Quantum Cryptography Protocols for Information Security

Information Representation With Quantum States

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.