  In microlensing of a Galactic star by a brown dwarf or other compact object,
the amplified image really consists of two unresolved images with slightly
different light-travel times. The difference (of order a microsecond) is GM/c^3
times a dimensionless factor depending on the total magnification. Since
magnification is well-measured in microlensing events, a single time-delay
measurement would provide the mass of the lens, without degeneracies. The
challenge is to find an observable that varies on sub-microsecond time scales.
This paper notes that the narrow-band intensity of the unresolved image pair
will show photon bunching (the Hanbury Brown and Twiss effect), and argues that
the lensed intensity will have an auto-correlation peak at the lensing time
delay. The ultrafast photon-counting technology needed for this type of
measurement exists, but the photon numbers required to give sufficient
signal-to-noise appear infeasible at present. Preliminary estimates suggest
time-delayed photon bunching may be measurable for lensed early-type
main-sequence stars at 10 kpc, with the help of 30 m-class telescopes.
