Application of Optimal Band-limited Control Protocols to Quantum Noise Sensing

Nanoscale sensing techniques are in increasing demand across medical, metrological and industrial applications. In the context of quantum devices, the extreme sensitivity to the environment can in principle provide excellent sensor performance. However, common sensing protocols for qubit-based sensors, as implemented through control operations that manipulate their response in frequency space, suffer from out-of-band spectral leakage which complicates the interpretation of a sensor’s signal. We demonstrate a novel type of sensing protocols based on the provably optimal band-limited Slepian functions. Experiments with trapped ion qubits deploying this method reveal orders of magnitude improvement over conventional sensing protocols. Combining the narrowband control protocols with concepts from RF engineering to tune the frequency sensitivity band of our qubits, we demonstrate how complex coherent amplitude noise spectra can be reconstructed using classical post-processing techniques like multitaper and Bayesian spectrum estimation. Contributions to qubit infidelity due to noise in different axes are identified using tomographic measurements. Lastly, we apply these techniques to measure the intrinsic noise in our qubit-drive synthesis chain with sensitivities down to 0.001 dB.