1. An apparatus for distributing a sequence of symbols between an emitter station and a receiver station connected by a quantum channel and a conventional channel, wherein the apparatus assesses the amount of information an eavesdropper having access to both of the channels can have obtained on the sequence, the apparatus comprising an emitter station having a control means so as to enable the emitter station to command and control its own components in cooperation with associated components of the receiver station and a quantum source for generating a stream of quantum systems, wherein the quantum systems have a coherent phase relationship between systems located in nearby positions of the stream, wherein some of the quantum systems are prepared in a quantum state belonging to a first set of quantum states, this set comprising at least two non-orthogonal quantum states, and the quantum states of this first set being associated with symbol values, and wherein the emitter station is adapted to produce some of the quantum systems in a quantum state belonging to a second set of quantum states, this second set comprising at least one state, the states of this second set being non-orthogonal to some of the states of the first set, the states of this second set not being a superposition of some of the states of the first set, the states of this second set being selected in such a way that they are perturbed by a measurement adapted, when applied to a quantum system prepared in a state belonging to the first set, to determine at least in some cases in which state this quantum system was prepared.