arXiv subjects

Analia Zwick

Publications and source records attributed to Analia Zwick.

18 recordsLinked to original sources

arXiv2025

Witnessing non-stationary and non-Markovian environments with a quantum sensor

Quantum sensors offer exceptional sensitivity to nanoscale magnetic fluctuations, where non-stationary effects -- such as spin diffusion -- and non-Markovian dynamics arising from coupling to few environmental degrees of freedom play critical roles. Because fully reconstructing the microscopic structure of realistic spin baths is often infeasible, a practical challenge is to identify the dynamical features that are actually encoded in the sensor's decoherence signal. Here, we demonstrate how quantum sensors can operationally characterize the statistical nature of environmental noise, distinguishing between stationary and non-stationary behaviors, as well as Markovian and non-Markovian dynamics. Using nitrogen-vacancy (NV) centers in diamond as a platform, we develop a physical noise model that captures the essential dynamical features of realistic environments relevant to sensor observables -- independently of the microscopic bath details -- and provides analytical predictions for Ramsey decay across different regimes. These predictions are experimentally validated through controlled noise injection with tunable correlation properties. Our results showcase the capability of quantum sensors to isolate and identify key dynamical properties of complex environments, without requiring full microscopic bath reconstruction. This work clarifies the operational signatures of non-stationarity and non-Markovian behavior at the nanoscale and lays the foundation for strategies that mitigates decoherence while exploiting environmental dynamics for enhanced quantum sensing.

quant-ph↗

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

Optimising microstructural characterisation of white-matter phantoms: impact of gradient waveform modulation on Non-uniform Oscillating Gradient Spin-Echo sequences

Changes in the nervous system due to neurological diseases take place at very small spatial scales, on the order of the micro and nanometers. Developing non-invasive imaging methods for obtaining this microscopic information as quantitative biomarkers is therefore crucial for improved medical diagnosis. In this context, diffusion-weighted magnetic resonance imaging has shown significant advances in revealing tissue microstructural features by probing molecular diffusion processes. Implementing modulated gradient spin-echo sequences allows monitoring time-dependent diffusion processes to reveal such detailed information. In particular, one of those sequences termed Non-uniform Oscillating Gradient Spin-Echo (NOGSE), has shown to selectively characterise microstructure sizes by generating an image contrast based on a signal decay-shift rather than on the conventionally used signal decay rate. In this work, we prove that such decay-shift is more pronounced with instantaneous switches of the magnetic field gradient strength sign. As fast gradient ramps need to be avoided in clinical settings, due to potential patient discomfort and artefacts in imaging, we evaluate the method's efficacy for estimating microstructure sizes using both idealised, sharp gradient modulations and more realistic, smooth modulations. In this more realistic scenario, we find that the signal decay shift might be lost as the diffusion time increases, likely hindering the accurate estimation of microstructural characteristics. We demonstrate, by a combination of numerical simulations, information theory analysis and proof-of-principle experiments with white-matter phantoms, that optimal sequence design to estimate microstructure size distributions can be achieved using either sharp or smooth gradient spin-echo modulations. This approach simplifies the translation of the NOGSE method for its use in clinical settings.

physics.med-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↗

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↗

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↗

arXiv2019

Enhanced precision bound of low-temperature quantum thermometry via dynamical control

High-precision low-temperature thermometry is a challenge for experimental quantum physics and quantum sensing. Here we consider a thermometer modelled by a dynamically-controlled multilevel quantum probe in contact with a bath. Dynamical control in the form of periodic modulation of the energy-level spacings of the quantum probe can dramatically increase the maximum accuracy bound of low-temperatures estimation, by maximizing the relevant quantum Fisher information. As opposed to the diverging relative error bound at low temperatures in conventional quantum thermometry, periodic modulation of the probe allows for low-temperature thermometry with temperature-independent relative error bound. The proposed approach may find diverse applications related to precise probing of the temperature of many-body quantum systems in condensed matter and ultracold gases, as well as in different branches of quantum metrology beyond thermometry, for example in precise probing of different Hamiltonian parameters in many-body quantum critical systems.

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

Thermal Baths as Quantum Resources: More Friends than Foes?

