er coin until we open the other box. In the quantum world, the coin states may be entangled. This means that opening only one box conditions the state of the coin in the other box. The point here is not about the advance knowledge of the entanglement possibilities, as represented by the four possible EPR-Bell states. Rather, it is the matter that a single measurement in one physical system determines "for absolute certain" the state of the other physical system. As described later in this chapter, the properties of EPR-Bell state entanglement are the key to applications called quantum superdense coding and quantum teleportation, both of which having no counterpart in the classical world. Bell state measurement In this section, I describe a 2-qubit measurement scheme, referred to as measurement in the Bell basis, or Bell state measurement. The principle is illustrated in Fig. 18.5. The circuit includes a Hadamard gate and a CNOT gate, followed by two qubit measurements. Qubit measurements, coding, and teleportation Alice Figure 18.6 Circuit used for superdense coding. The measurements, which are made in the base {|00), |01), |10), |11)}, output the clas- sical bits m, m'. What happens if we input to this gate circuit any of the EPR-Bell states 1Bmm')? It is left as a simple exercise to check that the pre-measurement states are |mm') and the resulting measurement cbits (short for "classical bits") are m, m', as shown in Table 18.2. This means that the apparatus is able to positively single out or identify any of the four EPR-Bell states, hence, the name "measurement in the Bell basis" or "Bell state measurement." Such a denomination may sound confusing, since the measurement is actually performed in the {|00), |01), |10), |11)} basis. The point is that while the circuit represents a true EPR-Bell state measurement apparatus, it can be used for other types of application, as will be shown next when describing superdense coding and quantum teleportation. Superdense coding In this section, I describe a first applicati