arXiv subjects

Gonzalo A. Alvarez

Publications and source records attributed to Gonzalo A. Alvarez.

At least 19 recordsLinked to original sources

arXiv2025

Manifestation of critical effects in environmental parameter estimation using a quantum sensor under dynamical control

Quantum probes offer a powerful platform for exploring environmental dynamics, particularly through their sensitivity to decoherence processes. In this work, we investigate the emergence of critical behavior in the estimation of the environmental memory time $τ_c$, modeled as an Ornstein-Uhlenbeck process characterized by a Lorentzian spectral density. Using dynamically controlled qubit-based sensors -- realized experimentally via solid-state Nuclear Magnetic Resonance (NMR) and supported by numerical simulations -- we implement tailored filter functions to interrogate the environmental noise spectrum and extract $τ_c$ from its spectral width. Our results reveal a sharp transition in estimation performance between short-memory (SM) and long-memory (LM) regimes, reflected in a non-monotonic estimation error that resembles a phase transition. This behavior is accompanied by an avoided-crossing-like structure in the estimated parameter space, indicative of two competing solutions near the critical point. These features underscore the interplay between control, decoherence, and inference in open quantum systems. Beyond their fundamental significance, these critical phenomena offer a practical diagnostic tool for identifying dynamical regimes and optimizing quantum sensing protocols. By exploiting this criticality, our findings pave the way for adaptive control strategies aimed at enhancing precision in quantum parameter estimation -- particularly in complex or structured environments such as spin networks, diffusive media, and quantum materials.

quant-ph↗

arXiv2024

Maximizing information obtainable by quantum sensors through the Quantum Zeno Effect

Efficient quantum sensing technologies rely on precise control of quantum sensors, particularly two-level systems or qubits, to optimize estimation processes. We here exploit the Quantum Zeno Effect (QZE) as a tool for maximizing information obtainable by quantum sensors, with a specific focus on the level avoided crossing (LAC) phenomenon in qubit systems. While the estimation of the LAC energy splitting has been extensively studied, we emphasize the crucial role that the QZE can play in estimating the coupling strength. We introduce the concept of information amplification by the QZE for a LAC system under off-resonant conditions. The proposed approach has implications for AC magnetic field sensing and the caracterization of complex systems, including many-spin systems requiring the estimation of spin-spin couplings. Overall, our findings contribute to the advancement of quantum sensing by leveraging the QZE for improved control and information extraction.

quant-ph↗

arXiv2023

Cumulant expansion framework for internal gradient distributions tensors

Magnetic resonance imaging is a powerful, non invasive tool for medical diagnosis. The low sensitivity for detecting the nuclear spin signals, typically limits the image resolution to several tens of micrometers in preclinical systems and millimeters in clinical scanners. Other sources of information, derived from diffusion processes of intrinsic molecules as water in the tissues, allow getting morphological information at micrometric and submicrometric scales as potential biomarkers of several pathologies. Here we consider extracting this morphological information by probing the distribution of internal magnetic field gradients induced by the heterogeneous magnetic susceptibility of the medium. We use a cumulant expansion to derive the dephasing on the spin signal induced by the molecules that explore these internal gradients while diffuse. Based on the cumulant expansion, we define internal gradient distributions tensors (IGDT) and propose modulating gradient spin echo sequences to probe them. These IGDT contain microstructural morphological information that characterize porous media and biological tissues. We evaluate the IGDT effects on the magnetization decay with typical conditions of brain tissue and show their effects can be experimentally observed. Our results thus provide a framework for exploiting IGDT as quantitative diagnostic tools.

physics.bio-ph↗

arXiv2022

Path integral framework for characterizing and controlling decoherence induced by non-stationary environments on a quantum probe

