Quantum cryptography is a method of encryption using naturally occurring quantum mechanical properties to secure and transmit data in a way that cannot be hacked. In contrast to traditional public-key cryptography, which relies on the computational difficulty of specific mathematical functions, quantum cryptography relies on the foundations of quantum mechanics to alert users to eavesdropping during the communication process.
Quantum cryptography transfers information encoded into individual particles of light (photons), each representing a binary digit (bit). Quantum cryptography systems are secure against compromise from any attacker who doesn't have the knowledge of the sender/receiver. It is impossible to copy or view data encoded in a quantum state without affecting it and alerting users. It is predicted quantum cryptography will also remain safe against attacks from quantum computers. Properties of quantum mechanics guaranteeing security include:
- particles can exist in more than one place or state at a time;
- a quantum property cannot be observed without changing or disturbing it; and
- whole particles cannot be copied.
One of the best-known examples of quantum cryptography is quantum key distribution (QKD), which provides a secure method for key exchange.
Stephen Wiesner, a scientist working at Columbia University in New York City, is credited with introducing the world to quantum cryptography in 1968 after submitting a paper titled "Conjugate Coding" to the journal IEEE Information Theory. Wiesner's paper was rejected by IEEE Information theory but would later be published by SIGACT News in 1983. In his paper, Wiesner successfully demonstrated how to store or transmit two quantum encoded messages. Wiesner showed messages can be encoded using two conjugate observables, such as linear and circular polarized light, in a way that allows either one of the messages to be received and decoded, but not both messages. Wiesner is also credited with coming up with the concepts of quantum money and quantum conjugate coding.
In 1984, Charles H. Bennett and Gilles Brassard proposed the first QKD protocol, known as BB84. The protocol describes encoding digital information using polarized photons where it is theoretically impossible to intercept the information without a high probability of disturbing the transmission in a detectable way. In 1991, Artur Ekert introduces a quantum cryptography protocol, known as E91, based on Bell's theorem.
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