Information amplification via postselection: A parameter estimation perspective

TL;DR

This paper investigates whether weak measurement with postselection genuinely enhances information for parameter estimation, concluding that it does not reduce the fundamental estimation error bound despite amplifying measurement results.

Contribution

It clarifies that weak measurement with postselection does not improve the quantum Fisher information or lower the Cramér-Rao bound in parameter estimation.

Findings

Weak measurement amplifies measurement results but not information.

Weak measurement does not decrease the Cramér-Rao bound.

Quantum Fisher information remains unaffected by postselection.

Abstract

It is known that weak measurement can significantly amplify the mean of measurement results, sometimes out of the range limited in usual quantum measurement. This fact, as actively demonstrated recently in both theory and experiment, implies the possibility to estimate a very small parameter using the weak measurement technique. But does the weak measurement really bring about the increase of "information" for parameter estimation? This paper clarifies that, in a general situation, the answer is NO; more precisely, the weak measurement cannot further decrease the lower bound of the estimation error, i.e. the so-called Cram\'{e}r-Rao bound, which is proportional to the inverse of the quantum Fisher information.