In this article we argue that thermal reservoirs (baths) are potentially useful resources in processes involving atoms interacting with quantized electromagnetic fields and their applications to quantum technologies. One may try to suppress the bath effects by means of dynamical control, but such control does not always yield the desired results. We wish instead to take advantage of bath effects, that do not obliterate "quantumness" in the system-bath compound. To this end, three possible approaches have been pursued by us: (i) Control of a quantum system faster than the correlation time of the bath to which it couples: Such control allows us to reveal quasi-reversible/coherent dynamical phenomena of quantum open systems, manifest by the quantum Zeno or anti-Zeno effects (QZE or AZE, respectively). Dynamical control methods based on the QZE are aimed not only at protecting the quantumness of the system, but also diagnosing the bath spectra or transferring quantum information via noisy media. By contrast, AZE-based control is useful for fast cooling of thermalized quantum systems. (ii) Engineering the coupling of quantum systems to selected bath modes: This approach, based on field -atom coupling control in cavities, waveguides and photonic band structures, allows to drastically enhance the strength and range of atom-atom coupling through the mediation of the selected bath modes. More dramatically, it allows us to achieve bath-induced entanglement that may appear paradoxical if one takes the conventional view that coupling to baths destroys quantumness. (iii) Engineering baths with appropriate non-flat spectra: This approach is a prerequisite for the construction of the simplest and most efficient quantum heat machines (engines and refrigerators). We may thus conclude that often thermal baths are "more friends than foes" in quantum technologies.

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

Quantum state transfer in disordered spin chains: How much engineering is reasonable?

The transmission of quantum states through spin chains is an important element in the implementation of quantum information technologies. Speed and fidelity of transfer are the main objectives which have to be achieved by the devices even in the presence of imperfections which are unavoidable in any manufacturing process. To reach these goals, several kinds of spin chains have been suggested, which differ in the degree of fine-tuning, or engineering, of the system parameters. In this work we present a systematic study of two important classes of such chains. In one class only the spin couplings at the ends of the chain have to be adjusted to a value different from the bulk coupling constant, while in the other class every coupling has to have a specific value. We demonstrate that configurations from the two different classes may perform similarly when subjected to the same kind of disorder in spite of the large difference in the engineering effort necessary to prepare the system. We identify the system features responsible for these similarities and we perform a detailed study of the transfer fidelity as a function of chain length and disorder strength, yielding empirical scaling laws for the fidelity which are similar for all kinds of chain and all disorder models. These results are helpful in identifying the optimal spin chain for a given quantum information transfer task. In particular, they help in judging whether it is worthwhile to engineer all couplings in the chain as compared to adjusting only the boundary couplings.

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↗

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↗

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↗

arXiv2011

Robustness of spin-coupling distributions for perfect quantum state transfer

The transmission of quantum information between different parts of a quantum computer is of fundamental importance. Spin chains have been proposed as quantum channels for transferring information. Different configurations for the spin couplings were proposed in order to optimize the transfer. As imperfections in the creation of these specific spin-coupling distributions can never be completely avoided, it is important to find out which systems are optimally suited for information transfer by assessing their robustness against imperfections or disturbances. We analyze different spin coupling distributions of spin chain channels designed for perfect quantum state transfer. In particular, we study the transfer of an initial state from one end of the chain to the other end. We quantify the robustness of different coupling distributions against perturbations and we relate it to the properties of the energy eigenstates and eigenvalues. We find that the localization properties of the systems play an important role for robust quantum state transfer.

quant-ph↗

arXiv2011

Quantum state transfer in a XX chain with impurities

One spin excitation states are involved in the transmission of quantum states and entanglement through a quantum spin chain, the localization properties of these states are crucial to achieve the transfer of information from one extreme of the chain to the other. We investigate the bipartite entanglement and localization of the one excitation states in a quantum $XX$ chain with one impurity. The bipartite entanglement is obtained using the Concurrence and the localization is analyzed using the inverse participation ratio. Changing the strength of the exchange coupling of the impurity allows us to control the number of localized or extended states. The analysis of the inverse participation ratio allows us to identify scenarios where the transmission of quantum states or entanglement can be achieved with a high degree of fidelity. In particular we identify a regime where the transmission of quantum states between the extremes of the chain is executed in a short transmission time $\sim N/2$, where $N$ is the number of spins in the chain, and with a large fidelity.

quant-ph↗