Memory effects in device-dependent and device-independent cryptography

Abstract

An introductory note separating two kinds of memory effect in quantum cryptography. Memory attacks in the sense of Barrett, Colbeck and Kent require devices that retain state across protocol instances and are then reused. This note shows that memory effects within a single protocol instance, with no device reuse at all, are already enough to cause substantial difficulties in applying existing non-IID proof techniques, in device-dependent as well as device-independent QKD, so the assumption forbidding them is load-bearing even where it is usually waved through.