  We explore the application of heterodyne interferometry for a weak-field
coherent detection scheme. The methods detailed here will be used in ALPS II,
an experiment designed to search for weakly-interacting, sub-eV particles. For
ALPS II to reach its design sensitivity this detection system must be capable
of accurately measuring fields with equivalent amplitudes on the order of
10$^{-5}$ photons per second or greater. We present initial results of an
equivalent dark count rate on the order of $10^{-5}$ photons per second as well
as successful generation and detection of a signal with a field strength
equivalent to $10^{-2}$ photons per second.
