Tomography increases key rates of quantum-key-distribution protocols

We construct a practically implementable classical processing for the Bennett-Brassard 1984 (BB84) protocol and the six-state protocol that fully utilizes the accurate channel estimation method, which is also known as the quantum tomography. Our proposed processing yields at least as high a key rate as the standard processing by Shor and Preskill. We show two examples of quantum channels over which the key rate of our proposed processing is strictly higher than the standard processing. In the second example, the BB84 protocol with our proposed processing yields a positive key rate even though the so-called error rate is higher than the 25% limit.

Figure 1

(Color online) Comparison of the key rates against the parameter $p$ of the amplitude damping channel (see Eq. (18)). “Reverse” and “Direct” are the key rates when we use the reverse reconciliation and the direct reconciliation in our proposed classical processing respectively. “Conventional six-state” and “Conventional BB84” are the key rates of the six-state protocol and the BB84 protocol with the conventional classical processing. Note that the conventional classical processing involves the noisy preprocessing [14, 16].