d, therefore, they both would not need this QT apparatus. However, such a communication is complicated and quite resource consuming, should Bob need lots of qubits for his computations. And, most importantly, Alice would be able to communicate only qubits known to her, which is utterly restrictive in view of the QT potential. As a second observation, we note that QT does not violate the noncloning theorem. Indeed, the initial qubit lq) is collapsed by Alice's first measurement into the pure state |m), as seen from the top wire in Fig. 18.7. The benefits of QT being now understood, we may then have a few legitimate ques- tions. First, why use classical bits to determine which gates Bob should use? Indeed, Alice's measurements result in the collapse of her two qubits into the pure tensor state Qubit measurements, coding, and teleportation Figure 18.8 Principle of quantum teleportation with an all-quantum gate circuit. |m) |m'), and each of the |m), |m') qubits could be used as "automatic remote controls" for Bob's gates, i.e., controlled- and controlled-Z. This would give the all-quantum- gate QT circuit illustrated in Fig. 18.8. While such a circuit is logically equivalent to that of the QT shown in Fig. 18.7, it has several drawbacks. First, controlled-U gates are more complex than simple 2 X 2 U gates. But this is not as important as the fact that if Alice and Bob were located far apart, the control qubits |m), |m'), even as known to Alice, must be transported over this distance! It is, thus, far more sensible and straight- forward for Alice to send the two bits m, m' to Bob through any classical channel, such as the telephone or the Internet. On the other hand, if Bob is located nearby, or directly on Alice's premises, this whole qubit manipulation has no point, and the circuit shown in Fig. 18.8 reduces to an elegant, but needless CROSSOVER circuit (a circuit that just accurately transmits quantum information between Alice and Bob, who are in immediate reach of each other!). Regardless of these different obser