Figure 1. Secret key and reconciled key throughput for fixed-rate reconciliation with three LDPC codes of rates R = 0.67, R = 0.75, and R = 0.83.

Raw key throughput is for the BB84 protocol. Asymptotic secret key throughput is calculated assuming perfect reconciliation. Reconciled key throughput is constant while the code is correcting properly, since a constant number of decoding iterations are used without syndrome validation, and it starts to decrease as soon as the FER approaches 1 (i.e. errors grow beyond the capability of the code to correct them). This happens first for higher rate codes. The distance limit for the secret key throughput is higher for low rate codes since they have more redundancy. For the same reason throughput at lower distances is also smaller for low rate codes. Secret key throughput with low and high rate codes cross when the degradation due to increased FER in the higher rate code equals the information leakage in the lower rate curve.