Descubriendo el potencial de la computación cuántica

@article{nokey,

title = {Digital-Analog Quantum Machine Learning},

author = {Lamata, L.},

url = {https://advanced.onlinelibrary.wiley.com/doi/10.1002/aidi.202400023},

doi = {doi.org/10.1002/aidi.202400023},

year  = {2025},

date = {2025-02-13},

journal = {Advanced Intelligent Discovery},

volume = {1},

issue = {1},

abstract = {Machine learning algorithms are extensively used in an increasing number of systems, applications, technologies, and products, both in industry and in society as a whole. They enable computing devices to learn from previous experience and therefore improve their performance in a certain context or environment. In this way, many useful possibilities have been made accessible. However, dealing with an increasing amount of data poses difficulties for classical devices. Quantum systems may offer a way forward, possibly enabling to scale up machine learning calculations in certain contexts. On the contrary, quantum systems themselves are also hard to scale up, due to decoherence and the fragility of quantum superpositions. In the short and mid term, it has been evidenced that a quantum paradigm that combines evolution under large analog blocks with discrete quantum gates, may be fruitful to achieve new knowledge of classical and quantum systems with no need of having a fault-tolerant quantum computer. In this perspective, we review some recent works that employ this digital-analog quantum paradigm to carry out efficient machine learning calculations with current quantum devices.},

keywords = {US},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Multipartite entanglement in the diagonal symmetric subspace},

author = {Romero-Pallejà, J. and Ahiable, J. and Marconi, C. and Sanpera, A. },

url = {https://pubs.aip.org/aip/jmp/article-abstract/66/2/022203/3335365/Multipartite-entanglement-in-the-diagonal?redirectedFrom=fulltext},

doi = {doi.org/10.1063/5.0240964},

year  = {2025},

date = {2025-02-11},

journal = {Journal of Mathematical Physics},

volume = {66},

pages = {22203},

abstract = {We investigate the entanglement properties in the symmetric subspace of N-partite d-dimensional systems (qudits). As it happens already for bipartite diagonal symmetric states, also in the multipartite case the local dimension d plays a crucial role. Here, we demonstrate that there is no bound entanglement for d = 3, 4 and N = 3. Using different techniques, we present strong analytical evidence that no bound entanglement exist for any N if d ≤ 4. Interestingly, bound entanglement of diagonal symmetric states exist for any number of parties, N ≥ 2, and local dimensions d ≥ 5.},

keywords = {UAB},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Quantum Bayesian Inference with Renormalization for Gravitational Waves},

author = {Escrig, G. and Campos, R. and Qi, H. and Martin-Delgado, M.A. },

url = {https://iopscience.iop.org/article/10.3847/2041-8213/ada6ae},

doi = {10.3847/2041-8213/ada6ae},

year  = {2025},

date = {2025-01-28},

journal = {The Astrophysical Journal Letters},

volume = {979},

issue = {2},

pages = {L36},

abstract = {Advancements in gravitational-wave (GW) interferometers, particularly the next generation, are poised to enable the detections of orders of magnitude more GWs from compact binary coalescences. While the surge in detections will profoundly advance GW astronomy and multimessenger astrophysics, it also poses significant computational challenges in parameter estimation. In this work, we introduce a hybrid quantum algorithm qBIRD, which performs quantum Bayesian inference with renormalization and downsampling to infer GW parameters. We validate the algorithm using both simulated and observed GWs from binary black hole mergers on quantum simulators, demonstrating that its accuracy is comparable to classical Markov Chain Monte Carlo methods. Currently, our analyses focus on a subset of parameters, including chirp mass and mass ratio, due to the limitations from classical hardware in simulating quantum algorithms. However, qBIRD can accommodate a broader parameter space when the constraints are eliminated with a small-scale quantum computer of sufficient logical qubits.},

keywords = {UCM-4.3},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Digital-analog quantum convolutional neural networks for image classification},

author = {Simen, A. and Flores-Garrigos, C. and Hegade, N.N. and Montalban, I. and Vives-Gilabert, Y. and Michon, E. and Zhang, Q. and Solano, E. and Martín-Guerrero, J.D. 

2,3,§},

url = {https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.6.L042060},

doi = {doi.org/10.1103/PhysRevResearch.6.L042060},

year  = {2024},

date = {2024-12-11},

journal = {Physical Review Research},

volume = {6},

issue = {4},

pages = {2060},

abstract = {We propose digital-analog quantum kernels for enhancing the detection of complex features in the classification of images. We consider multipartite-entangled analog blocks, stemming from native Ising interactions in neutral-atom quantum processors, and individual operations as digital steps to implement the protocol. To further improve the detection of complex features, we apply multiple quantum kernels by varying the qubit connectivity according to the hardware constraints. An architecture that combines nontrainable quantum kernels and standard convolutional neural networks is used to classify realistic medical images, from breast cancer and pneumonia diseases, with a significantly reduced number of parameters. Despite this fact, the model exhibits better performance than its classical counterparts and achieves comparable metrics according to public benchmarks. These findings highlight the potential of digital-analog quantum convolutions in extracting complex and meaningful features from images, positioning them as a candidate model for addressing challenging classification problems.},

keywords = {UV},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Quantum search algorithm on weighted databases},

author = {Sun, Y. and Wu, L.A. },

url = {https://www.nature.com/articles/s41598-024-81701-7},

doi = {doi.org/10.1038/s41598-024-81701-7},

year  = {2024},

date = {2024-12-04},

urldate = {2024-12-03},

abstract = {The Grover algorithm is a crucial solution for addressing unstructured search problems and has emerged as an essential quantum subroutine in various complex algorithms. By using a different approach with previous studies, this research extensively investigates Grover’s search methodology within non-uniformly distributed databases, a scenario frequently encountered in practical applications. Our analysis reveals that the behavior of the Grover evolution differs significantly when applied to non-uniform databases compared to uniform or ‘unstructured databases’. Based on the property of differential equation, it is observed that the search process facilitated by this evolution does not consistently result in a speed-up, and we have identified specific criteria for such situations. Furthermore, we have extended this investigation to databases characterized by coherent states, confirming the speed-up achieved through Grover evolution via rigorous numerical verification. In conclusion, our study provides an enhancement to the original Grover algorithm, offering insights to optimize implementation strategies and broaden its range of applications.},

keywords = {UPV/EHU},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Randomness-free detection of non-projective measurements: qubits & beyond},

author = {Rout, S. and Sankar Bhattacharya, S. and Horodecki, P.},

url = {https://iopscience.iop.org/article/10.1088/1367-2630/adc0b4},

doi = {10.1088/1367-2630/adc0b4},

year  = {2024},

date = {2024-11-29},

urldate = {2024-11-29},

abstract = {Non-projective measurements play a crucial role in various information-processing protocols. In this work, we propose an operational task to identify measurements that are neither projective nor classical post-processing of data obtained from projective measurements. Our setup involves space-like separated parties with access to a shared state with bounded local dimensions. Specifically, in the case of qubits, we focus on a bipartite scenario with different sets of target correlations. While some of these correlations can be obtained through non-projective measurements on a shared two-qubit state, it is impossible to generate these correlations using projective simulable measurements on bipartite qubit states, or equivalently, by using one bit of shared randomness and local post-processing. For certain target correlations, we show that detecting qubit non-projective measurements is robust under arbitrary depolarizing noise, except in the limiting case. We extend this task for qutrits and demonstrate that some correlations achievable via local non-projective measurements cannot be reproduced by both parties performing the same qutrit projective simulable measurements on their pre-shared state. We provide numerical evidence for the robustness of this scheme under arbitrary depolarizing noise. For a more generic consideration (bipartite and tripartite scenario), we provide numerical evidence for a projective-simulable bound on the reward function for our task. We also show a violation of this bound by using qutrit positive operator valued measures. From a foundational perspective, we extend the notion of non-projective measurements to general probabilistic theories (GPTs) and use a randomness-free test to demonstrate that a class of GPTs, called square-bits or box-world are unphysical.},

