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

Abstract

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.

Nonvanishing effect of detuning errors in dynamical-decoupling-based quantum sensing experiments

“…For instance, this model estimates T * 2n = 24±7 µs and d NV = 11.8±0.3 nm from the ∆/2π = +2 MHz data, against T * 2n = 12 ± 1 µs and d NV = 8.9 ± 0.1 nm at ∆ = 0. We stress that such detunings are sometimes unavoidable, due to inhomogeneous broadening (a 2 MHz ODMR linewidth is not uncommon in dense layers of near-surface NV centres [52]) or hyperfine shifts, and as such this motivates the inclusion of detuning in the analysis of NMR data, or the development of pulse sequences that are less sensitive to detuning [41,60,61].…”

Non-Gaussian noise spectroscopy with a superconducting qubit sensor

“…Quantum noise spectroscopy (QNS) leverages the fact that open-loop control modulation is akin to shaping the filter function that determines the sensor's response in frequency space [8][9][10][11][12] and, in it simplest form, aims to characterize the spectral properties of environmental noise as sensed by a single qubit sensor. By now, QNS protocols employing both pulsed and continuous control modalities have been explored, and experimental implementations have been reported across a wide variety of qubit platforms -including NMR [13], superconducting quantum circuits [14][15][16][17], semiconductor quantum dots [18][19][20][21], diamond nitrogen vacancy centers [22,23], and trapped ions [24]. Notably, knowledge of the underlying noise spectrum has already enabled unprecedented coherence times to be achieved via tailored error suppression [25].…”

Measuring Environmental Quantum Noise Exhibiting a Nonmonotonic Spectral Shape

“…We briefly compare our work to three other recent papers that we have become aware of during the preparation of this manuscript [39][40][41]. In [39] adaptive XY-n sequence (AXY-n) are introduced, which are non-equallyspaced XY-n sequences.…”