ements is independent of their sequence order, as is an easy exercise to verify. In this perspective, the EPR protocol is not a matter of mere key "exchange," since, in fact, the key remains fully undetermined until both measurements from Alice and Bob have been duly performed, results compared, and valid cbits finally identified. In view of making an inventory of common secret bits, by sharing measurement bases over a public channel, the EPR protocol appears to be very similar to BB84. The difference, however, is that Alice and Bob must share beforehand a wealth of entangled states. This allows them to perform independent measurements, regardless of time sequence and their roles as originator or recipient in the quantum channel. With BB84, one or the other must agree on their roles (originator Alice or recipient Bob) and about the protocol sequence, and only single qubits are "exchanged." Although it is more complex to implement, because of the need for entangled EPR-Bell states as opposed to single qubits, EPR can be viewed as the conceptual "crown jewel" of QKD. Like the other protocols, however, information reconciliation and privacy amplification by SKD is required, to eliminate the effects of quantum noise and eavesdropping. Another major issue, which concerns all QKD protocols, is distance. The different quantum states that are exchanged (BB84, B92) or shared (EPR) by Alice and Bob must, throughout the process, remain immune to various forms of physical perturbation, SO as to keep their coherence, a term to mean their initial qubit description. Those perturbations introduce decoherence, whose nature is to ruin the protocol implementation and its 24 Here is a classical analogy (or Gedanken experiment) to illustrate the concept of nonlocality, which may provoke animated discussions. Assume that Alice prepares two different envelopes, each including either a picture of a cat or that of a dog, and seals them. Alice asks a third party to pick up one of the two envelopes. This envelope is to be sent to Bob, an a