Activating hidden metrological usefulness

TL;DR

This paper demonstrates that bipartite entangled states, which are not useful in linear interferometry alone, can be made more effective for quantum metrology through multiple copies or ancilla addition, revealing new advantages of entanglement.

Contribution

The authors introduce a general method to identify optimal local Hamiltonians for enhancing quantum metrological usefulness of entangled states.

Findings

All bipartite entangled pure states outperform separable states in metrology.

States previously considered useless can be activated with multiple copies or ancillas.

Analytic solutions for optimal Hamiltonians are provided for symmetric states.

Abstract

We consider bipartite entangled states that cannot outperform separable states in any linear interferometer. Then, we show that these states can still be more useful metrologically than separable states if several copies of the state are provided or an ancilla is added to the quantum system. We present a general method to find the local Hamiltonian for which a given quantum state performs the best compared to separable states. We obtain analytically the optimal Hamiltonian for some quantum states with a high symmetry. We show that all bipartite entangled pure states outperform separable states in metrology. Some potential applications of the results are also suggested.