Quantum key distribution (QKD) is a cryptographic method for two parties to establish a shared secret key over an insecure communication channel. The security of QKD is based on the principles of quantum mechanics, specifically the no-cloning theorem and the uncertainty principle.
The BB84 protocol is the first and most well-known QKD protocol. It was proposed by Charles Bennett and Gilles Brassard in 1984. The BB84 protocol works by using photons to encode the secret key. The photons are sent through an insecure channel, and the two parties, Alice and Bob, measure them using different bases.
If Alice and Bob have chosen the same basis, they will be able to decode the photons and share the secret key. However, if they have chosen different bases, the eavesdropper, Eve, will not be able to decode the photons and will not be able to learn the secret key.
The BB84 protocol
The BB84 protocol works as follows:
- Alice prepares a stream of photons, each of which is in one of two polarization states: horizontal (H) or vertical (V).
- Alice sends the photons to Bob through an insecure channel.
- Bob randomly chooses a basis, either H or V, and measures each photon.
- Alice and Bob publicly announce the bases they used.
- Alice and Bob keep the photons that were measured in the same basis as they announced. They discard the photons that were measured in the other basis.
- Alice and Bob now have a shared secret key, which is the string of bits corresponding to the photons they kept.
Security of the BB84 protocol
The security of the BB84 protocol is based on the no-cloning theorem and the uncertainty principle. The no-cloning theorem states that it is impossible to create a perfect copy of an unknown quantum state. This means that Eve cannot simply copy the photons sent by Alice and learn the secret key.
The uncertainty principle states that it is impossible to measure both the position and momentum of a particle with perfect accuracy. This means that if Eve tries to measure the polarization of the photons, she will inevitably disturb them. This disturbance will be detectable by Alice and Bob, and they will know that Eve has been eavesdropping.
Variants of the BB84 protocol
There are many variants of the BB84 protocol. Some of the most common variants are:
- The B92 protocol: This protocol uses only one polarization state, which makes it less susceptible to errors.
- The SARG04 protocol: This protocol uses four polarization states, which makes it more secure against certain attacks.
- The decoy-state protocol: This protocol adds decoy photons to the signal photons, which makes it more difficult for Eve to eavesdrop.
Applications of QKD
QKD has a number of potential applications, including:
- Secure communication: QKD can be used to create secure communication channels that are resistant to eavesdropping. This could be used for government and military communications, as well as for financial transactions.
- Quantum cryptography: QKD can be used to create new cryptographic protocols that are more secure than traditional cryptographic protocols.
- Quantum computing: QKD can be used to distribute quantum states between different parts of a quantum computer. This could be used to improve the performance of quantum computers.
QKD is a promising technology with a wide range of potential applications. The security of QKD is based on the principles of quantum mechanics, which makes it resistant to conventional attacks. However, QKD is still a relatively new technology, and there are a number of challenges that need to be addressed before it can be widely deployed.