keywords = {UAB},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Role of coherence in many-body Quantum Reservoir Computing},

author = {Palacios, A. and Martínez-Pena, R. and Soriano, M.C. and Giorgi, G.L. and Zambrini, R. },

url = {https://www.nature.com/articles/s42005-024-01859-4},

doi = {doi.org/10.1038/s42005-024-01859-4},

year  = {2024},

date = {2024-11-14},

urldate = {2024-11-14},

journal = {Communications Physics},

volume = {7},

number = {369},

abstract = {Quantum Reservoir Computing (QRC) offers potential advantages over classical reservoir computing, including inherent processing of quantum inputs and a vast Hilbert space for state exploration. Yet, the relation between the performance of reservoirs based on complex and many-body quantum systems and non-classical state features is not established. Through an extensive analysis of QRC based on a transverse-field Ising model we show how different quantum effects, such as quantum

coherence and correlations, contribute to improving the performance in temporal tasks, as measured by the Information Processing Capacity. Additionally, we critically assess the impact of finite measurement resources and noise on the reservoir’s dynamics in different regimes, quantifying the limited ability to exploit quantum effects for increasing damping and noise strengths. Our results reveal a monotonic relationship between reservoir performance and coherence, along with the importance of quantum effects in the ergodic regime.},

keywords = {UIB},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Exploring ground states of Fermi-Hubbard model on honeycomb lattices with counterdiabaticity},

author = {Tang, J. and Xu, R. and Ding, Y. and Xu, X. and Ban, Y. and Yung, M.H. and Pérez-Obiol, A. and Platero, G. Chen, Xi.},

url = {https://www.nature.com/articles/s41535-024-00697-5},

doi = {doi.org/10.1038/s41535-024-00697-5},

year  = {2024},

date = {2024-11-07},

journal = {Quantum Materials },

volume = {9},

number = {87},

abstract = {Exploring the ground state properties of many-body quantum systems conventionally involves adiabatic processes, alongside exact diagonalization, in the context of quantum annealing or adiabatic quantum computation. Shortcuts to adiabaticity by counter-diabatic driving serve to accelerate these processes by suppressing energy excitations. Motivated by this, we develop variational quantum algorithms incorporating the auxiliary counter-diabatic interactions, comparing them with digitized adiabatic algorithms. These algorithms are then implemented on gate-based quantum circuits to explore the ground states of the Fermi-Hubbard model on honeycomb lattices, utilizing systems with up to 26 qubits. The comparison reveals that the counter-diabatic inspired ansatz is superior to traditional Hamiltonian variational ansatz. Furthermore, the number and duration of Trotter steps are analyzed to understand and mitigate errors. Given the model’s relevance to materials in condensed matter, our study paves the way for using variational quantum algorithms with counterdiabaticity to explore quantum materials in the noisy intermediate-scale quantum era.},

keywords = {UPV/EHU},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Quantum and classical dynamics correspondence and the brachistochrone problem},

author = {Fanchiotti, H. and García-Canal, C.A. and Mayosky, M. and Pérez, A. and Veiga, A. },

url = {https://journals.aps.org/pra/abstract/10.1103/PhysRevA.110.042219},

doi = {doi.org/10.1103/PhysRevA.110.042219},

issn = {2469-9926 },

year  = {2024},

date = {2024-10-22},

journal = {Physical Review A},

volume = {110},

issue = {4},

abstract = {The decomplexification procedure, which allows showing mathematically the isomorphism between classical and quantum dynamics of systems with a finite number of basis states, is exploited to propose resonant electric circuits with gyrator-based couplings and to experimentally study the quantum brachistochrone problem, particularly the passage time in Hermitian and parity-time-symmetric cases.},

keywords = {UV},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Equivalence of constacyclic codes with shift constants of different orders},

author = {Dastbasteh, R. and Padashnick, F. and Crespo, P. and Grassl, M. and Sharafi, J. 

},

url = {https://link.springer.com/article/10.1007/s10623-024-01512-9},

doi = {doi.org/10.48550/arXiv.2403.04600},

year  = {2024},

date = {2024-10-18},

urldate = {2025-10-18},

journal = {Designs, Codes and Cryptography},

abstract = {Let a and b be two non-zero elements of a finite field F_q, where q > 2. It has been shown that if a and b have the same multiplicative order in F_q, then the families of a-constacyclic and b-constacyclic codes over F_q are monomially equivalent. In this paper, we investigate the monomial equivalence of a-constacyclic and b-constacyclic codes when a and b have distinct multiplicative orders. We present novel conditions for establishing monomial equivalence in such constacyclic codes, surpassing previous methods of determining monomially equivalent constacyclic and cyclic codes. As an application, we use these results to search for new linear codes more systematically. In particular, we present more than 70 new record-breaking linear codes over various finite fields, as well as new binary quantum codes.},

keywords = {TECNUN},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Dirac quantum walk on tetrahedra},

author = {Ugo Nzongani, U. and Eon, N. and Márquez-Martín, I. and Pérez, A. and Di Molfetta, G. and Arrighi, P.},

url = {https://journals.aps.org/pra/abstract/10.1103/PhysRevA.110.042418},

doi = {doi.org/10.1103/PhysRevA.110.042418},

issn = {2469-9926 },

year  = {2024},

date = {2024-10-16},

journal = {Physical Review A},

volume = {110},

issue = {4},

abstract = {Discrete-time quantum walks (QWs) are transportation models of single quantum particles over a lattice. Their evolution is driven through causal and local unitary operators. QWs are a powerful tool for quantum simulation of fundamental physics, as some of them have a continuum limit converging to well-known physics partial differential equations, such as the Dirac or the Schrödinger equation. In this paper, we show how to recover the Dirac equation in (3+1) dimensions with a QW evolving in a tetrahedral space. This paves the way to simulate the Dirac equation on a curved space-time. This also suggests an ordered scheme for propagating matter over a spin network, of interest in loop quantum gravity, where matter propagation has remained an open problem.},

keywords = {UV},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Robust generation of N-partite N-level singlet states by identical particle interferometry},

author = {Piccolini, M. and Karczewski, M. and Winter, A. and Lo Franco, R. },

url = {https://iopscience.iop.org/article/10.1088/2058-9565/ad8214},

doi = {10.1088/2058-9565/ad8214},

year  = {2024},

date = {2024-10-15},

journal = {Quantum Science and Technology},

volume = {10},

number = {15013 },

issue = {1},

abstract = {We propose an interferometric scheme for generating the totally antisymmetric state of N identical bosons with N internal levels (generalized singlet). This state is a resource for various problems with dramatic quantum advantage. The procedure uses a sequence of Fourier multi-ports, combined with coincidence measurements filtering the results. Successful preparation of the generalized singlet is confirmed when the N particles of the input state stay separate (anti-bunch) on each multiport. The scheme is robust to local lossless noise and works even with a totally mixed input state.},

keywords = {UAB},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Decoding algorithms for surface codes},

author = {deMarti iOlius, A. and Fuentes, P. and Orús, R. and Crespo, P. and Etxezarreta Martinez, J. },

