Agnostic phase estimation for quantum sensing and quantum computation

Phase estimation is crucial to quantum-information processing. Several quantum algorithms use phase estimation as a subroutine for finding unitary operators’ eigenvalues. Furthermore, phase estimation is used in quantum metrology, the field of using quantum systems to probe, measure, and estimate unknown physical parameters. I will tackle the problem of how to measure the strength of a totally unknown unitary. Imagine a unitary rotation generated by a magnetic field. If the direction of the magnetic field is unknown, no strategy with classically separable probes enables optimal measurements of the rotation angle. However, a quantum strategy, relying on entanglement, does. Our strategy leverages quantum simulations of hypothetical closed timelike curves (CTCs) to overcome limitations in phase estimation. Colleagues and I demonstrate this on a superconducting processor. I will describe how our technique can be used in tasks ranging from sensing to quantum-computer calibration. Further, I will discuss extensions and applications of our agnostic phase-estimation protocol. [1] PhysRevLett.132.260801 [2] PhysRevLett.131.150202