BB84
Introduction
The BB84 protocol, named after its inventors Charles Bennett and Gilles Brassard and the year it was proposed (1984), is a quantum key distribution scheme. It is one of the first and most well-known protocols in the field of quantum cryptography, which aims to provide secure communication by leveraging the principles of quantum mechanics.
Background
Quantum cryptography, and by extension the BB84 protocol, is a response to the vulnerabilities inherent in classical cryptographic systems. Classical systems often rely on the computational difficulty of certain problems (such as the factoring of large prime numbers) for their security. However, with the advent of quantum computers, these problems could potentially be solved much more quickly, rendering classical cryptographic systems insecure. Quantum cryptography, and the BB84 protocol, aim to provide a solution to this problem by using the principles of quantum mechanics to ensure the security of a cryptographic key.
Principles of Operation
The BB84 protocol operates on the principles of quantum mechanics, specifically the Heisenberg uncertainty principle and the no-cloning theorem. The uncertainty principle states that certain pairs of physical properties, such as position and momentum, cannot both be accurately and simultaneously measured. The no-cloning theorem states that it is impossible to create an identical copy of an arbitrary unknown quantum state. These two principles form the basis of the security of the BB84 protocol.
In the BB84 protocol, two parties, usually referred to as Alice and Bob, wish to share a secret key. Alice begins by randomly selecting a string of bits and a corresponding string of bases (either rectilinear or diagonal). She then prepares a string of quantum bits (or qubits) according to these strings and sends them over a quantum channel to Bob. Bob, upon receiving these qubits, measures them in a basis he chooses randomly for each qubit.
After Bob has made his measurements, Alice and Bob communicate over a classical channel to compare their bases. For each bit where Bob chose the same basis as Alice, his measurement will match Alice's original bit with high probability. These matching bits form the raw key.
Security of the BB84 Protocol
The security of the BB84 protocol comes from the principles of quantum mechanics mentioned above. If an eavesdropper, usually referred to as Eve, tries to intercept the qubits sent by Alice, she must measure them. However, due to the uncertainty principle, any measurement she makes will disturb the qubits, and this disturbance can be detected by Alice and Bob.
Furthermore, due to the no-cloning theorem, Eve cannot make a perfect copy of the qubits to measure without disturbing the original qubits. This means that any successful eavesdropping attempt will leave detectable traces, allowing Alice and Bob to know if their key has been compromised.
Practical Implementation and Challenges
While the BB84 protocol is theoretically secure, practical implementation presents several challenges. These include the need for a reliable quantum channel, the practical difficulty of preparing and measuring single qubits, and the potential for loss and errors in transmission.
Despite these challenges, there have been several successful implementations of the BB84 protocol. These implementations have used various technologies, including photons for the qubits and fiber optic cables or free space for the quantum channel.