Coherent Detection of Ultra-weak Electromagnetic Fields

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.