The quantum relay and quantum signal source exploit quantum properties of photon streams. A pair of spatially separated and polarization-entangled photon streams is used. The pair collectively exist in a quantum superposition state by virtue of their mutual entanglement. In the relay, an encoder establishes a modulation control signal corresponding to the information to be conveyed. An optical quantum circuit is placed in the path of one of the pair of streams, so that the first stream passes through it. The optical quantum circuit alters the quantum polarization state of the photon passing through it based on the control signal. In this way information is encoded into quantum probability distributions of the superposition state through quantum parallelism and quantum interference, whereby information is conveyed in the photon streams. In the signal source a second information source is used to modulate the other of the pair of streams, thus enhancing the information density of the system, by encoding both in the probability distribution and in the correlation of the photon arrival times.