on of quantum entanglement, which enables quantum superdense coding. The key concept behind this appellation is the possibility for Alice to send to Bob two classical bits m, m', which she has in her possession, by means of communicating only a single qubit. The generic circuit for superdense coding is represented in Fig. 18.6. Alice and Bob must share an EPR-Bell state, for instance 1Boo). Alice controls a gate U, which she can change at will into I, Z, X, and iY. Bob has the control of a Bell-state measurement apparatus, as shown by the shaded subsystem in the figure. It can be checked as an easy exercise that the choice of U transforms 1Boo) into any EPR-Bell state 1Boo), (B10), |B01), 1B11), according to (18.19) Z|Boo) = (B10); iY|Boo) = |B11). Thus, by switching the U gate into I, Z, X, and iY, Alice can send Bob the classical bit pairs 00, 01, 10, and 11, respectively, at an information rate of two bits per qubit, which represents twice the classical information rate, hence the name superdense coding. Note that from any realistic telecom viewpoint, twofold improvements in information- rate capacities do not deserve such superlatives as "super" or "dense," or, worst of all, "superdense," notwithstanding the conceptual importance of the potential capacity expansion of the "old-fashioned" classical bit. 18.5 Quantum teleportation Alice Figure 18.7 Circuit used by Alice and Bob for quantum teleportation. Quantum teleportation The strange technique referred to as quantum teleportation (QT) makes it possible to transmit an unknown qubit instantaneously over any distance. Here, "distance" means any range spanning from next door's office to the opposite end of the Solar System, or even beyond in the Universe, despite the limitations imposed by Einstein's theory of relativity, the physical finiteness of the speed of light, and its invariance regardless of any reference frame. To a wider audience, the word "teleportation" may raise false expectations in terms of the capability of moving physical objects, which we may