Microlensing and Photon Bunching: The impact of decoherence
  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.
