Optimal control of a quantum sensor: from an analytic solution to a fast algorithm

Quantum sensors can show unprecedented sensitivities, provided they are controlled in a very specific, optimal way. In this talk, I will consider a spin sensor of time-varying fields in the presence of dephasing noise, and show that the problem of finding the optimal pulsed control field can be mapped to the determination of the ground state of a spin chain. I will discuss an approximate but analytic solution of this problem: it provides a lower bound for the sensor sensitivity, and a pulsed control sequence very close to optimal. Such sequence can be further improved via a fast simulated annealing algorithm. I will finally show a comparison with experimental data, to demonstrate the sensitivity improvement for a spin-qubit magnetometer based on a nitrogen-vacancy (NV) center in diamond.