Optimal Control for One-Qubit Quantum Sensing

F Poggiali,P Cappellaro,N Fabbri

doi:10.1103/physrevx.8.021059

Abstract

Quantum systems can be exquisite sensors thanks to their sensitivity to external perturbations. This same characteristic also makes them fragile to external noise. Quantum control can tackle the challenge of protecting quantum sensors from environmental noise, while leaving their strong coupling to the target field to be measured. As the compromise between these two conflicting requirements does not always have an intuitive solution, optimal control based on numerical search could prove very effective. Here we adapt optimal control theory to the quantum sensing scenario, by introducing a cost function that, unlike the usual fidelity of operation, correctly takes into account both the unknown field to be measured and the environmental noise. We experimentally implement this novel control paradigm using a Nitrogen Vacancy center in diamond, finding improved sensitivity to a broad set of time varying fields. The demonstrated robustness and efficiency of the numerical optimization, as well as the sensitivity advantaged it bestows, will prove beneficial to many quantum sensing applications.

Highlights

Quantum control has been demonstrated to be a crucial tool both in quantum information processing [1] and in quantum sensing [2,3] on a variety of experimental platforms, ranging from trapped ions [4,5] to ultracold atoms [6,7] and superconducting qubits [8,9], as well as nuclear [10,11] and electronic spin qubits [12,13]
Optimal control theory [14,15] exploits numerical optimization methods [16,17,18,19,20] to find the best control fields that steer the dynamics of a system towards the desired goal
We tackle the complex task of measuring multichromatic ac target fields and different significant waveforms, such as trains of magnetic impulses, which are relevant for applications in biology, physiology, and neuroscience [34,35,36,37]. In these cases, optimal control demonstrates better performance than traditional dynamical decoupling since it allows for both a larger accumulation of the spin phase that encodes the field information and for an improved compensation of environment-induced decoherence, boosting the qubit’s sensitivity and enabling detection of very weak magnetic fields

Summary

Introduction

Quantum control has been demonstrated to be a crucial tool both in quantum information processing [1] and in quantum sensing [2,3] on a variety of experimental platforms, ranging from trapped ions [4,5] to ultracold atoms [6,7] and superconducting qubits [8,9], as well as nuclear [10,11] and electronic spin qubits [12,13]. Quantum sensing poses peculiar challenges to control, as sensor qubits need to interact strongly with the target field to be probed, but this leads to undesired coupling with external noise of the same nature of the target field, which often gives rise to either energy losses or decoherence. A paradigmatic scenario is when one wants to measure a frequency shift of a spin qubit sensor, as due to a magnetic field, in the presence of magnetic dephasing noise.

Methods

Results

Conclusion