional Z boson. A benchmark Beyond the Standard Model (BSM) process, pp → t¯tv1, is also considered, where v1 is a massive vector mediator. The events are simulated with MadGraph, a program that generates particle-collision events from scattering amplitudes, and the analysis is performed at tree level, meaning that only the leading-order diagrams without loop corrections are included. For this purpose, this thesis also extends the MadGraph density matrix extraction to three selected particles in the final state. The reconstructed spin states are then studied through their purity, reduced bipartite entanglement and multipartite entanglement diagnostics. The results show that proton-proton samples can hide different underlying spin structures. The separated q¯q and gg production channels display different degrees of mixedness and different multipartite behaviour. In the proposed BSM process, a χ2 analysis over the mt¯t distribution shows that quantities extracted from the spin density matrix, such as spin correlations and entanglement measures, can be sensitive to the new contribution. Overall, the results show that density matrices provide access to quantum state information in simulated collider processes, extending the analysis beyond the standard bipartite t¯t setting.
Toward ML-Based State Discrimination on FPGA in Open-Source Quantum Control Electronics
(2026-07) Barillas Rodriguez, Ender Jose; Pérez Díaz, Joel
Low-latency and high-fidelity qubit state measurement is essential for superconducting quantum computing, particularly for enabling mid-circuit measurements and real-time feedback. While commercial control hardware increasingly supports onFPGA state discrimination, these platforms operate on closed firmware that precludes custom signal processing pipelines. Open-source FPGA frameworks offer an alternative path: full programmatic access to the readout chain, letting researchers integrate custom algorithms, including machine learning, directly into the measurement workflow. This thesis documents the setup and characterisation of a QICK-based [1] readoutsystem on a Xilinx ZCU216 RFSoC platform, covering FPGA configuration, DDS based waveform generation, and the analogue front-end signal path. The work establishes a functional loopback signal chain, validated end-to-end from pulse generation to demodulated IQ data, together with the calibration workflow needed to drive and read out superconducting resonators once a device is attached, and examines the constraints the hardware places on dispersive qubit readout at the target resonator frequencies. Using existing single-shot readout data from a superconducting qubit device, we design and evaluate a machine-learning-assisted state discrimination pipeline. On integrated IQ data, linear discriminant analysis is already near-optimal: none of the twenty MLP architectures we tested improves on it, as expected from the Gaussian structure of the IQ clouds. The only meaningful gain comes from a lightweight 1D CNN trained on synthetic raw ADC traces built from the real device's IQ statistics, which recovers shots corrupted by mid-readout T1 relaxation that integrated methods cannot distinguish. On a synthetic test set the CNN reaches 99.75% overall delity and 98.4% accuracy on relaxation shots, against 44.3% for LDA on the same 122 shots. This model is trained and evaluated entirely offine; real-time deployment within the QICK firmware was not achieved in this work. The thesis instead closes by outlining the path toward it: the data collection procedure required on the ZCU216, the model adaptation and quantisation constraints imposed by the programmable logic fabric, and the methodology needed to test whether deployment improves classi cation speed, fidelity, or both.
Noise Simulation of a Tunable Fluxonium : Modeling and Analysis of Markovian and Non-Markovian Effects with Open Quantum Systems
(2026-07) Aribó Herrera, Marta Xiulan; Palacios de Luis, Ana; Riera, Arnau
This thesis presents an open-system simulator of decoherence in a tunable fluxonium device. The simulator incorporates both standard Markovian noise channels and an additional set of near-resonant two-level-system (TLS) defects as explicit quantum degrees of freedom to capture possible memory effects. The fluxonium is modeled as a qutrit to account for leakage, and its dynamics are simulated both at fixed flux biases and along a time-dependent annealing schedule, with the results compared to the corresponding Markovian dynamics. At fixed bias, the effect of the explicit TLSs depends strongly on the operating point: it is negligible where the relevant charge matrix elements are suppressed, but can reduce T1 by over an order of magnitude and produce non-exponential decay where the qutrit couples strongly to the defects. Along the schedule considered here, a coherent operating window is identified in which the evolution is both adiabatic and coherence-preserving; within it, TLS memory effects do not become significant, and Markovian rate estimates suffice to choose the annealing time.
Study of chiral phonon induced electronic excitation in 2D materials
(2025-07) López Fernández, Javier; Canonico, Luis M.; Romeral Martínez, José Luis; Cummings, Aron W.
In this thesis, we study the interaction between electrons and chiral phonons in the non-equilibrium regime, on honeycomb lattice models. To this end, we employ a real–space tight-binding model, and we model phonons as background lattice oscillations. The electron dynamics is studied using a non-equilibrium, linear–scaling method that can account for non-perturbative carrier dynamics. We first use the methodology to simulate a simpler optical phonon mode. Then, we examine the nature of the allowed transitions due to electron–chiral phonon interaction. Lastly, we show that, in contrast to the usual thought of the lattice as a spin sink, chiral phonons can produce a net spin-polarized excited electron density.
Exploring the limits of many-body quantum metrology via adiabatic dynamics
(2025-07) Luszczak André, Erik; Perarnau Llobet, Martí; Calsamiglia Costa, John
The fundamental limits of thermal equilibrium (and ground state) metrology have been recently derived, thus providing an upper bound on the maximum precision we can extract when trying to estimate an unknown Hamiltonian parameter. Nevertheless, approaching and connecting these bounds with the dynamical Heisenberg limit still remains an open challenge. In this thesis, we first show how to approach the equilibrium bound via control on two-body interactions, and then we build an explicit connection between this result and the dynamical bound. In particular, we consider adiabatic protocols on the controllable part of the Hamiltonian, which make the system follow the ground state of the Hamiltonian throughout the evolution. If we impose adiabaticity locally, we manage to saturate both the Heisenberg and ground state metrology bounds in some particular models. We first exemplify these insights with a spin-squeezing Hamiltonian, and then we relate them to a paradigmatic model in condensed matter: the Heisenberg XXZ chain.
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.

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