The Holevo Cram\'er-Rao bound is at most thrice the Helstrom version

TL;DR

This paper proves that the Holevo Cramér-Rao bound is at most three times larger than the Helstrom bound, indicating limited improvement and supporting the continued use of the simpler Helstrom bound in quantum multi-parameter estimation.

Contribution

The paper establishes an upper limit on the ratio between the Holevo and Helstrom bounds, clarifying the practical advantage of the Holevo bound in quantum metrology.

Findings

Holevo bound is at most three times the Helstrom bound

Helstrom bound remains a good approximation for multiple parameters

The ratio limit simplifies quantum parameter estimation analysis

Abstract

In quantum metrology, the Holevo Cram\'er-Rao bound has attracted renewed interest in recent years due to its superiority over the Helstrom Cram\'er-Rao bound and its asymptotic attainability for multi-parameter estimation. Its evaluation, however, is often much more difficult than that of the Helstrom version, calling into question the actual improvement offered by the Holevo CRB and whether it is worth the trouble. Here I prove that the Holevo bound is at most thrice the Helstrom version, so the improvement must be limited. The result also shows that the Helstrom version remains a pretty good bound even for multiple parameters and can be approached asymptotically to within a factor of 3.