A triggering device sensitive to a single photon is discussed. It is based on a balanced quantum nondemolition (QND) measurement proposed by Chuang and Yamamoto [Phys. Rev. Lett. 76, 4281 (1996)]. The balanced measurement measures the total photon number and obtains no which-path/mode information. Hence, the timing of the photon can be determined without destroying its wave function or entangling the probe field. This could have extensive use in the realization of long-distance quantum communications systems.
J. C. Howell and J. A. Yeazell, Nondestructive Single-Photon Trigger, Phys. Rev. A 62(3), 032311. https://doi.org/10.1103/PhysRevA.62.032311
American Physical Society