For questions related to quantum cryptography protocol BB84.

BB84 is a quantum key distribution protocol for quantum cryptography invented by Charles Bennett and Gilles Brassard in 1984. It is used for safe random key distribution between two parties. It also allows to detect an eavesdropper trying to catch the key.

The protocol works as follows:

Alice generates a qubit in one of 4 states $\{|1\rangle, |0\rangle, |+\rangle, |-\rangle \}$ and sends it to Bob. The states $|1\rangle$ and $|+\rangle$ mean bit $1$ and the states $|0\rangle$ and $|-\rangle$ mean bit $0$. Upon receiving the qubit Bob measures it in $\{0,1\}$ basis or in $\{-,+\}$ basis, chosen randomly. If Bob measured in say $\{0,1\}$ basis and Alice sent $|0\rangle$ or $|1\rangle$ qubit, then Alice and Bob shared 1 bit of a secret key. What remains is to check that Bob used the right basis. This is done by using an open classical channel, for example Bob sends information which basis he used and Alice replies (also in open) is this basis right or not. If the basis is wrong, the bit is discarded.

More information can be found in the original paper Quantum cryptography: Public key distribution and coin tossing or on Wikipedia.