  The phase space structure of certain quantum states reveals structure on a
scale that is small compared to the Planck area. Using an analog between the
wavefunction of a single photon and the electric field of a classical
ultrashort optical pulse we show that spectral shearing interferometry enables
measurement of such structures directly. Thereby extending the idea of
Praxmeyer et al. In particular, we use multiple-shear spectral interferometry
to fully characterize a pulse consisting of two sub-pulses which are temporally
and spectrally disjoint, without a relative-phase ambiguity. This enables us to
compute the Wigner distribution of the pulse. This spectrographic
representation of the pulse field features fringes that are tilted with respect
to both the time- and frequency axes, showing that in general the shortest
sub-Planck distances may not be in the directions of the canonical (and easily
experimentally accessible) directions. Further, independent of this
orientation, evidence of the sub-Planck scale of the structure maybe extracted
directly from the measured signal.
