We choose the BB84 protocol, which is the first proposed QKD scheme and still the most widely used in practice.
BB84 assumes that the polarization of photons is used as a carrier of information, so Alice defines a random key with a length Q and uses a randomly selected polarization from the alphabet as a carrier of the key.
Efficiency is 25% for B92 and 50% for BB84. But the complexity order is increased to 4.
Two-Stage QKD Protocol (Stage 1: B92 and Stage 2: BB84).
This process is called deterministic one-step quantum key distribution using the
BB84 protocol.
Six-state protocol and BB84 protocol were generalised in case of using d-level quantum systems-qudits instead qubits (Cerf et al.
In figure 1 we can see that at low qudit dimension (up to d ~ 16) the protocol's security against noncoherent attack is higher when d+1 bases are used (when d = 2 it corresponds as noted above to greater security of six-state protocol than BB84 protocol).
The QKD protocol that they proposed uses four quantum states and is called BB84 (named after the Bennett-Brassard 1984 paper).
In this section we will review the BB84 and B92 QKD protocols and discuss the infrastructure that we have designed and implemented to support high-speed QKD and quantum networks.
The keys produced using QKD are guaranteed to be secret--as is proved by
BB84 protocol [7], [8] and may be used in conjunction with any classical cryptographic system (CCS).