BB84 Implementation and Computer Reality

Quantum Key Distribution (QKD) principles and software implementations are now well-known. Focussing on CASCADE algorithm, a major step of the key distillation algorithm (BB84), we show that the mathematical optimality may not mean practical efficiency because of the design of computers and their processors. As pointed out by John Backus , the father of FORTRAN, most of the work of the processor of a Von Neumann computer is dedicated to transfer to and from memory and the computer bus appears as a bottleneck. Thus, it is preferable to repeat simple computations than to randomly access the memory. This will impact the design of the permutations required by CASCADE. Moreover, computers are designed to work with bytes, not with bits. This will imply a reparametrization of CASCACDE different from the mathematically optimal parametrization.