Classical and quantum: some mutual clarifications

This paper presents two unconventional links between quantum and classical physics. The first link appears in the study of quantum cryptography. In the presence of a spy, the quantum correlations shared by Alice and Bob are imperfect. One can either process the quantum information, recover perfect correlations and finally measure the quantum systems; or, one can perform the measurements first and then process the classical information. These two procedures tolerate exactly the same error rate for a wide class of attacks by the spy. The second link is drawn between the quantum notions of ”no-cloning theorem” and ”weak-measurements with post-selection”, and simple experiments using classical polarized light and ordinary telecom devices.