  Precise information about the temporal mode of optical states is crucial for
optimizing their interaction efficiency between themselves and/or with matter
in various quantum communication devices. Here we propose and experimentally
demonstrate a method of determining both the real and imaginary components of a
single photon's temporal density matrix by measuring the autocorrelation
function of the photocurrent from a balanced homodyne detector at multiple
local oscillator frequencies. We test our method on single photons heralded
from biphotons generated via four-wave mixing in an atomic vapor and obtain
excellent agreement with theoretical predictions for several settings.
