Quantum Metrology Assisted by Abstention

The main goal of quantum metrology is to obtain accurate values of physical parameters using quantum probes. In this context, we show that abstention, i.e., the possibility of getting an inconclusive answer at readout, can drastically improve the measurement precision and even lead to a change in its asymptotic behavior, from the shot-noise to the Heisenberg scaling. We focus on phase estimation and quantify the required amount of abstention for a given precision. We also develop analytical tools to obtain the asymptotic behavior of the precision and required rate of abstention for arbitrary pure states.

Figure 1

(a) Plot of $NS=2N(1-F)$ vs $Q$ (solid line) for an asymptotically large number $N$ of parallel spins, Eq. (13). The dots have been obtained by numerical optimization with $N=100$. (b) Profile of the transformed fiducial state $|{\tilde{\Psi }}_0⟩$ for $\lambda =1.5$ ($Q=0.56$). The thin (thick) line corresponds to $N=20$ ($N=80$). The points are obtained by numerical optimization. The dashed lines represent the function $\lambda \psi (t)$ that defines the primal feasibility condition $\phi (t)\le \lambda \psi (t)$, where $\psi (t)$ is in (14).