Reliable processing of quantum information is a milestone to achieve for the deployment of quantum technologies. Uncontrolled, out-of-equilibrium sources of decoherence need to be characterized in detail for designing the control of quantum devices to mitigate the loss of quantum information. However, quantum sensing of such environments is still a challenge due to their non-stationary nature that in general can generate complex high-order correlations. We here introduce a path integral framework to characterize non-stationary environmental fluctuations by a quantum probe. We found the solution for the decoherence decay of non-stationary, generalized Gaussian processes that induce pure dephasing. This dephasing when expressed in a suitable basis, based on the non-stationary noise eigenmodes, is defined by the overlap of a generalized noise spectral density and a filter function that depends on the control fields. This result thus extends the validity to out-of-equilibrium environments, of the similar general expression for the dephasing of open quantum systems coupled to stationary noises. We show physical insights for a broad subclass of non-stationary noises that are local-in-time, in the sense that the noise correlation functions contain memory based on constraints of the derivatives of the fluctuating noise paths. Spectral and non-Markovian properties are discussed together with implementations of the framework to treat paradigmatic environments that are out-of-equilibrium, e.g. due to a quench and a pulsed noise. We show that our results provide tools for probing the spectral and time-correlation properties, and for mitigating decoherence effects of out-of-equilibrium -- non-stationary -- environments.

quant-ph↗

arXiv2021

Decoherence scaling transition in the dynamics of quantum information scrambling

Reliable processing of quantum information for developing quantum technologies requires precise control of out-of-equilibrium many-bodysystems. This is a highly challenging task as the fragility of quantum states to external perturbations increases with the system-size. Here, we report on a series of experimental quantum simulations that quantify the sensitivity of a controlled Hamiltonian evolution to perturbations that drive the system away from the targeted evolution. Based on out-of-time ordered correlations, we demonstrate that the decay-rate of the process fidelity increases with the effective number $K$ of correlated qubits as $K^α$. As a function of the perturbation strength, we observe a decoherence scaling transition of the exponent $α$ between two distinct dynamical regimes. In the limiting case below the critical perturbation strength, the exponent $α$ drops sharply below 1, and there is no inherent limit to the number of qubits that can be controlled. This resilient quantum feature of the controlled dynamics of quantum information is promising for reliable control of large quantum systems.

quant-ph↗

arXiv2020

Non-invasive quantitative imaging of selective microstructure-sizes with magnetic resonance

Extracting reliable and quantitative microstructure information of living tissue by non-invasive imaging is an outstanding challenge for understanding disease mechanisms and allowing early stage diagnosis of pathologies. Magnetic Resonance Imaging is the favorite technique to pursue this goal, but still provides resolution of sizes much larger than the relevant microstructure details on in-vivo studies. Monitoring molecular diffusion within tissues, is a promising mechanism to overcome the resolution limits. However, obtaining detailed microstructure information requires the acquisition of tens of images imposing long measurement times and results to be impractical for in-vivo studies. As a step towards solving this outstanding problem, we here report on a method that only requires two measurements and its proof-of-principle experiments to produce images of selective microstructure sizes by suitable dynamical control of nuclear spins with magnetic field gradients. We design microstructure-size filters with spin-echo sequences that exploit magnetization "decay-shifts" rather than the commonly used decay-rates. The outcomes of this approach are quantitative images that can be performed with current technologies, and advance towards unravelling a wealth of diagnostic information based on microstructure parameters that define the composition of biological tissues.

physics.med-ph↗

arXiv2019

Precision limits of tissue microstructure characterization by Magnetic Resonance Imaging

Characterization of microstructures in live tissues is one of the keys to diagnosing early stages of pathology and understanding disease mechanisms. However, the extraction of reliable information on biomarkers based on microstructure details is still a challenge, as the size of features that can be resolved with non-invasive Magnetic Resonance Imaging (MRI) is orders of magnitude larger than the relevant structures. Here we derive from quantum information theory the ultimate precision limits for obtaining such details by MRI probing of water-molecule diffusion. We show that already available MRI pulse sequences can be optimized to attain the ultimate precision limits by choosing control parameters that are uniquely determined by the expected size, the diffusion coefficient and the spin relaxation time $T_{2}$. By attaining the ultimate precision limit per measurement, the number of measurements and the total acquisition time may be drastically reduced compared to the present state of the art. These results will therefore allow MRI to advance towards unravelling a wealth of diagnostic information.

quant-ph↗

arXiv2015

Criticality of environmental information obtainable by dynamically controlled quantum probes

