Wikipedia / BB84
Wikipedia / BB84
The BB84 protocol, named after its inventors Charles Bennett and Gilles Brassard in 1984, is a prepare-and-measure quantum key distribution (QKD) protocol, in which, one party (e.g. Alice) performs the encoding by preparing the quantum states, and the other party (e.g., Bob) measures them. The BB84 QKD scheme is the first quantum cryptography protocol, and has become one of the most well-studied QKD protocols. The protocol is provably secure assuming a perfect implementation, relying on two conditions: (1) the quantum property that information gain is only possible at the expense of disturbing the signal if the two states one is trying to distinguish are not orthogonal (see no-cloning theorem); and (2) the existence of an authenticated public classical channel. As such, the security of the BB84 protocol is fundamentally based on the principle that two non-orthogonal quantum states cannot be perfectly distinguished. This inherent limitation means that the states cannot be reliably copied, thereby ensuring a robust framework for secure quantum communication. The BB84 QKD protocol is usually explained as a method of securely communicating a private key from one party to another for use in one-time pad encryption. The proof of BB84 QKD scheme depends on a perfect implementation. Side channel attacks exist, taking advantage of non-quantum sources of information. Since this information is non-quantum, it can be intercepted without measuring or cloning quantum particles. The BB84 protocol provides a significant advance in the field of quantum cryptography and represents a pioneering step toward achieving secure communication in the quantum era.
Overview
BB84 QKD system transmits individual photons through a fiber optic cable, with each photon representing a bit of data (zero or one). Polarizing filters on the sender's side set each photon's orientation, while the receiver uses beam splitters to read it. The sender and receiver then compare their photon orientations, with the matching set becoming the cryptographic key. However, encoding with other degrees of freedom, e.g., phase, is also possible, and the procedures are similar.
Description
In the BB84 scheme, Alice wishes to send a private key to Bob. She begins with two strings of bits-qubit state written as:
give us an index into the following four qubit states:
.
over a public and authenticated quantum channel, it makes it virtually impossible for either Bob or Eve to distinguish the states of the qubits. Also, after Bob has received the qubits, we know that Eve cannot be in possession of a copy of the qubits sent to Bob, by the no-cloning theorem, unless she has made measurements. Her measurements, however, risk disturbing a particular qubit with probability if she guesses the wrong basis.
do not match.
bits and discloses her choices over the public channel. Both Alice and Bob announce these bits publicly and run a check to see whether more than a certain number of them agree. If this check passes, Alice and Bob proceed to use information reconciliation and privacy amplification techniques to create some number of shared secret keys. Otherwise, they cancel and start over.
See also
E91 – quantum cryptographic communication protocol
References