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arXiv:2403.11339 · Physical Review Applied

Maximizing information obtainable by quantum sensors through the quantum Zeno effect

Bruno Ronchi, Analía Zwick, Gonzalo A. Álvarez

Published Sep 25, 2024• 4 citations
arXivHTMLPDF

Abstract

Efficient quantum sensing technologies rely on the 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 characterization 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.

DOI
10.1103/physrevapplied.22.034058

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Efficient quantum sensing technologies rely on the 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 characterization 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.
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Quantum control is a fundamental tool for quantum technologies, and quantum Zeno (QZE) and anti-Zeno (AZE) effects, respectively denoting the slowdown and speedup of quantum system evolution by frequent interruptions/operations, represent quantum control paradigms. Indeed, they allow either protecting (QZE) or steering (AZE) the state of a quantum system via the interplay between frequent operations (system control) and the coupling of the system to its environment. Here we present two experiments demonstrating the possibility to exploit QZE and AZE to extract information on noise processes occurring in a quantum channel. In the first experiment, we realize the first noise diagnostics scheme based on repeated quantum measurements, showing how a single photon undergoing a noise process (in our case, random polarization fluctuations) can quantify non-Markovian temporal correlations within such a noise. In the second experiment, we show how a QZE-protection mechanism applied to a quantum state can become a noise sensing technique, allowing to estimate the statistical distribution of decoherence effects occurring on a photonic qubit during its propagation in a noisy quantum channel.
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Quantum sensors take advantage of the inherent sensitivity of quantum systems to external environment perturbations to precisely measure target signals. The same interaction with the environment, however, limits the device sensitivity by reducing the coherence of the quantum states. Quantum control can play a crucial role in improving the sensor performance. We explore quantum control tools to improve the performance of a NitrogenVacancy (NV) center as a single-qubit sensor of ultra-weak magnetic fields in noisy environments. We present a method to obtain a precise spectroscopic characterization of the NV environment, instrumental to design robust sensing strategies with improved noise protection. Then, we exploit optimal quantum control based on a set of numerical methods to optimize the temporal dependence of the sensor driving field adapted to quantum sensing purposes, and prove its advantage to achieve best compromises between signal acquisition and noise cancellation.
We show how the quantum Zeno effect can be exploited to control quantum many-body dynamics for quantum information and computation purposes. In particular, we consider a one dimensional array of three level systems interacting via a nearest-neighbour interaction. By encoding the qubit on two levels and using simple projective frequent measurements yielding the quantum Zeno effect, we demonstrate how to implement a well defined quantum register, quantum state transfer on demand, universal two-qubit gates and two-qubit parity measurements. Thus, we argue that the main ingredients for universal quantum computation can be achieved in a spin chain with an always-on and constant many-body Hamiltonian. We also show some possible modifications of the initially assumed dynamics in order to create maximally entangled qubit pairs and single qubit gates.
Quantum sensors play an important role in many branches of modern science, and they occupy a huge segment of the growing market for quantum devices. Quantum sensors use qubits and their analogs as detecting and analyzing quantum elements. Some sensors can be based on a single qubit, which is often presented as a system making its evolution on the so-called Bloch sphere. Different criteria are used to evaluate the efficiency of the sensing process. One of the most popular is the Quantum Fisher Information Matrix (QFIM) based on Fisher information. The magnitudes of the QFIM elements are strongly related to the precision of the sensing. As an analog of the classical Cramér theorem, one can define the quantum Cramér-Rao bound for the variance V, which is equal to V = 1/NF where F is the corresponding quantum Fisher information element, and N stands for the number of repeated sensory measurements. In this work, we develop our quantum Fisher information-based approach for a single feedback-driven qubit-type element for sensing external magnetic fields. We demonstrate the efficiency of our algorithm and discuss its further possible improvement. The approach developed here can be easily extended to other sensing schemes: collective spin systems and multi-qubit-based sensors. Alternative control algorithms can be applied to drive the probe state vector for maximization of the QFIM components. The particular choice of the control algorithm is defined by the specific experimental set-up.