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1984

Quantum Cryptography

Charles H. Bennett (b. 1943), Gilles Brassard (b. 1955)

“In a 1984 paper, “Quantum Cryptography: Public Key Distribution and Coin Tossing,” physicists Charles Bennett and Gilles Brassard came up with a new approach for distributing cryptographic keys that relied on the fundamental laws of quantum physics, rather than on couriers carrying locked suitcases or the difficulty of factoring large numbers. Thus, unlike public key cryptography, quantum key distribution (QKD) is unaffected by advances in number theory, factoring, or quantum computers.

Bennett and Brassard described a protocol, now called BB84, for sending photons between two people, typically referred to as Alice and Bob. Alice, the sender, modulates the quantum state of each photon with a randomly generated bit using a randomly chosen measurement basis. Bob receives the photons and measures each, choosing his own random measurement basis for each. Bob and Alice now share all of their measurement bases: for the photons where Alice and Bob randomly chose the same bases—approximately 50 percent of the time—the encoded bits that Bob measured should match the bits that Alice sent. To find out, Bob tells Alice half of the values that he measured. If these match, then Bob and Alice use the second half of the bits as their cryptographic key.

What if there is a person in the middle, eavesdropping on the communications? If the attacker merely measures the photons, then under Heisenberg’s uncertainty principle, there’s a significant chance that each photon’s state may be changed, and the intrusion will be detected when Bob tells Alice his measurements.

Building a working system requires precise control over single photons. That wasn’t possible in 1999, when MagiQ Technologies, Inc.®, a Somerville, Massachusetts, startup, incorporated to build a working system. But it was possible four years later, when MagiQ announced the commercial availability of its equipment in 2003.

Since then, QKD and other approaches of using quantum mechanisms to secure data are increasingly catching the interest of financial institutions, research firms, and governments. In 2017, the Chinese government demonstrated quantum cryptography using an alternative protocol based on quantum entanglement.”

SEE ALSO: Vernam Cipher (1917), Quantum Computer Factors “15” (2001)

The eye of an observer reflected in a mirror in quantum cryptography apparatus. Photographed at the lab of Anton Zeilinger at Vienna University, Austria.