url = {https://quantum-journal.org/papers/q-2024-10-10-1498/pdf/},

doi = {doi.org/10.22331/q-2024-10-10-1498},

isbn = {2521-327X},

year  = {2024},

date = {2024-10-10},

journal = {Quantum},

volume = {8},

pages = {1498},

abstract = {Quantum technologies have the potential to solve certain computationally hard problems with polynomial or super-polynomial speedups when compared to classical methods. Unfortunately, the unstable nature of quantum information makes it prone to errors. For this reason, quantum error correction is an invaluable tool to make quantum information reliable and enable the ultimate goal of fault-tolerant quantum computing. Surface codes currently stand as the most promising candidates to build near term error corrected qubits given their two-dimensional architecture, the requirement of only local operations, and high tolerance to quantum noise. Decoding algorithms are an integral component of any error correction scheme, as they are tasked with producing accurate estimates of the errors that affect quantum information, so that they can subsequently be corrected. A critical aspect of decoding algorithms is their speed, since the quantum state will suffer additional errors with the passage of time. This poses a connundrum, where decoding performance is improved at the expense of complexity and viceversa. In this review, a thorough discussion of state-of-the-art decoding algorithms for surface codes is provided. The target audience of this work are both readers with an introductory understanding of the field as well as those seeking to further their knowledge of the decoding paradigm of surface codes. We describe the core principles of these decoding methods as well as existing variants that show promise for improved results. In addition, both the decoding performance, in terms of error correction capability, and decoding complexity, are compared. A review of the existing software tools regarding surface codes decoding is also provided.},

keywords = {TECNUN},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Digital-analog quantum genetic algorithm using Rydberg-atom arrays},

author = {Lamata, L. and Llenas, A. },

url = {https://arxiv.org/pdf/2407.09308},

doi = {doi.org/10.1103/PhysRevA.110.042603},

year  = {2024},

date = {2024-10-03},

urldate = {2024-10-03},

journal = {Physical Review A},

volume = {110},

issue = {4},

abstract = {Digital-analog quantum computing (DAQC) combines digital gates with analog operations, offering an alternative paradigm for universal quantum computation. This approach leverages the higher fidelities of analog operations and the flexibility of local single-qubit gates. In this paper, we propose a quantum genetic algorithm within the DAQC framework using a Rydberg-atom emulator. The algorithm employs single-qubit operations in the digital domain and a global driving interaction based on the Rydberg Hamiltonian in the analog domain. We evaluate the algorithm performance by estimating the ground-state energy of Hamiltonians, with a focus on molecules such as H2, LiH, and BeH2. Our results show energy estimations with less than 1% error and state overlaps nearing 1, with computation times ranging from a few minutes for H2 (2-qubit circuits) to one to two days for LiH and BeH2 (6-qubit circuits). The gate fidelities of global analog operations further underscore DAQC as a promising quantum computing strategy in the noisy intermediate-scale quantum era.},

keywords = {US},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Photonic counterdiabatic quantum optimization algorithm},

author = {Pranav, C. and Koushik, P. and Garcia-de-Andoin, M. and Ban, Y. and Sanz, M. and Chen, X.},

url = {https://www.nature.com/articles/s42005-024-01807-2.pdf},

doi = {doi.org/10.1038/s42005-024-01807-2},

isbn = {2399-3650},

year  = {2024},

date = {2024-09-30},

journal = {Communications Physics},

number = {315},

abstract = {One of the key applications of near-term quantum computers has been the development of quantum optimization algorithms. However, these algorithms have largely been focused on qubit-based technologies. Here, we propose a hybrid quantum-classical approximate optimization algorithm for photonic quantum computing, specifically tailored for addressing continuous-variable optimization problems. Inspired by counterdiabatic protocols, our algorithm reduces the required quantum operations for optimization compared to adiabatic protocols. This reduction enables us to tackle non-convex continuous optimization within the near-term era of quantum computing. Through illustrative benchmarking, we show that our approach can outperform existing state-of-the-art hybrid adiabatic quantum algorithms in terms of convergence and implementability. Our algorithm offers a practical and accessible experimental realization, bypassing the need for high-order operations and overcoming experimental constraints. We conduct a proof-of-principle demonstration on Xanadu’s eight-mode nanophotonic quantum chip, successfully showcasing the feasibility and potential impact of the algorithm.},

keywords = {UPV/EHU},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Identification of malfunctioning quantum devices},

author = {Skotiniotis, M. and Llorens, S. and Hotz, R. and Calsamiglia, J. Muñoz-Tapia, R. },

url = {https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.6.033329},

doi = {doi.org/10.1103/PhysRevResearch.6.033329},

year  = {2024},

date = {2024-09-23},

urldate = {2025-09-23},

journal = {Physical Review Research},

volume = {6},

abstract = {We consider the problem of correctly identifying a malfunctioning quantum device that forms part of a network of 𝑁 such devices, which can be considered as the quantum analog of classical anomaly detection. In the case where the devices in question are sources assumed to prepare identical quantum pure states, with the faulty source producing a different anomalous pure state, we show that the optimal probability of successful identification requires a global quantum measurement. We also put forth several local measurement strategies—both adaptive and nonadaptive—that achieve the same optimal probability of success in the limit where the number of devices to be checked is large. In the case where the faulty device performs a known unitary operation, we show that the use of entangled probes provides an improvement that even allows perfect identification for values of the unitary parameter that surpass a certain threshold. Finally, if the faulty device implements a known qubit channel, we find that the optimal probability for detecting the position of rank-one and rank-two Pauli channels can be achieved by product state inputs and separable measurements for any size of network, whereas for rank-three and general amplitude damping channels, optimal identification requires entanglement with 𝑁 qubit ancillas.},

keywords = {UAB},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Engineered dissipation to mitigate barren plateaus},

author = { Sannia, A. and Tacchino, F. and Tavernelli, I. Giorgi, G.L. and Zambrini, R.},

url = {https://www.nature.com/articles/s41534-024-00875-0#citeas},

doi = {doi.org/10.1038/s41534-024-00875-0},

year  = {2024},

date = {2024-09-04},

journal = {NPJ/Quantum Information},

volume = {10},

issue = {81},

abstract = {Variational quantum algorithms represent a powerful approach for solving optimization problems on noisy quantum computers, with a broad spectrum of potential applications ranging from chemistry to machine learning. However, their performances in practical implementations crucially depend on the effectiveness of quantum circuit training, which can be severely limited by phenomena such as barren plateaus. While, in general, dissipation is detrimental for quantum algorithms, and noise itself can actually induce barren plateaus, here we describe how the inclusion of properly engineered Markovian losses after each unitary quantum circuit layer allows for the trainability of quantum models. We identify the required form of the dissipation processes and establish that their optimization is efficient. We benchmark the generality of our proposal in both a synthetic and a practical quantum chemistry example, demonstrating its effectiveness and potential impact across different domains.},

keywords = {CSIC-4.7},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Quantum multi-anomaly detection},

author = {Llorens, S. and Sentís, G. and Muñoz-Tapia, R.},

url = {https://quantum-journal.org/papers/q-2024-08-28-1452/},

doi = {doi.org/10.22331/q-2024-08-28-1452},

year  = {2024},

date = {2024-08-28},

urldate = {2024-08-28},

journal = {Quantum},

volume = {8},

pages = {1452},

abstract = {A source assumed to prepare a specified reference state sometimes prepares an anomalous one. We address the task of identifying these anomalous states in a series of 