A universal approach to decoherence control combined with quantum estimation theory reveals a critical behavior, akin to a phase transition, of the information obtainable by a qubit probe concerning the memory time of environmental fluctuations. This criticality emerges only when the probe is subject to dynamical control. It gives rise to a sharp transition between two dynamical phases characterized by either a short or long memory time compared to the probing time. This phase-transition of the environmental information is a fundamental feature that facilitates the attainment of the highest estimation precision of the environment memory-time and the characterization of probe dynamics.

quant-ph↗

arXiv2015

Maximizing information on the environment by dynamically controlled qubit probes

We explore the ability of a qubit probe to characterize unknown parameters of its environment. By resorting to quantum estimation theory, we analytically find the ultimate bound on the precision of estimating key parameters of a broad class of ubiquitous environmental noises ("baths") which the qubit may probe. These include the probe-bath coupling strength, the correlation time of generic bath spectra, the power laws governing these spectra, as well as their dephasing times T2. Our central result is that by optimizing the dynamical control on the probe under realistic constraints one may attain the maximal accuracy bound on the estimation of these parameters by the least number of measurements possible. Applications of this protocol that combines dynamical control and estimation theory tools to quantum sensing are illustrated for a nitrogen-vacancy center in diamond used as a probe.

quant-ph↗

arXiv2015

Robustness of spin-chain state-transfer schemes

This is a shortened and slightly edited version of a chapter in the collection "Quantum State Transfer and Network Engineering", edited by G.M. Nikolopoulos and I. Jex, where we review our own research about the robustness of spin-chain state-transfer schemes along with other approaches to the topic. Since our own research is documented elsewhere to a large extent we here restrict ourselves to a review of other approaches which might be useful to other researchers in the field.

quant-ph↗

arXiv2015

Optimized dynamical control of state transfer through noisy spin chains

We propose a method of optimally controlling the tradeoff of speed and fidelity of state transfer through a noisy quantum channel (spin-chain). This process is treated as qubit state-transfer through a fermionic bath. We show that dynamical modulation of the boundary-qubits levels can ensure state transfer with the best tradeoff of speed and fidelity. This is achievable by dynamically optimizing the transmission spectrum of the channel. The resulting optimal control is robust against both static and fluctuating noise in the channel's spin-spin couplings. It may also facilitate transfer in the presence of diagonal disorder (on site energy noise) in the channel.

quant-ph↗

arXiv2014

Local and bulk 13C hyperpolarization in NV-centered diamonds at variable fields and orientations

Polarizing nuclear spins is of fundamental importance in biology, chemistry and physics. Methods for hyperpolarizing 13C nuclei from free electrons in bulk, usually demand operation at cryogenic temperatures. Room-temperature approaches targeting diamonds with nitrogen-vacancy (NV) centers could alleviate this need, but hitherto proposed strategies lack generality as they demand stringent conditions on the strength and/or alignment of the magnetic field. We report here an approach for achieving efficient electron->13C spin alignment transfers, compatible with a broad range of magnetic field strengths and field orientations with respect to the diamond crystal. This versatility results from combining coherent microwave- and incoherent laser-induced transitions between selected energy states of the coupled electron-nuclear spin manifold. 13C-detected Nuclear Magnetic Resonance (NMR) experiments demonstrate that this hyperpolarization can be transferred via first-shell or via distant 13Cs, throughout the nuclear bulk ensemble. This method opens new perspectives for applications of diamond NV centers in NMR, and in quantum information processing.

quant-ph↗

arXiv2014

Experimental observation of a phase transition in the evolution of many-body systems with dipolar interactions

Non-equilibrium dynamics of many-body systems is important in many branches of science, such as condensed matter, quantum chemistry, and ultracold atoms. Here we report the experimental observation of a phase transition of the quantum coherent dynamics of a 3D many-spin system with dipolar interactions, and determine its critical exponents. Using nuclear magnetic resonance (NMR) on a solid-state system of spins at room-temperature, we quench the interaction Hamiltonian to drive the evolution of the system. The resulting dynamics of the system coherence can be localized or extended, depending on the quench strength. Applying a finite-time scaling analysis to the observed time-evolution of the number of correlated spins, we extract the critical exponents v = s = 0.42 around the phase transition separating a localized from a delocalized dynamical regime. These results show clearly that such nuclear-spin based quantum simulations can effectively model the non-equilibrium dynamics of complex many-body systems, such as 3D spin-networks with dipolar interactions.

