  Expansion dynamics at the yoctosecond timescale affect the evolution of the
quark gluon plasma (QGP) created in heavy ion collisions. We show how these
dynamics are accessible through Hanbury Brown and Twiss (HBT) intensity
interferometry of direct photons emitted from the interior of the QGP. A
detector placed close to the beam axis is particularly sensitive to early polar
momentum anisotropies of the QGP. Observing a modification of the HBT signal at
the proposed FoCal detector of the LHC ALICE experiment would allow to measure
the isotropization time of the plasma and could provide first experimental
evidence for photon double pulses at the yoctosecond timescale.
