Microlensing and Photon Bunching: The impact of decoherence

TL;DR

This paper investigates how decoherence affects photon bunching measurements in gravitational microlensing, concluding that decoherence generally destroys the correlation signals needed to measure lensing time delays.

Contribution

It demonstrates that decoherence from extended sources prevents using photon bunching correlations to analyze microlensing time delays.

Findings

Decoherence erases photon bunching correlations in typical microlensing scenarios.

Long effective baselines at the lens plane are impractical for celestial objects with current technology.

Photon bunching signals are unlikely to be useful for measuring microlensing time delays in most cases.

Abstract

Gravitational microlensing within the Galaxy offers the prospect of probing the details of distant stellar sources, as well as revealing the distribution of compact (and potentially non-luminous) masses along the line-of-sight. Recently, it has been suggested that additional constraints on the lensing properties can be determined through the measurement of the time delay between images through the correlation of the bunching of photon arrival times; an application of the Hanbury-Brown Twiss effect. In this paper, we revisit this analysis, examining the impact of decoherence of the radiation from a spatially extended source along the multiple paths to an observer. The result is that, for physically reasonable situations, such decoherence completely erases any correlation that could otherwise be used to measure the gravitational lensing time delay. Indeed, the divergent light paths traverse extremely long effective baselines at the lens plane, corresponding to extremes of angular resolving power well beyond those attainable with any terrestrial technologies; the drawback being that few conceivable celestial objects would be sufficiently compact with high enough surface brightness to yield usable signals.