arXiv subjects

Carlos H. S. Vieira

Publications and source records attributed to Carlos H. S. Vieira.

17 recordsLinked to original sources

arXiv2026

Quantum-State Texture Dynamics: Theory and Experiment

Quantum-state texture (QST) has found applications in several fields from quantum foundations and computation to quantum criticality. However, a general theory of QST dynamics under arbitrary physical processes remains unavailable, limiting both its practical application and experimental exploration. Here, we demonstrate that the QST response to an arbitrary finite-dimensional channel is fully encoded in the dual evolution of a single reference state. This description yields necessary and sufficient conditions for texture preservation and implies exact conservation under all free-unital dynamics. Using a nuclear magnetic resonance quantum processor, we experimentally verify these predictions across distinct channel classes. Furthermore, we show that local QST measurements provide an operational signature of entangling gates in circuit layers. Our results establish quantum-state texture as a resource and a practical diagnostic tool in quantum information processing.

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arXiv2026

Weak-to-Strong Measurement Transition with Thermal Instabilities: From Anomalous Amplification to Metrological Sensitivity

Quantum measurement is physically realized through a finite dynamical interaction between a system and a measuring apparatus, giving rise to a continuous transition from weak to strong regimes. While this crossover is well understood under ideal conditions, the combined role of thermal instabilities and pre- and post-selection open dynamics has not been systematically addressed. Here, we develop a generalized open-system framework to analyze the weak-to-strong measurement transition in the simultaneous presence of environmental decoherence and thermal noise. We model the probe as a thermal Gaussian state, explicitly incorporating temperature-dependent fluctuations in the measuring device, and include open-system evolution of the measured system prior to post-selection. By deriving the apparatus's final state, we show that the measurement statistics are modified in a nontrivial, highly sensitive manner by the temperature regime of the system's thermal instabilities, the probe's thermal properties, and the particular choice of pre- and post-selection. This approach allows us to characterize how thermal effects reshape the weak-value condition, the anomalous amplification, and the resulting metrological sensitivity, demonstrating the protocol's practical utility for precision measurements across the full measurement crossover.

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arXiv2026

Experimental observation and application of the genuine Quantum Mpemba Effect

Coherence is an inherently quantum property that deeply affects microscopic processes, including thermalization phenomena. A striking example is the quantum Mpemba effect (QME), in which a system can exhibit anomalous relaxation, thermalizing faster from a state initially farther from equilibrium than from one closer. Here, we experimentally investigate the genuine QME and observe how the dynamics of a spin-1/2 system interacting with a heat sink can be sped-up to equilibrium. The mechanism underlying this speed-up is uncovered through detailed analysis of the heat exchange process. Furthermore, we apply the QME in a quantum Otto refrigerator, thereby increasing its cooling power. This proof-of-concept experiment unveils new practical paths for improving quantum thermal tasks.

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arXiv2026

Boosting State Discrimination in Quantum Brownian Motion Channel via Memory-Induced Coherence Preservation

Preserving quantum resources in dissipative environments is a fundamental challenge in quantum information processing. While environmental interactions usually degrade quantum resources, we theoretically show that in a Quantum Brownian Motion (QBM) channel, continuous-variable state discrimination can be improved by increasing, rather than minimizing, the initial thermal noise. Specifically, without suppressing the inherent environmental dissipation, when combined with squeezing, this initial noise induces a coherence preservation mechanism driven by the transient non-thermalization of the probe with the bath. This preservation translates into a pronounced reduction in error probabilities for state discrimination between orthogonal squeezing directions. Furthermore, we also show that quadrature homodyne detection achieves near-optimal performance, approaching the Helstrom limit. These results highlight the advantage of exploiting thermal-squeezed states, offering a robust physical architecture for quantum communication in high-temperature environments.

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arXiv2026

Towards gravimetry enhancement with squeezed states

We investigate the sensitivity of gravitational acceleration estimation using squeezed probe states in a quantum metrology framework. In particular, we analyze how the squeezing phase, beyond its amplitude, affects the attainable precision. We show that probes squeezed along the canonical phase-space quadratures can surpass the shot-noise limit only in specific time regimes, whereas position-momentum correlated input states can consistently overcome this limit across all interaction times. Furthermore, we demonstrate that optimal sensitivity can be achieved by combining projective momentum measurements with a time-dependent adjustment of the squeezing phase. Our results highlight the fundamental role of phase-engineered squeezing in quantum gravimetry protocols and provide new insights into the design of optimized sensing strategies.

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arXiv2026

Multiparameter estimation with position-momentum correlated Gaussian probes

Gaussian quantum probes have been widely used in quantum metrology and thermometry, where the goal is to estimate the temperature of an environment with which the probe interacts. It was recently shown that introducing initial position-momentum (PM) correlations in such probes can enhance the estimation precision compared to standard, uncorrelated Gaussian states. Motivated by these findings, we investigate whether PM correlations can also be advantageous in a simultaneous estimation setting, specifically, when estimating both the PM correlations themselves and the effective environment temperature that interacts with the probe. Using the Quantum Fisher Information Matrix, we derive new precision bounds for this joint estimation task. Additionally, we demonstrate that such correlations can serve as a resource to improve temperature estimation within this multiparameter context. Finally, we analyze the compatibility between the two parameters, establishing conditions under which the derived bounds can be saturated.

