  We prepare heralded single photons from a photon pair source based on
non-degenerate four-wave mixing in a cold atomic ensemble via a cascade decay
scheme. Their statistics shows strong antibunching with g(2)(0) < 0.03,
indicating a near single photon character. In an optical homodyne experiment,
we directly measure the temporal envelope of these photons and find, depending
on the heralding scheme, an exponentially decaying or rising profile. The
rising envelope will be useful for efficient interaction between single photons
and microscopic systems like single atoms and molecules. At the same time,
their observation illustrates the breakdown of a realistic interpretation of
the heralding process in terms of defining an initial condition of a physical
system.