n preparations with k anomalies. We analyze the minimum-error protocol and the zero-error (unambiguous) protocol and obtain closed expressions for the success probability when both reference and anomalous states are known to the observer and anomalies can appear equally likely in any position of the preparation series. We find the solution using results from association schemes theory, thus establishing a connection between graph theory and quantum hypothesis testing. In particular, we use the Johnson association scheme which arises naturally from the Gram matrix of this problem. We also study the regime of large n and obtain the expression of the success probability that is non-vanishing. Finally, we address the case in which the observer is blind to the reference and the anomalous states. This scenario requires a universal protocol for which we prove that in the asymptotic limit, the success probability corresponds to the average of the known state scenario.},

keywords = {UAB},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Noise-Aware Variational Eigensolvers: A Dissipative Route for Lattice Gauge Theories},

author = {Cobos, J. and Locher, D.F. and Bermudez, A. and Müller, M. and Rico, E. },

url = {https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.5.030340},

doi = {doi.org/10.1103/PRXQuantum.5.030340},

issn = {2691-3399},

year  = {2024},

date = {2024-08-26},

urldate = {2024-08-26},

journal = {PRX Quantum},

volume = {5},

issue = {3},

pages = {030340},

abstract = {We propose a novel variational ansatz for the ground-state preparation of the lattice gauge theory (LGT) in quantum simulators. It combines dissipative and unitary operations in a completely deterministic scheme with a circuit depth that does not scale with the size of the considered lattice. We find that, with very few variational parameters, the ansatz can achieve precision in energy in both the confined and deconfined phase of the LGT. We benchmark our proposal against the unitary Hamiltonian variational ansatz showing a reduction in the required number of variational layers to achieve a target precision. After performing a finite-size scaling analysis, we show that our dissipative variational ansatz can predict accurate critical exponents without requiring a number of layers that scales with the system size, which is the standard situation for unitary ansätze. Furthermore, we investigate the performance of this variational eigensolver subject to circuit-level noise, determining variational error thresholds that fix the error rate below which it would be beneficial to increase the number of layers. In light of these quantities and for typical gate errors in current quantum processors, we provide a detailed assessment of the prospects of our scheme to explore the LGT on near-term devices.},

keywords = {CSIC-4.8},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Reducing the number of qubits in quantum simulations of one dimensional many-body Hamiltonians},

author = {Mehrabankar, S. and García-March, M.A. and Almudéver, C.G. and Pérez, A. },

url = {https://iopscience.iop.org/article/10.1088/1367-2630/ad6d84},

doi = {10.1088/1367-2630/ad6d84},

year  = {2024},

date = {2024-08-22},

urldate = {2024-08-22},

journal = {New Journal of Physics },

abstract = {We investigate the Ising and Heisenberg models using the block renormalization group method (BRGM), focusing on its behavior across different system sizes. The BRGM reduces the number of spins by a factor of 1/2 (1/3) for the Ising (Heisenberg) model, effectively preserving essential physical features of the model while using only a fraction of the spins. Through a comparative analysis, we demonstrate that as the system size increases, there is an exponential convergence between results obtained from the original and renormalized Ising Hamiltonians, provided the coupling constants are redefined accordingly. Remarkably, for a spin chain with 24 spins, all physical features, including magnetization, correlation function, and entanglement entropy, exhibit an exact correspondence with the results from the original Hamiltonian. The study of the Heisenberg model also shows this tendency, although complete convergence may appear for a size much larger than 24 spins, and is therefore beyond our computational capabilities. The success of BRGM in accurately characterizing the Ising model, even with a relatively small number of spins, underscores its robustness and utility in studying complex physical systems, and facilitates its simulation on current NISQ computers, where the available number of qubits is largely constrained.



},

keywords = {UV},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Security of quantum position-verification limits Hamiltonian simulation via holography},

url = {https://link.springer.com/article/10.1007/JHEP08(2024)152},

doi = {doi.org/10.1007/JHEP08(2024)152},

year  = {2024},

date = {2024-08-20},

journal = {Journal of High Energy Physics},

volume = {2024},

number = {152},

issue = {8},

abstract = {We investigate the link between quantum position-verification (QPV) and holography established in [1] using holographic quantum error correcting codes as toy models. By inserting the “temporal” scaling of the AdS metric by hand via the bulk Hamiltonian interaction strength, we recover a toy model with consistent causality structure. This leads to an interesting implication between two topics in quantum information: if position-based verification is secure against attacks with small entanglement then there are new fundamental lower bounds for resources required for one Hamiltonian to simulate another.},

keywords = {UCM-4.2},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Robust two-qubit gate with reinforcement learning and dropout},

author = {Xu, T.N. and Ding, Y. and Martín-Guerrero, J.D. and Chen, X.},

url = {https://journals.aps.org/pra/abstract/10.1103/PhysRevA.110.032614},

doi = {doi.org/10.1103/PhysRevA.110.032614},

year  = {2024},

date = {2024-08-13},

urldate = {2024-08-13},

journal = {Physical Review A},

volume = {110},

issue = {3},

abstract = {In the realm of quantum control, reinforcement learning, a prominent branch of machine learning, emerges as a competitive candidate for computer-assisted optimal experiment design. This paper investigates the extent to which guidance from human experts is necessary for effectively implementing reinforcement learning in the design of quantum control protocols. Specifically, our focus lies on engineering a robust two-qubit gate, utilizing a combination of analytical solutions as prior knowledge and techniques from computer science. Through thorough benchmarking of various models, we identify dropout—a widely used method for mitigating overfitting in machine learning—as a particularly robust approach. Our findings demonstrate the potential of integrating computer science concepts to propel the development of advanced quantum technologies.},

howpublished = {Preprint},

keywords = {UV},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Nonequilibrium transition between dissipative time crystals},

author = {Cabot, A. and Giorgi, G.L. and Zambrini, R.},

url = {https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.5.030325},

doi = {doi.org/10.1103/PRXQuantum.5.030325},

year  = {2024},

date = {2024-08-06},

journal = {PRX Quantum},

volume = {5},

number = {30325},

issue = {3},

abstract = {We show a dissipative phase transition in a driven nonlinear quantum oscillator in which a discrete time-translation symmetry is spontaneously broken in two different ways. The corresponding regimes display either discrete or incommensurate time-crystal order, which we analyze numerically and analytically beyond the classical limit, addressing observable dynamics, phenomenology in different (laboratory and rotating) frames, Liouvillian spectral features, and quantum fluctuations. Via an effective semiclassical description, we show that phase diffusion dominates in the incommensurate time crystal (or continuous time crystal in the rotating frame), which manifests as a band of eigenmodes with a lifetime growing linearly with the mean-field excitation number. Instead, in the discrete time-crystal phase, the leading fluctuation process corresponds to quantum activation with a single mode that has an exponentially growing lifetime. Interestingly, the transition between these two regimes manifests itself already in the quantum regime as a spectral singularity, namely, as an exceptional point mediating between phase diffusion and quantum activation. Finally, we discuss this transition between different time-crystal orders in the context of synchronization phenomena.},

keywords = {CSIC-4.7},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Retrieving past quantum features with deep hybrid classical-quantum reservoir computing},

author = {Nokkala, J. and Giorgi, G.L. and Zambrini, R. },

url = {https://iopscience.iop.org/article/10.1088/2632-2153/ad5f12},

doi = {10.1088/2632-2153/ad5f12},

year  = {2024},

date = {2024-07-19},

journal = {Machine Learning: Science and Technology},

volume = {5},

abstract = {Machine learning techniques have achieved impressive results in recent years and the possibility of harnessing the power of quantum physics opens new promising avenues to speed up classical learning methods. Rather than viewing classical and quantum approaches as exclusive alternatives, their integration into hybrid designs has gathered increasing interest, as seen in variational quantum algorithms, quantum circuit learning, and kernel methods. Here we introduce deep hybrid classical-quantum reservoir computing for temporal processing of quantum states where information about, for instance, the entanglement or the purity of past input states can be extracted via a single-step measurement. We find that the hybrid setup cascading two reservoirs not only inherits the strengths of both of its constituents but is even more than just the sum of its parts, outperforming comparable non-hybrid alternatives. The quantum layer is within reach of state-of-the-art multimode quantum optical platforms while the classical layer can be implemented in silico.},

