Comment on J.Qin et al., Unconditional and Robust Quantum Metrological Advantage beyond N00N States, PRL 130, 070801 (2023) and J.A. H. Nielsen et.al., Deterministic Quantum Phase Estimation beyond N00N States. PRL 130, 123603 (2023)

Abstract

The papers by Qin et al. [1] and Nielsen et al. [2] report an unconditional quantum metrological advantage based on squeezed-vacuum phase sensing. The principal evidence for this claim is the Quantum Fisher Information (QFI) evaluated per trial or per detected photon, which is subsequently interpreted as a measure of metrological performance. This interpretation is statistically unjustified because it does not account for the total experimental resources required to construct an estimator. Consequently, the reported protocols do not provide an unconditional metrological advantage over classical phase-sensing strategies when resource accounting is performed consistently.