Beating the Ramsey limit on sensing with continuous, deterministic qubit control (Part 2: Experiment)

We present experimental results demonstrating a protocol for enhancing sensitivity of a measurement of qubit frequency. We achieve a significant improvement in both signal and signal per root evolution time compared to standard Ramsey interferometry with a superconducting transmon qubit. We use a continuous qubit drive to exactly cancel decoherence on one Bloch vector component and use the orthogonal Bloch component for sensing the qubit frequency. Compared to standard Ramsey interferometry, we achieve a boost in signal by a factor of 1.62 +/- 0.13, and improvement of signal per root evolution time by a factor of 1.11+/-0.32. Our protocol is robust to parameter miscalibrations and requires no feedback, extra control or measurement resources. Accordingly, our protocol is applicable in a wide variety of quantum computing and quantum sensor technologies.