quant-ph↗

arXiv2014

Coherent dynamical recoupling of diffusion-driven decoherence in magnetic resonance

During recent years, dynamical decoupling (DD) has gained relevance as a tool for manipulating quantum systems and extracting information from them. This is particularly relevant for spins involved in nuclear magnetic resonance (NMR), where DD sequences can be used to prolong quantum coherences, or for selectively couple/decouple the effects imposed by random environmental fluctuations. In this Letter, we show that one can exploit these concepts in order to selectively recouple diffusion processes in restricted spaces. The ensuing method provides a novel tool to measure restriction lengths in confined systems such as capillaries, pores or cells. The principles of this method for selectively recoupling diffusion-driven decoherence, its standing within the context of diffusion NMR, and corroborating experiments, are presented.

quant-ph↗

arXiv2013

Dynamical control of state transfer through noisy quantum channels: optimal tradeoff of speed and fidelity

We propose a method of optimally controlling state transfer through a noisy quantum channel (spin-chain). This process is treated as qubit state-transfer through a fermionic bath. We show that dynamical modulation of the boundary-qubits levels suffices to ensure fast and high-fidelity state transfer. This is achievable by dynamically optimizing the transmission spectrum of the channel. The resulting optimal control is robust against both static and fluctuating noise.

quant-ph↗

arXiv2013

Quantum simulations of localization effects with dipolar interactions

Quantum information processing often uses systems with dipolar interactions. We use a nuclear spin-based quantum simulator, to study the spreading of information in such a dipolar-coupled system and how perturbations to the dipolar couplings limit the spreading, leading to localization. In [Phys. Rev. Lett. 104, 230403 (2010)], we found that the system reaches a dynamic equilibrium size, which decreases with the square of the perturbation strength. Here, we study the impact of a disordered Hamiltonian with dipolar 1/r^3 interactions. We show that the expansion of the coherence length of the cluster size of the spins becomes frozen in the presence of large disorder, reminiscent of Anderson localization of non-interacting waves in a disordered potential.

quant-ph↗

arXiv2012

Concatenated dynamical decoupling with virtual pulses

The loss of quantum information due to interaction with external degrees of freedom, which is known as decoherence, remains one of the main obstacles for large-scale implementations of quantum computing. Accordingly, different measures are being explored for reducing its effect. One of them is dynamical decoupling (DD) which offers a practical solution because it only requires the application of control pulses to the system qubits. Starting from basic DD sequences, more sophisticated schemes were developed that eliminate higher-order terms of the system-environment interaction and are also more robust against experimental imperfections. A particularly successful scheme, called concatenated DD (CDD), gives a recipe for generating higher order sequences by inserting lower order sequences into the delays of a generating sequence. Here, we show how this scheme can be improved further by converting some of the pulses to virtual (and thus ideal) pulses. The resulting scheme, called vCDD, has lower power deposition and is more robust against pulse imperfections than the original CDD scheme.

quant-ph↗

arXiv2012

Spin chains for robust state transfer: Modified boundary couplings vs. completely engineered chains

Quantum state transfer in the presence of noise is one of the main challenges in building quantum computers. We compare the quantum state transfer properties for two classes of qubit chains under the influence of static randomness. In fully engineered chains all nearest-neighbor couplings are tuned in such a way that a single-qubit state can be transferred perfectly between the ends of the chain, while in boundary-controlled chains only the two couplings between the transmitting and receiving qubits and the remainder of the chain can be optimized. We study how the noise in the couplings affects the state transfer fidelity depending on the noise model and strength as well as the chain type and length. We show that the desired level of fidelity and transfer time are important factors in designing a chain. In particular we demonstrate that transfer efficiency comparable or better than that of the most robust engineered systems can also be reached in boundary-controlled chains without the demanding engineering of a large number of couplings.

quant-ph↗