Quantum Nonlocality in Weak-Thermal-Light Interferometry

TL;DR

This paper demonstrates that in low-photon flux thermal-light interferometry, nonlocal quantum measurements outperform local measurements, revealing a fundamental quantum nonlocality in classical optics experiments.

Contribution

It shows that nonlocal quantum measurements are fundamentally superior to local measurements for estimating mutual coherence in low-light conditions, highlighting a quantum nonlocality signature.

Findings

Nonlocal measurements outperform local ones in low-photon regimes.

Quantum nonlocality affects classical optics experiments.

Local detection methods are fundamentally limited by quantum mechanics.

Abstract

In astronomy, interferometry of light collected by separate telescopes is often performed by physically bringing the optical paths together in the form of Young's double-slit experiment. Optical loss severely limits the efficiency of this so-called direct detection method, motivating the fundamental question of whether one can achieve a comparable performance using separate optical measurements at the two telescopes before combining the measurement results. Using quantum mechanics and estimation theory, here I show that any such spatially local measurement scheme, such as heterodyne detection, is fundamentally inferior to coherently nonlocal measurements, such as direct detection, for estimating the mutual coherence of bipartite thermal light when the average photon flux is low. This surprising result reveals an overlooked signature of quantum nonlocality in a classic optics experiment.