Cryptographic quantum key distribution (QKD) can be utilized for generating and transmitting and/or receiving cryptographic keys with assured secrecy. QKD is based on the quantum nature of light, and any attempts to intercept and clone the quantum key information encoded in transmitted photons is forbidden due to the linearity of quantum mechanics and the Heisenberg uncertainty principle. In QKD, the quantum state of a photon may be associated with qubits (0 or 1), and these qubits can be used to form the bits associated with a cryptographic key.
Photonic QKD may take place over fiber optic networks, through free-space optical transmission, or through any other network capable of relaying photons. For free-space transmission, at least two telescopes are needed: one to transmit the QKD signal and one to receive the QKD signal. The sizes of these telescopes are determined by a number of parameters, including transmission distance, desired signal-to-noise ratio, chosen optical wavelength, and atmospheric conditions. Due to a number of these factors, including the size and weight of the QKD telescope, QKD has been precluded from use in satellite communications systems.