A photon source capable of emitting, for example, a single photon or a single pair of photons on demand. The photon source may include an excitation region where a single instance of a quantum system is excited using excitation energy. A Stimulated Raman Adiabatic Passage (STIRAP) technique can be used for exciting the quantum system to a desired energy level. The photon source may include a photon emission region physically displaced from the excitation region. A transport device can be used for controllably moving an excited quantum system from the excitation region to the photon emission region. The photon emission region may include a resonant cavity tuned to the de-excitation frequency of the quantum system for inducing de-excitation of the quantum system and emission of a photon. The photon emission resonant cavity may be switchably coupled to an output port by a tunable resonant cavity coupling device.