Two interferometric quantum-nondemolition (QND) devices have been proposed: (1) a polarization- independent device and (2) a polarization- preserving device. The prolarization- independent device works on an input state of up to two photons, whereas the polarization- preserving device works on a superposition of vacuum and single-photon states. The overall function of the device would be to probabilistically generate a unique detector output only when its input electromagnetic mode was populated by a single photon, in which case its output mode would also be populated by a single photon.
Like other QND devices, the proposed devices are potentially useful for a variety of applications, including such areas of NASA interest as quantum computing, quantum communication, detection of gravity waves, as well as pedagogical demonstrations of the quantum nature of light. Many protocols in quantum computation and quantum communication require the possibility of detecting a photon without destroying it. The only prior single-photon-detecting QND device is based on quantum electrodynamics in a resonant cavity and, as such, it depends on the photon frequency. Moreover, the prior device can distinguish only between one photon and no photon. The proposed interferometric QND devices would not depend on frequency and could distinguish between (a) one photon and (b) zero or two photons.
The first proposed device is depicted schematically in Figure 1. The input electromagnetic mode would be a superposition of a zero-, a one-, and a two-photon quantum state. The overall function of the device would be to probabilistically generate a unique detector output only when its input electromagnetic mode was populated by a single photon, in which case its output mode also would be populated by a single photon.
The input mode would first be divided by a 50:50 beam splitter. The two resulting modes would be directed into two separate Mach-Zehnder (MZ) interferometers, each of which would contain elements that would produce a phase shift of π/2 radians between its two arms. At the same time, single-photon probes would enter through secondary input ports of the MZ interferometers. Whenever a single-input photon entered through the primary port of either interferometer, the single-photon probe would effect a relative-phase change. The primary outputs of the MZ interferometers would be mixed in a beam splitter and detected by photodetectors D1 and D2. A coincidence in the outputs of these photodetectors could signal the presence of one or two photons in the input mode. The interferometers would be balanced in such a way that a vacuum input could not result in a coincidence in detectors D1 and D2.