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Cybernetics and Physics

Control over performance of qubit-based sensors

Sergey Borisenok

Published Nov 30, 2018• 5 citations
Publisher

Abstract

The extreme sensitivity of quantum systems towards the external perturbations and in the same time their ability to be strongly coupled to the measured target field makes them to be stable under the environmental noise. A high quality quantum sensor can be engineered even on the platform of a single trapped qubit. There is a variety of optimal and sub-optimal algorithms for effective control over the quantum system states. Here we discuss the opportunity to improve the efficiency of the external field quantum sensor based on a single qubit via its feedback tracking.

DOI
10.35470/2226-4116-2018-7-3-93-95

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The extreme sensitivity of quantum systems towards the external perturbations and in the same time their ability to be strongly coupled to the measured target field makes them to be stable under the environmental noise. A high quality quantum sensor can be engineered even on the platform of a single trapped qubit. There is a variety of optimal and sub-optimal algorithms for effective control over the quantum system states. Here we discuss the opportunity to improve the efficiency of the external field quantum sensor based on a single qubit via its feedback tracking.
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.
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.
This paper proposes and analyzes an adaptive quantum feedback control system designed to optimize the performance of quantum devices. The core of the system lies in a real-time monitoring and adjustment mechanism utilizing a quantum sensor to measure the state of the quantum system. This measurement data is then fed back into a control algorithm, which dynamically adjusts the system parameters to achieve precise control. Unlike traditional control systems that are sensitive to parameter uncertainties, this adaptive approach leverages the inherent properties of quantum systems to enhance stability and performance. The theoretical framework presented here establishes a foundation for future research and development in adaptive quantum control, addressing key challenges in quantum device operation and manipulation. The system aims to provide a robust and flexible solution for controlling quantum systems with varying characteristics. The primary focus is on the control loop design and the feedback algorithm implementation, with an emphasis on achieving optimal performance under conditions of parameter uncertainty. This work contributes to the growing field of quantum control by introducing a novel adaptive strategy that can significantly improve the operational capabilities of quantum devices.
Quantum systems based on superconducting circuits with qubits with quantized energy levels serve as an efficient physical background for constructing different quantum devices, for instance, quantum sensors. The important class of such engineering devices belongs to the systems with quantum tunneling effects, like Josephson junctions, where the set of energy levels is influenced by the external fields. These fields change the phase properties of the measuring qubits, and, in this way, they can be detected. The weakly anharmonic oscillator is often referred as a transmon qubit. Such qubits are constructed with superconducting capacitor structures connected by Josephson junctions, they can be controlled by microwave pulses of a few nanoseconds. The quantum sensing mechanism studied here is based on coupling the measuring transmon superconducting qubits with magnetic materials. The sensing protocol of Ramsey Fringes interferometry covers several stages and demands high-quality control over the sensor coherence. Although the weakly nonlinear properties of transmons prevent the sensing procedure from the strong influence of external perturbations, nevertheless, the extension of the coherence time interval and minimization of the dephasing effects are still a matter of great importance. Here we discuss the improvement of the quantum sensor performance by the application of feedback control in the form of gradient descent algorithm over the dephasing factor-function.
Here we discuss as an important particular case the hybrid quantum electromagnetic sensor represented by a nitrogen-vacancy (NV) ensemble of N gt;gt; 1 centers in the doped diamond coupled to a high-quality factor dielectric resonator. The existing NV ensemble-based devices exhibit sensitivities that are several orders of magnitude away from theoretical limits. On the one hand, improvements in spin dephasing time, readout accuracy, and properties of the base diamond material can significantly improve the quality of operation of a quantum sensor. On the other hand, sensing optimization schemes also significantly affect its performance. We focus on the application of feedback to control the response in the cavity quantum electromagnetic sensor. We use the standard Tavis-Cummings model for the system and then introduce control over the target attractor method. In this model they are considered to be noninteracting two-level systems with transition frequencies wj, distributed inhomogeneously due to heterogeneous local magnetic and strain environments as well as hyperfine coupling with 14N nuclear spins. We adopted an operator form of closed-loop algorithm to the semi-classical Tavis-Cummings model in its ‘weak microwave drive’ limit. We used here target attractor Kolesnikov’s formulation of feedback to track the sensor response r or, alternatively, the reflected signal to the input drive field. In both cases, we achieve the tracking goal in the exponentially fast convergence.