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arXiv2025

Temperature and non-Markovian parameter estimation in quantum Brownian motion

We investigate a quantum metrological protocol operating in a non-Markovian environment by employing the quantum Brownian motion (QBM) model, in which the system is linearly coupled to a reservoir of harmonic oscillators. Specifically, we use a position-momentum (PM) correlated Gaussian state as a probe to examine how memory effects influence the evolution of the system's covariance matrix in the weak coupling regime under both high- and low-temperature conditions. To confirm the presence of non-Markovian behavior, we apply two well-established non-Markovianity quantifiers. Furthermore, we estimate both the channel's sample temperature and its non-Markovianity witness parameter. Our results demonstrate that non-Markovianity and PM correlations can jointly be valuable resources to enhance metrological performance.

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arXiv2025

Probing parameters estimation with Gaussian non-commutative measurements

Gaussian quantum states and channels are pivotal across many branches of quantum science and their applications, including the processing and storage of quantum information, the investigation of thermodynamics in the quantum regime, and quantum computation. The great advantage is that Gaussian states are experimentally accessible via their first and second statistical moments. In this work, we investigate parameter estimation for Gaussian states, in which the probe-state preparation stage involves two noncommutative Gaussian measurements on the position and momentum observables, introducing tunable parameters. The influence of these noncommutative Gaussian measurements is investigated through the quantum Fisher information (QFI). We showed that the QFI for characterizing Gaussian channels can be increased by adjusting the uncertainty parameters in the preparation of the probe state. Furthermore, if the probe is initially in a thermal state, probe-state preparation may generate quantum coherence in its energy basis. We showed that not only does the amount of coherence affect the improvement of the QFI, but also the rate of change of the coherence with respect to the parameter to be estimated. The proposed probe-state protocol is applied to two paradigmatic single-mode Gaussian channels, the attenuator and amplification channels, which are building blocks of Gaussian quantum information. Our results contribute to the use of coherence in quantum metrology and are experimentally feasible in quantum-optical devices.

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arXiv2024

Enhancing Gaussian quantum metrology with position-momentum correlations

Quantum metrology offers significant improvements in several quantum technologies. In this work, we propose a Gaussian quantum metrology protocol assisted by initial position-momentum correlations (PM). We employ a correlated Gaussian wave packet as a probe to examine the dynamics of Quantum Fisher Information (QFI) and purity based on PM correlations to demonstrate how to estimate the PM correlations and, more importantly, to unlock its potential applications such as a resource to enhance quantum thermometry. In the low-temperature regime, we find an improvement in the thermometry of the surrounding environment when the original system exhibits a non-null initial correlation (correlated Gaussian state). In addition, we explore the connection between the loss of purity and the gain in QFI during the process of estimating the effective environment coupling and its effective temperature.

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arXiv2024

Two-stroke thermal machine using spin squeezing operation

Quantum thermal machines are powerful platforms for investigating how quantum effects impact the energy flow between different systems. Here, we investigate a two-stroke cycle in which spin-squeezing effects are intrinsically switched on during the entire operation time. By using Kitagawa and Ueda's parameter and the l1-norm to compute the degree of spin squeezing and the quantum coherence, we first show that the greater the spin squeezing effect, the greater the amount of coherence in the energy basis. Then, we investigate the engine performance given the amount of spin squeezing into the system. Our results show that even assuming an always-on spin squeezing, which is directly associated with the amount of entropy production in the cycle, it is possible to find a better set of efficiency and extracted power for the engine provided a high level of control over the relevant parameters, i.e., the operation time and the squeezing intensity.

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arXiv2024

Improving parameters estimation in Gaussian channels using quantum coherence

Gaussian quantum channels are relevant operations in continuous variable systems. In general, given an arbitrary state, the action on it is well-known provided that the quantum channels are completely characterized. In this work, we consider the inverse problem, i.e., the estimation of channel parameters employing probes in which quantum coherence is used as a resource. Two paradigmatic bosonic Gaussian channels are treated, the thermal attenuator and the thermal amplifier. We also consider the degradation of the coherence due to a Markovian bath. The quantum Fisher information for each relevant parameter is computed and we observed that the rate of change of coherence concerning the channel parameter, rather than the amount of coherence, can produce a parameter estimation gain. Finally, we obtain a direct relation between the quantum Fisher information and the relative entropy or coherence, allowing in principle an experimental implementation based on the measurement of the covariance matrix of the probe system.