keywords = {CSIC-4.7},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Benchmarking quantum computers: towards a standard performance evaluation approach},

author = {Acuaviva, A. and Aguirre, D. and Peña, R. and Sanz, M. },

url = {https://iopscience.iop.org/article/10.1088/2058-9565/ae4269},

doi = {10.1088/2058-9565/ae4269},

year  = {2024},

date = {2024-07-17},

urldate = {2024-07-17},

journal = {Quantum Science and Technology},

abstract = {The technological development of increasingly larger quantum processors on different quantum platforms raises the problem of how to fairly compare their performance, known as quantum benchmarking of quantum processors. This is a challenge that computer scientists have already faced when comparing classical processors, leading to the development of various mathematical tools to address it, but also to the identification of the limits of this problem. In this work, we briefly review the most important aspects of both classical processor benchmarks and the metrics comprising them, providing precise definitions and analyzing the quality attributes that they should exhibit. Subsequently, we analyze the intrinsic properties that characterize the paradigm of quantum computing and hinder the naive transfer of strategies from classical benchmarking. However, we can still leverage some of the lessons learned such as the quality attributes of a good benchmark such as relevance, reproducibility, fairness, verifiability and usability. Additionally, we review some of the most important metrics and benchmarks for quantum processors proposed in the literature, assessing what quality attributes they fulfill. Finally, we propose general guidelines for quantum benchmarking. These guidelines aim to pave the way for establishing a roadmap towards standardizing the performance evaluation of quantum devices, ultimately leading to the creation of an organization akin to the Standard Performance Evaluation Corporation (SPEC).},

keywords = {UPV/EHU},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Synthetic Z2 gauge theories based on parametric excitations of trapped ions},

author = {Bǎzǎvan, O. and Saner, S. and Tirrito, E. and Araneda, G. and Srinivas, R. and Bermudez, A.},

url = {https://www.nature.com/articles/s42005-024-01691-w#citeas},

doi = {doi.org/10.1038/s42005-024-01691-w},

year  = {2024},

date = {2024-07-12},

urldate = {2025-07-12},

journal = {Communications Physics},

volume = {7},

pages = {229},

abstract = {Resource efficient schemes for the quantum simulation of lattice gauge theories can benefit from hybrid encodings of gauge and matter fields that use the native degrees of freedom, such as internal qubits and motional phonons in trapped-ion devices. We propose to use a parametric scheme to induce a tunneling of the phonons conditioned to the internal qubit state which, when implemented with a single trapped ion, corresponds to a minimal  Z2 gauge theory. To evaluate the feasibility of this scheme, we perform numerical simulations of the state-dependent tunneling using realistic parameters, and identify the leading sources of error in future experiments. We discuss how to generalize this minimal case to more complex settings by increasing the number of ions, moving from a single link to a Z2 plaquette, and to an entire  Z2 chain. We present analytical expressions for the gauge-invariant dynamics and the corresponding confinement, which are benchmarked using matrix product state simulations.},

keywords = {CSIC-4.7},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Hawking-Page transition on a spin chain},

author = {Pérez-García D. and Santilli, L. and Tierz, M 

},

url = {https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.6.033007},

doi = {doi.org/10.1103/PhysRevResearch.6.033007},

year  = {2024},

date = {2024-07-01},

urldate = {2024-07-01},

journal = {Physical Review Research},

volume = {6},

issue = {3},

pages = {033007},

abstract = {The accessibility of the Hawking-Page transition in AdS5 through a one-dimensional (1D) Heisenberg spin chain is demonstrated. We use the random matrix formulation of the Loschmidt echo for a set of spin chains, and randomize the ferromagnetic spin interaction. It is shown that the thermal Loschmidt echo, when averaged, detects the predicted increase in entropy across the Hawking-Page transition. This suggests that a 1D spin chain exhibits characteristics of black hole physics in 4+1 dimensions. We show that this approach is equally applicable to free fermion systems with a general dispersion relation.},

keywords = {UCM-4.3},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Learning Quantum Processes Without Input Control},

url = {https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.5.020367},

doi = {doi.org/10.1103/PRXQuantum.5.020367},

year  = {2024},

date = {2024-06-27},

journal = {PRX Quantum},

volume = {5},

number = {20367},

issue = {2},

abstract = {We introduce a general statistical learning theory for processes that take as input a classical random variable and output a quantum state. Our setting is motivated by the practical situation in which one desires to learn a quantum process governed by classical parameters that are out of one’s control. This framework is applicable, for example, to the study of astronomical phenomena, disordered systems and biological processes not controlled by the observer. We provide an algorithm for learning with high probability in this setting with a finite amount of samples, even if the concept class is infinite. To do this, we review and adapt existing algorithms for shadow tomography and hypothesis selection, and combine their guarantees with the uniform convergence on the data of the loss functions of interest. As a byproduct, we obtain sufficient conditions for performing shadow tomography of classical-quantum states with a number of copies, which depends on the dimension of the quantum register, but not on the dimension of the classical one. We give concrete examples of processes that can be learned in this manner, based on quantum circuits or physically motivated classes, such as systems governed by Hamiltonians with random perturbations or data-dependent phase shifts.},

keywords = {UAB},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Floquet-Rydberg quantum simulator for confinement in gauge theories},

author = {Domanti, E.C. and Zappalà, D. and Bermudez, A. and Amico, L. },

url = {https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.6.L022059},

doi = {doi.org/10.1103/PhysRevResearch.6.L022059},

year  = {2024},

date = {2024-06-11},

journal = {Physical Review Research},

volume = {6},

abstract = {Recent advances in the field of quantum technologies have opened up the road for the realization of small-scale quantum simulators of lattice gauge theories which, among other goals, aim at improving our understanding on the nonperturbative mechanisms underlying the confinement of quarks. In this work, considering periodically driven arrays of Rydberg atoms in a tweezer ladder geometry, we devise a scalable Floquet scheme for the quantum simulation of the real-time dynamics in a LGT, in which hardcore bosons/spinless fermions are coupled to dynamical gauge fields. Resorting to an external magnetic field to tune the angular dependence of the Rydberg dipolar interactions, and by a suitable tuning of the driving parameters, we manage to suppress the main gauge-violating terms and show that an observation of gauge-invariant confinement dynamics in the Floquet-Rydberg setup is at reach of current experimental techniques. Depending on the lattice size, we present a thorough numerical test of the validity of this scheme using either exact diagonalization or matrix-product-state algorithms for the periodically modulated real-time dynamics.},

keywords = {CSIC-4.7},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Quantum Phase Transitions in periodically quenched systems},

author = {Sáiz, Á. and Khalouf‑Rivera, J. and Arias, J. M. and Pérez‑Fernández, P. and Casado‑Pascual, J.},

url = {https://quantum-journal.org/papers/q-2024-06-11-1365/},

doi = {doi.org/10.22331/q-2024-06-11-1365},

year  = {2024},

date = {2024-06-11},

urldate = {2024-06-11},

journal = {Quantum},

volume = {8},

pages = {1365},

abstract = {Quantum phase transitions encompass a variety of phenomena that occur in quantum systems exhibiting several possible symmetries. Traditionally, these transitions are explored by continuously varying a control parameter that connects two different symmetry configurations. Here we propose an alternative approach where the control parameter undergoes abrupt and time-periodic jumps between only two values. This approach yields results surprisingly similar to those obtained by the traditional one and may prove experimentally useful in situations where accessing the control parameter is challenging.},

