  A fundamental property of a three-dimensional Bose-Einstein condensate (BEC)
is long-range coherence, however, in systems of lower dimensionality, not only
is the long range coherence destroyed, but additional states of matter are
predicted to exist. One such state is a `transverse condensate', first
predicted by van Druten and Ketterle [Phys. Rev. Lett. 79, 549 (1997)], in
which the gas condenses in the transverse dimensions of a highly anisotropic
trap while remaining thermal in the longitudinal dimension. Here we detect the
transition from a three-dimensional thermal gas to a gas undergoing transverse
condensation by probing Hanbury Brown--Twiss correlations.
