Fig. 1: Experimental setting.
a Ramsey interferometry experiment for the estimation of an unknown phase parameter β. The quantum system is prepared in an initial resource state \(\left|{\psi }_{\theta }(0)\right\rangle\), the evolution of which results in a phase parameter β. The projective measurement on the final state \(\left|{\psi }_{\theta }(\beta )\right\rangle\) allows to determine the value of the parameter β. b The QFI of the final state \(\left|{\psi }_{\theta }(\beta )\right\rangle\) reveals the information content relative to the unknown phase parameter β. The larger QFI (right) implies the better distinguishability between the states \(\left|{\psi }_{\theta }(\beta )\right\rangle\) and \(\left|{\psi }_{\theta }(\beta +d\beta )\right\rangle\) that have an infinitesimal parametric difference dβ → 0. c The energy level structure of the NV center spin in diamond under an external magnetic field. The two-level quantum system is encoded by the ground state spin sublevels ms = 0, −1. d Rabi oscillations: the population in the spin state ms = 0 as a function of time, which facilitates efficient coherent control of the NV center spin state.
