Quantum states are energy levels of molecules, atoms, and photons. Two quantum states for which a state transition exists can be used to represent an information bit, if the energy levels of both states can be measured. A bit defined by quantum states is called a quantum bit or qubit. However, quantum states are probabilistic. When the energy level of a molecule, an atom, or a photon is measured, the outcome is one of all possible energy levels, and each possible outcome is associated with a probability. The sum of the probabilities of all possible measurement outcomes is, of course, 1. A qubit is thus also probabilistic. The binary values 0 and 1 are represented by two possible quantum states of a qubit. If the measurement probabilities of these two quantum states are p0 and p1, then p0+p1=1.
Raw Key: Two parties agree on a shared binary secret from a sequence of randomly polarized photons.
Avalanche Photodiode: A photodetector which can be regarded as the semiconductor analog to a photomultiplier.
QPN: A registered abbreviation for Quantum Private Network, which is a VPN link where session key agreement is based on a QKD protocol.
Polarization: The plane in which an electromagnetic wave propagates.
Photon: A discrete packet of electromagnetic energy.
Teleportation: Transfer of an unknown qubit state.
Quantum Gate: A device for changing the state of one or of multiple qubits.