Quantum states with a positive partial transpose are useful for metrology

TL;DR

This paper demonstrates that certain multipartite quantum states with positive partial transpose can surpass separable states in precision for linear interferometry, challenging the notion that Bell inequality violation is necessary for quantum advantage.

Contribution

It introduces a new iterative method to identify PPT states useful for quantum metrology and explores their properties and robustness.

Findings

PPT states can outperform separable states in metrology.

Some PPT states are highly robust to noise.

Metrological usefulness does not require Bell inequality violation.

Abstract

We show that multipartite quantum states that have a positive partial transpose with respect to all bipartitions of the particles can outperform separable states in linear interferometers. We introduce a powerful iterative method to find such states. We present some examples for multipartite states and examine the scaling of the precision with the particle number. Some bipartite examples are also shown that possess an entanglement very robust to noise. We also discuss the relation of metrological usefulness to Bell inequality violation. We find that quantum states that do not violate any Bell inequality can outperform separable states metrologically. We present such states with a positive partial transpose, as well as with a non-positive positive partial transpose.