keywords = {US},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {TensorKrowch: Smooth integration of tensor networks in machine learning},

author = {Pareja Monturiol, J.R. and Pérez-García, D. and Pozas-Kerstjens, A. },

url = {https://quantum-journal.org/papers/q-2024-06-11-1364/},

doi = {doi.org/10.22331/q-2024-06-11-1364},

year  = {2024},

date = {2024-06-11},

journal = {Quantum},

volume = {8},

pages = {1364 },

abstract = {Tensor networks are factorizations of high-dimensional tensors into networks of smaller tensors. They have applications in physics and mathematics, and recently have been proposed as promising machine learning architectures. To ease the integration of tensor networks in machine learning pipelines, we introduce TensorKrowch, an open source Python library built on top of PyTorch. Providing a user-friendly interface, TensorKrowch allows users to construct any tensor network, train it, and integrate it as a layer in more intricate deep learning models. In this paper, we describe the main functionality and basic usage of TensorKrowch, and provide technical details on its building blocks and the optimizations performed to achieve efficient operation.},

keywords = {UCM-4.2},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Invariant measures on p-adic Lie groups: the p-adic quaternion algebra and the Haar integral on the p-adic rotation groups},

author = {Aniello, P. and L’Innocente, S. and Mancini, S. and Parisi, V. and Svampa, I. and Winter, A. },

url = {https://link.springer.com/article/10.1007/s11005-024-01826-8},

doi = {doi.org/10.1007/s11005-024-01826-8},

year  = {2024},

date = {2024-06-06},

urldate = {2024-06-06},

journal = {Letters in Mathematical Physics},

volume = {114},

number = {78},

issue = {3},

abstract = {We provide a general expression of the Haar measure—that is, the essentially unique translation‑invariant measure—on a p‑adic Lie group. We then argue that this measure can be regarded as the measure naturally induced by the invariant volume form on the group, as it happens for a standard Lie group over the reals.



As an important application, we next consider the problem of determining the Haar measure on the p‑adic special orthogonal groups in dimension two, three, and four (for every prime number p). In particular, the Haar measure on SO(2, ℚₚ) is obtained by a direct application of our general formula. As for SO(3, ℚₚ) and SO(4, ℚₚ), instead, we show that Haar integrals on these two groups can conveniently be lifted to Haar integrals on certain p‑adic Lie groups from which the special orthogonal groups are obtained as quotients. This construction involves a suitable quaternion algebra over the field ℚₚ and is reminiscent of the quaternionic realization of the real rotation groups. Our results should pave the way to the development of harmonic analysis on the p‑adic special orthogonal groups, with potential applications in p‑adic quantum mechanics and in the recently proposed p‑adic quantum information theory.},

keywords = {UAB},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {The Bethe Ansatz as a Quantum Circuit},

author = {Ruiz, R. and Sopena, A. and Hunter Gordon, M. and Sierra, G. and López, E.},

url = {https://quantum-journal.org/papers/q-2024-05-23-1356/#},

doi = {doi.org/10.22331/q-2024-05-23-1356},

year  = {2024},

date = {2024-05-23},

journal = {Quantum},

volume = {8},

pages = {1356 },

abstract = {The Bethe ansatz represents an analytical method enabling the exact solution of numerous models in condensed matter physics and statistical mechanics. When a global symmetry is present, the trial wavefunctions of the Bethe ansatz consist of plane wave superpositions. Previously, it has been shown that the Bethe ansatz can be recast as a deterministic quantum circuit. An analytical derivation of the quantum gates that form the circuit was lacking however. Here we present a comprehensive study of the transformation that brings the Bethe ansatz into a quantum circuit, which leads us to determine the analytical expression of the circuit gates. As a crucial step of the derivation, we present a simple set of diagrammatic rules that define a novel Matrix Product State network building Bethe wavefunctions. Remarkably, this provides a new perspective on the equivalence between the coordinate and algebraic versions of the Bethe ansatz.},

keywords = {CSIC-4.7},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Physics-Informed Neural Networks for an optimal counterdiabatic quantum computation},

author = {Ferrer-Sánchez, A. and Flores-Garrigós, C. and Hernani-Morales, C. and Orquín-Marqués, J.J. and Hegade, N.N. and Cadavid, A.G., Montalban, I. and Solano, E. and Vives-Gilabert, Y. and Martín-Guerrero, J.D. },

url = {https://iopscience.iop.org/article/10.1088/2632-2153/ad450f},

doi = {10.1088/2632-2153/ad450f},

year  = {2024},

date = {2024-05-09},

urldate = {2023-09-08},

journal = {Machine Learning: Science and Technology},

volume = {5},

abstract = {We introduce a novel methodology that leverages the strength of Physics-Informed Neural Networks (PINNs) to address the counterdiabatic (CD) protocol in the optimization of quantum circuits comprised of systems with NQ qubits. The primary objective is to utilize physics-inspired deep learning techniques to accurately solve the time evolution of the different physical observables within the quantum system. To accomplish this objective, we embed the necessary physical information into an underlying neural network to effectively tackle the problem. In particular, we impose the hermiticity condition on all physical observables and make use of the principle of least action, guaranteeing the acquisition of the most appropriate counterdiabatic terms based on the underlying physics. The proposed approach offers a dependable alternative to address the CD driving problem, free from the constraints typically encountered in previous methodologies relying on classical numerical approximations. Our method provides a general framework to obtain optimal results from the physical observables relevant to the problem, including the external parameterization in time known as scheduling function, the gauge potential or operator involving the non-adiabatic terms, as well as the temporal evolution of the energy levels of the system, among others. The main applications of this methodology have been the H2 and LiH molecules, represented by a 2-qubit and 4-qubit systems employing the STO-3G basis. The presented results demonstrate the successful derivation of a desirable decomposition for the non-adiabatic terms, achieved through a linear combination utilizing Pauli operators. This attribute confers significant advantages to its practical implementation within quantum computing algorithms.},

keywords = {UV},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Nuclear Physics in the Era of Quantum Computing and Quantum Machine Learning},

author = {García-Ramos, J.E. and Sáiz, A. and Arias, J.M. and Lamata, L. and Pérez Fernández, P. },

url = {https://advanced.onlinelibrary.wiley.com/doi/10.1002/qute.202300219},

doi = {doi.org/10.1002/qute.202300219},

year  = {2024},

date = {2024-05-03},

journal = {Advanced Quantum Technologies},

volume = {6},

issue = {1},

abstract = {In this paper, the application of quantum simulations and quantum machine learning is explored to solve problems in low-energy nuclear physics. The use of quantum computing to address nuclear physics problems is still in its infancy, and particularly, the application of quantum machine learning (QML) in the realm of low-energy nuclear physics is almost nonexistent. Three specific examples are presented where the utilization of quantum computing and QML provides, or can potentially provide in the future, a computational advantage: i) determining the phase/shape in schematic nuclear models, ii) calculating the ground state energy of a nuclear shell model-type Hamiltonian, and iii) identifying particles or determining trajectories in nuclear physics experiments.},

keywords = {US},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Gradient-annihilated PINNs for solving Riemann problems: Application to relativistic hydrodynamics},

author = {Ferrer-Sánchez, A. and Martín-Guerrero, J.D. Ruiz de Austri-Bazan, R. and Torres-Forné, A. and Font, J.A. },

url = {https://www.sciencedirect.com/science/article/pii/S0045782524001622?via%3Dihub},

