Noise-dependent optimal strategies for quantum metrology

TL;DR

This paper investigates how optimal quantum metrology strategies vary with noise levels, revealing a crossover from entangled probes to ancilla-assisted methods due to noise robustness differences.

Contribution

It demonstrates that the optimal strategy in noisy quantum metrology depends on noise level, highlighting a transition between entangled and ancilla-assisted approaches.

Findings

Separable states outperform entangled probes at high noise levels.

Ancilla-assisted strategies are optimal in certain noisy regimes.

There is a non-trivial crossover point between strategies.

Abstract

We show that for some noisy channels, the optimal entanglement-assisted strategy depends on the noise level. We note that there is a non-trivial crossover between the parallel-entangled strategy and the ancilla-assisted strategy - in the former the probes are all entangled, the latter the probes are entangled with a noiseless ancilla but not amongst themselves. The transition can be explained by the fact that, separable states are more robust against noise and therefore are optimal in the high noise limit, but they are in turn outperformed by ancilla-assisted ones.