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arXiv2024

Gouy phase and quantum interference with cross-Wigner functions for matter-waves

The Gouy phase is essential for accurately describing various wave phenomena, ranging from classical electromagnetic waves to matter waves and quantum optics. In this work, we employ phase-space methods based on the cross-Wigner transformation to analyze spatial and temporal interference in the evolution of matter waves characterized initially by a correlated Gaussian wave packet. First, we consider the cross-Wigner of the initial function with its free evolution, and second for the evolution through a double-slit arrangement. Different from the wave function which acquires a global Gouy phase, we find that the cross-Wigner acquires a Gouy phase difference due to different evolution times. The results suggest that temporal like-Gouy phases are important for an accurate description of temporal interference. Furthermore, we propose a technique based on the Wigner function to reconstruct the cross-Wigner from the spatial intensity interference term in a double-slit experiment with matter waves.

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arXiv2024

The role of position momentum correlations in coherence freezing and purity behavior

We explore the effects of Markovian bath coupling and initial position-momentum correlations on the coherence and purity of Gaussian quantum states. Our analysis focuses on the roles these factors play in the dynamics of quantum coherence, coherence lengths, and state purity. Our results reveal that initial position-momentum correlations have a remarkable impact on the quantum properties of the mixed state. These correlations lead to opposing behaviors in coherence and purity: as quantum coherence increases in response to stronger correlations, purity diminishes, and vice versa. This inverse relationship illustrates the phenomenon where, governed by these initial correlations, a state with greater mixing can display enhanced quantum coherence compared to a less mixed state. We also observe an unanticipated coherence freezing phenomenon, quantified by the relative entropy of coherence. Notably, this freezing is driven by initial position-momentum correlations, although the final frozen value is independent of these correlations.

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arXiv2023

Exploring quantum thermodynamics with NMR

Quantum thermodynamics seeks to extend non-equilibrium stochastic thermodynamics to small quantum systems where non-classical features are essential to its description. Such a research area has recently provided meaningful theoretical and experimental advances by exploring the wealth and the power of quantum features along with informational aspects of a system's thermodynamics. The relevance of such investigations is related to the fact that quantum technological devices are currently at the forefront of science and engineering applications. This short review article provides an overview of some concepts in quantum thermodynamics highlighting test-of-principles experiments using nuclear magnetic resonance techniques.

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arXiv2022

Coherence behavior of strongly coupled bosonic modes

We study the effect of the intermode coupling in the generation of coherence when two bosonic modes are bilinearly coupled. We consider the case for which the two modes are weakly coupled and the rotating-wave approximation (RWA) applies and the case for which they are strongly coupled and the (RWA) does not apply. Then, in the regime of validity of (RWA), there is no coherence generation solely due to squeezing effects, which means there is an exchange of excitation between the modes and negligible squeezing. On the other hand, if the two modes are strongly coupled coherence is generated by the squeezing interaction. For the system of two bosonic modes weakly coupled with a Markovian bath at temperature $T$ the coherence decreases with $T$. In general, when both kinds of couplings (exchange of excitation and squeezing) are turned on the exchange of excitation contributes to generating more coherence in comparison with a purely squeezing coupling. Thus, the coherence decreases more slowly with the temperature when both couplings are present. Finally, we explore the case when only one of the two coupled modes interacts with a Markovian bath while the other one remains free from the environment. We observe that the intermode coupling induces oscillations in the coherence and fidelity dynamics similar to the behavior for the coupling with a non-Markovian environment.

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arXiv2020

Negativity-Mutual Information conversion and coherence in two-coupled harmonic oscillators

Quantum information is a common topic of research in many areas of quantum physics, such as quantum communication and quantum computation, as well as quantum thermodynamics. It can be encoded in discrete or continuous variable systems, with the appropriated formalism to treat it generally depending on the quantum system to be chosen. For continuous variable systems, it is convenient to employ a quasi-probability function to represent quantum states and non-classical signatures. The Wigner function is a special quasi-probability function because it allows to describe a quantum system in the phase space very similar to the classical one. In this work we consider some informational aspects of two-mode continuous variables systems. In particular we illustrate how the mutual information may be useful to transfer the non-Gaussianity property between two states. Moreover, the coherence of a Gaussian state is studied when the system is coupled to a thermal reservoir but also with an extra single-mode Gaussian state. Our results may be, in principle, investigated in trapped ions setups, where the two-mode system can be encoded in the vibrational modes of the ion.

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arXiv2020

Bell Non-Locality in Many Body Quantum Systems with Exponential Decay of Correlations

Using Bell-inequalities as a tool to explore non-classical physical behaviours, in this paper we analyze what one can expect to find in many-body quantum physics. Concretely, framing the usual correlation scenarios as a concrete spin-lattice, we want to know whether or not it is possible to violate a Bell-inequality restricted to this scenario. Using clustering theorems, we are able to show that a large family of quantum many-body systems behave almost locally, violating Bell-inequalities (if so) only by a non-significant amount. We also provide examples, explain some of our assumptions via counter-examples and present all the proofs for our theorems. We hope the paper is self-contained.

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