There is more to quantum interferometry than entanglement

TL;DR

This paper shows that quantum entanglement alone does not fully explain the sensitivity in quantum interferometry, highlighting the importance of broader quantum correlations beyond entanglement, supported by theoretical proofs and NMR experiments.

Contribution

It introduces a new interferometric resource measure called interferometric power, which captures quantum correlations beyond entanglement and proves its properties and invariance under certain operations.

Findings

Entanglement does not fully determine quantum interferometric sensitivity.

Interferometric power captures quantum correlations beyond entanglement.

Experimental NMR results confirm theoretical predictions.

Abstract

Entanglement has long stood as one of the characteristic features of quantum mechanics, yet recent developments have emphasized the importance of quantumness beyond entanglement for quantum foundations and technologies. We demonstrate that entanglement cannot entirely capture the worst-case sensitivity in quantum interferometry, when quantum probes are used to estimate the phase imprinted by a Hamiltonian, with fixed energy levels but variable eigenbasis, acting on one arm of an interferometer. This is shown by defining a bipartite entanglement monotone tailored to this interferometric setting and proving that it never exceeds the so-called interferometric power, a quantity which relies on more general quantum correlations beyond entanglement and captures the relevant resource. We then prove that the interferometric power can never increase when local commutativity-preserving operations are applied to qubit probes, an important step to validate such a quantity as a genuine quantum correlations monotone. These findings are accompanied by a room-temperature nuclear magnetic resonance experimental investigation, in which two-qubit states with extremal (maximal and minimal) interferometric power at fixed entanglement are produced and characterized.