Application of optimal band-limited control protocols to quantum noise sensing

• Published in: Nature communications Vol. 8; no. 1; pp. 2189 - 8
• Main Authors: Frey, V. M., Mavadia, S., Norris, L. M., de Ferranti, W., Lucarelli, D., Viola, L., Biercuk, M. J.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 19.12.2017; Nature Publishing Group; Nature Portfolio
• Subjects: 639/766/483/1255; 639/766/483/2802; 639/925/927/511; Bayesian analysis; Humanities and Social Sciences; Leakage; multidisciplinary; Narrowband; Noise; Noise control; Noise spectra; Qubits (quantum computing); Routines; Science; Science (multidisciplinary); Spectral analysis
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-017-02298-2

Essential to the functionality of qubit-based sensors are control protocols, which shape their response in frequency space. However, in common control routines out-of-band spectral leakage complicates interpretation of the sensor’s signal. In this work, we leverage discrete prolate spheroidal sequences (a.k.a. Slepian sequences) to synthesize provably optimal narrowband controls ideally suited to spectral estimation of a qubit’s noisy environment. Experiments with trapped ions demonstrate how spectral leakage may be reduced by orders of magnitude over conventional controls when a near resonant driving field is modulated by Slepians, and how the desired narrowband sensitivity may be tuned using concepts from RF engineering. We demonstrate that classical multitaper techniques for spectral analysis can be ported to the quantum domain and combined with Bayesian estimation tools to experimentally reconstruct complex noise spectra. We then deploy these techniques to identify previously immeasurable frequency-resolved amplitude noise in our qubit’s microwave synthesis chain. Control of qubits’ frequency response by dynamical decoupling is usually vexed by control’s out-of-band harmonics, a problem known in metrology as “spectral leakage”. Here, the authors reduce this problem by orders of magnitude exploiting discrete prolate spheroidal sequences to control a trapped-ion qubit.