doi = {doi.org/10.1016/j.cma.2024.116906},

year  = {2024},

date = {2024-05-01},

journal = {Computer Methods in Applied Mechanics and Engineering},

volume = {424},

abstract = {We present a novel methodology based on Physics-Informed Neural Networks (PINNs) for solving systems of partial differential equations admitting discontinuous solutions. Our method, called Gradient-Annihilated PINNs (GA-PINNs), introduces a modified loss function that forces the model to partially ignore high-gradients in the physical variables, achieved by introducing a suitable weighting function. The method relies on a set of hyperparameters that control how gradients are treated in the physical loss. The performance of our methodology is demonstrated by solving Riemann problems in special relativistic hydrodynamics, extending earlier studies with PINNs in the context of the classical Euler equations. The solutions obtained with the GA-PINN model correctly describe the propagation speeds of discontinuities and sharply capture the associated jumps. We use the relative 𝓁² error to compare our results with the exact solution of special relativistic Riemann problems, used as the reference “ground truth”, and with the corresponding error obtained with a second-order, central, shock-capturing scheme. In all problems investigated, the accuracy reached by the GA-PINN model is comparable to that obtained with a shock-capturing scheme, achieving a performance superior to that of the baseline PINN algorithm in general. An additional benefit worth stressing is that our PINN-based approach sidesteps the costly recovery of the primitive variables from the state vector of conserved variables, a well-known drawback of grid-based solutions of the relativistic hydrodynamics equations. Due to its inherent generality and its ability to handle steep gradients, the GA-PINN methodology discussed in this paper could be a valuable tool to model relativistic flows in astrophysics and particle physics, characterized by the prevalence of discontinuous solutions.},

keywords = {UV},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Bounding the Joint Numerical Range of Pauli Strings by Graph Parameters},

author = {Xu, Z.P. and Schwonnek, R. and Winter, A.

},

url = {https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.5.020318},

doi = {doi.org/10.1103/PRXQuantum.5.020318},

year  = {2024},

date = {2024-04-22},

urldate = {2024-04-22},

journal = {PRX Quantum},

volume = {5},

number = {20318},

issue = {2},

abstract = {The relations among a given set of observables on a quantum system are effectively captured by their so-called joint numerical range, which is the set of tuples of jointly attainable expectation values. Here we explore geometric properties of this construct for Pauli strings, whose pairwise commutation and anticommutation relations determine a graph 𝐺. We investigate the connection between the parameters of this graph and the structure of minimal ellipsoids encompassing the joint numerical range, and we develop this approach in different directions. As a consequence, we find counterexamples to a conjecture by de Gois et al. [Phys. Rev. A 107, 062211 (2023)], and answer an open question raised by Hastings and O’Donnell [STOC 2022: Proceedings of the 54th Annual ACM SIGACT Symposium on Theory of Computing, pp. 776–789], which implies a new graph parameter that we call “𝛽⁡(𝐺).” Furthermore, we provide new insights into the perennial problem of estimating the ground-state energy of a many-body Hamiltonian. Our methods give lower bounds on the ground-state energy, which are typically hard to come by, and might therefore be useful in a variety of related fields.},

keywords = {UAB},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Satellite-based entanglement distribution and quantum teleportation with continuous variables},

author = {Gonzalez-Raya, T. and Pirandola, S. and Sanz, M. },

url = {https://www.nature.com/articles/s42005-024-01612-x.pdf},

doi = {doi.org/10.1038/s42005-024-01612-x},

isbn = {2399-3650},

year  = {2024},

date = {2024-04-11},

journal = {Communications Physics},

volume = {7},

number = {126},

abstract = {Advances in satellite quantum communications aim at reshaping the global telecommunication network by increasing the security of the transferred information. Here, we study the effects of atmospheric turbulence in continuous-variable entanglement distribution and quantum teleportation in the optical regime between a ground station and a satellite. More specifically, we study the degradation of entanglement due to various error sources in the distribution, namely, diffraction, atmospheric attenuation, turbulence, and detector inefficiency, in both downlink and uplink scenarios. As the fidelity of a quantum teleportation protocol using these distributed entangled resources is not sufficient, we include an intermediate station for either state generation, or beam refocusing, in order to reduce the effects of atmospheric turbulence and diffraction, respectively. The results show the feasibility of free-space entanglement distribution and quantum teleportation in downlink paths up to the LEO region, but also in uplink paths with the help of the intermediate station. Finally, we complete the study with microwave-optical comparison in bad weather situations, and with the study of horizontal paths in ground-to-ground and inter-satellite quantum communication.},

keywords = {UPV/EHU},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Quantum fidelity kernel with a trapped-ion simulation platform},

author = {Martínez-Peña, R. and Soriano, M.C. and Zambrini, R. 

},

url = {https://journals.aps.org/pra/abstract/10.1103/PhysRevA.109.042612},

doi = {10.1103/PhysRevA.109.042612},

issn = {2469-9926},

year  = {2024},

date = {2024-04-08},

urldate = {2024-04-08},

journal = {Physical Review A},

volume = {109},

issue = {4},

abstract = {Quantum kernel methods leverage a kernel function computed by embedding input information into the Hilbert space of a quantum system. However, large Hilbert spaces can hinder generalization capability, and the scalability of quantum kernels becomes an issue. To overcome these challenges, various strategies under the concept of inductive bias have been proposed. Bandwidth optimization is a promising approach that can be implemented using quantum simulation platforms. We propose trapped-ion simulation platforms as a means to compute quantum kernels, filling a gap in the previous literature and demonstrating their effectiveness for binary classification tasks. We compare the performance of the proposed method with an optimized classical kernel and evaluate the robustness of the quantum kernel against noise. The results show that ion trap platforms are well-suited for quantum kernel computation and can achieve high accuracy with only a few qubits.},

keywords = {CSIC-4.7},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Topological obstructions to quantum computation with unitary oracles},

author = {Skotiniotis, M. and Llorens, S. and Calsamiglia, J. and Muñoz-Tapia, R. },

url = {https://journals.aps.org/pra/abstract/10.1103/PhysRevA.109.032625},

doi = {doi.org/10.1103/PhysRevA.109.032625},

year  = {2024},

date = {2024-03-28},

urldate = {2024-03-28},

journal = {Physical Review Research},

volume = {9},

issue = {3},

pages = {32625},

abstract = {Algorithms with unitary oracles can be nested, which makes them extremely versatile. An example is the phase estimation algorithm used in many candidate algorithms for quantum speedup. The search for new quantum algorithms benefits from understanding their limitations: Some tasks are impossible in quantum circuits, although their classical versions are easy, for example, cloning. An example with a unitary oracle 𝑈 is the if clause, the task to implement controlled 𝑈 (up to the phase on 𝑈). In classical computation the conditional statement is easy and essential. In quantum circuits the if clause was shown impossible from one query to 𝑈. Is it possible from polynomially many queries? Here we unify algorithms with a unitary oracle and develop a topological method to prove their limitations: No number of queries to 𝑈 and 𝑈† lets quantum circuits implement the if clause, even if admitting approximations, postselection, and relaxed causality. We also show limitations of process tomography, oracle neutralization, and dim⁡𝑈√𝑈, 𝑈𝑇, and 𝑈† algorithms. Our results strengthen an advantage of linear optics, challenge the experiments on relaxed causality, and motivate new algorithms with many-outcome measurements.},

keywords = {UAB},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Efficient quantum amplitude encoding of polynomial functions},

author = {Gonzalez-Conde, J. and Watts, T.W. and Rodriguez-Grasa, P. and Sanz, M.},

