Cryptographic tasks and protocols using quantum systems. Quantum key distribution is one application; it should not be confused with post-quantum cryptography, which studies classical schemes intended to resist quantum computation.
A verification scheme using quantum states as part of a banknote or token. Secret-basis qubits can make extra copies hard to produce because their encodings are nonorthogonal. Security claims must specify the adversary's information, access to original tokens and the verification procedure; the no-cloning theorem alone does not fix every forging probability.
If a known classical bit is independently encoded in a secret uniformly chosen X or Z eigenbasis, the averaged verification operator is . Its largest eigenvalue is the displayed optimal acceptance probability for a counterfeit prepared without the original qubit. The maximizing state bisects the two candidate Bloch directions. For independent secret bases, the maximal all-pass probability is the th power, because the total acceptance operator is a tensor product. Randomly choosing one of the legitimate encodings instead passes with probability per bit.
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Quantum cryptography is a cutting-edge field of cryptography that leverages the principles of quantum mechanics to provide secure communication that is theoretically immune to eavesdropping. The main feature of quantum cryptography is its use of quantum bits, or qubits, which can exist in multiple states simultaneously due to the phenomenon of superposition.