Beating the Ramsey limit on sensing with continuous, deterministic qubit control (Part 1: Theory)

We present a protocol for unconditionally enhancing the sensitivity of a measurement of a qubit's frequency. We use a continuous, predetermined qubit drive to exactly cancel decoherence on one Bloch vector component, which preserves partial coherence. This leads to a larger signal in the orthogonal Bloch component, enhancing sensitivity to an out-of-plane field. We present a theoretical analysis of the protocol, demonstrating an unconditional sensitivity enhancement compared to Ramsey interferometry. Our protocol predicts improvements in sensitivity per measurement shot by up to a factor of 1.96 and sensitivity per total evolution time by up to a factor of 1.18. We also show that our protocol is robust to parameter miscalibrations. Additionally, our protocol requires no feedback and no extra control or measurement resources, and can be applied in a wide variety of quantum computing and quantum sensor technologies.