url = {https://quantum-journal.org/papers/q-2024-03-21-1297/},

doi = {doi.org/10.22331/q-2024-03-21-1297},

isbn = {2521-327X},

year  = {2024},

date = {2024-03-21},

urldate = {2024-03-21},

journal = {Quantum},

volume = {8},

pages = {1297},

abstract = {Loading functions into quantum computers represents an essential step in several quantum algorithms, such as quantum partial differential equation solvers. Therefore, the inefficiency of this process leads to a major bottleneck for the application of these algorithms. Here, we present and compare two efficient methods for the amplitude encoding of real polynomial functions on n qubits. This case holds special relevance, as any continuous function on a closed interval can be uniformly approximated with arbitrary precision by a polynomial function. The first approach relies on the matrix product state representation. We study and benchmark the approximations of the target state when the bond dimension is assumed to be small. The second algorithm combines two subroutines. Initially we encode the linear function into the quantum registers with a shallow sequence of multi-controlled gates that loads the linear function's Hadamard-Walsh series, exploring how truncating the Hadamard-Walsh series of the linear function affects the final fidelity. Applying the inverse discrete Hadamard-Walsh transform transforms the series coefficients into an amplitude encoding of the linear function. Then, we use this construction as a building block to achieve a block encoding of the amplitudes corresponding to the linear function on k0 qubits and apply the quantum singular value transformation that implements a polynomial transformation to the block encoding of the amplitudes. This unitary together with the Amplitude Amplification algorithm will enable us to prepare the quantum state that encodes the polynomial function on k0 qubits. Finally we pad n−k0 qubits to generate an approximated encoding of the polynomial on n qubits, analyzing the error depending on k0. In this regard, our methodology proposes a method to improve the state-of-the-art complexity by introducing controllable errors.},

keywords = {UPV/EHU},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Sequential hypothesis testing for continuously-monitored quantum systems},

author = {Gasbarri, G. and Bilkis, M. and Roda-Salichs, E. and Calsamiglia, J. },

url = {https://quantum-journal.org/papers/q-2024-03-20-1289/#},

doi = {doi.org/10.22331/q-2024-03-20-1289},

year  = {2024},

date = {2024-03-20},

urldate = {2024-03-20},

journal = {Quantum},

volume = {8},

abstract = {We consider a quantum system that is being continuously monitored, giving rise to a measurement signal. From such a stream of data, information needs to be inferred about the underlying system's dynamics. Here we focus on hypothesis testing problems and put forward the usage of sequential strategies where the signal is analyzed in real time, allowing the experiment to be concluded as soon as the underlying hypothesis can be identified with a certified prescribed success probability. We analyze the performance of sequential tests by studying the stopping-time behavior, showing a considerable advantage over currently-used strategies based on a fixed predetermined measurement time.},

keywords = {UAB},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Dissipation as a resource for Quantum Reservoir Computing},

author = {Sannia, A. and Martínez-Peña, R. and Soriano, M. and Giorgi, G.L. and Zambrini, R. },

url = {https://quantum-journal.org/papers/q-2024-03-20-1291/#},

doi = {doi.org/10.22331/q-2024-03-20-1291},

year  = {2024},

date = {2024-03-20},

journal = {Quantum },

volume = {8},

pages = {1291},

abstract = {Dissipation induced by interactions with an external environment typically hinders the performance of quantum computation, but in some cases can be turned out as a useful resource. We show the potential enhancement induced by dissipation in the field of quantum reservoir computing introducing tunable local losses in spin network models. Our approach based on continuous dissipation is able not only to reproduce the dynamics of previous proposals of quantum reservoir computing, based on discontinuous erasing maps but also to enhance their performance. Control of the damping rates is shown to boost popular machine learning temporal tasks as the capability to linearly and non-linearly process the input history and to forecast chaotic series. Finally, we formally prove that, under non-restrictive conditions, our dissipative models form a universal class for reservoir computing. It means that considering our approach, it is possible to approximate any fading memory map with arbitrary precision.},

keywords = {UIB},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Digital-analog quantum computation with arbitrary two-body Hamiltonians},

author = {Garcia-de-Andoin, M. and Saiz, A. and Perez-Fernandez, P. and Lamata, L. and Oregi, I. and Sanz, M. },

url = {https://journals.aps.org/prresearch/pdf/10.1103/PhysRevResearch.6.013280},

doi = {doi.org/10.1103/PhysRevResearch.6.013280},

isbn = {2643-1564},

year  = {2024},

date = {2024-03-14},

urldate = {2024-03-14},

journal = {Physical Review Research},

volume = {6},

issue = {1},

abstract = {Digital-analog quantum computing is a computational paradigm which employs an analog Hamiltonian resource together with single-qubit gates to reach universality. Here, we design a new scheme which employs an arbitrary two-body source Hamiltonian, extending the experimental applicability of this computational paradigm to most quantum platforms. We show that the simulation of an arbitrary two-body target Hamiltonian of 𝑛 qubits requires 𝒪⁡(𝑛2) analog blocks with guaranteed positive times, providing a polynomial advantage compared to the previous scheme. Additionally, we propose a classical strategy which combines a Bayesian optimization with a gradient descent method, improving the performance by ∼55% for small systems measured in the Frobenius norm.},

keywords = {UPV/EHU},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Quantum memories for squeezed and coherent superpositions in a driven-dissipative nonlinear oscillator},

author = {Labay-Mora, A. and Zambrini, R. and Giorgi, G.L. 



},

url = {https://journals.aps.org/pra/abstract/10.1103/PhysRevA.109.032407},

doi = {doi.org/10.1103/PhysRevA.109.032407},

issn = {2469-9926},

year  = {2024},

date = {2024-03-08},

journal = {Physical Review A},

volume = {109},

issue = {3},

abstract = {Quantum oscillators with nonlinear driving and dissipative terms have gained significant attention due to their ability to stabilize cat states for universal quantum computation. Recently, superconducting circuits have been employed to realize such long-lived qubits stored in coherent states. We present a generalization of these oscillators, which are not limited to coherent states. The key ingredient lies in the presence of different nonlinearities in driving and dissipation, beyond the quadratic one. Through an extensive analysis of the asymptotic dynamical features for different nonlinearities, we identify the conditions for the storage and retrieval of quantum states, such as squeezed states, in both coherent and incoherent superpositions. We explore their applications in quantum computing, where squeezing prolongs the lifetime of memory storage for qubits encoded in the superposition of two symmetric squeezed states, and in quantum associative memory, which has so far been limited to the storage of classical patterns.},

keywords = {CSIC-4.7},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Dynamical quantum phase transitions from random matrix theory},

author = {Pérez-García, D. and Santilli, L. and Tierz, M.},

url = {https://quantum-journal.org/papers/q-2024-02-29-1271/},

doi = {doi.org/10.22331/q-2024-02-29-1271},

year  = {2024},

date = {2024-02-29},

journal = {Quantum},

volume = {8},

pages = {1271},

abstract = {We uncover a novel dynamical quantum phase transition, using random matrix theory and its associated notion of planar limit. We study it for the isotropic XY Heisenberg spin chain. For this, we probe its real-time dynamics through the Loschmidt echo. This leads to the study of a random matrix ensemble with a complex weight, whose analysis requires novel technical considerations, that we develop. We obtain three main results: 1) There is a third order phase transition at a rescaled critical time, that we determine. 2) The third order phase transition persists away from the thermodynamic limit. 3) For times below the critical value, the difference between the thermodynamic limit and a finite chain decreases exponentially with the system size. All these results depend in a rich manner on the parity of the number of flipped spins of the quantum state conforming the fidelity.},

keywords = {UCM-4.3},

pubstate = {published},

tppubtype = {article}

}