the information of one qubit measurement conditions that of the second qubit measurement, the EPR-Bell states are said to be entangled. In QIT jargon, EPR-Bell states are also referred to as ebits, for "entanglement bits" (one ebit = one single entangled pair). To provide a visual illustration of this, imagine two people A and B, who are tradi- tionally named Alice and Bob, and who live far apart. However, both have access to a quantum system, which is in the entangled EPR-Bell state |B01), as represented by a cloud in Fig. 18.4. The state remains in |B01) until Alice unilaterally takes the initiative to measure the first qubit, through the operator E11, for instance. This measurement immediately results in the EPR-Bell pair collapsing into the pure state |10). Bob's mea- surement of the second qubit can only yield |0) as a positive outcome. Bob and Alice can exchange roles. For instance, Bob may first measure the second qubit through E20, resulting in the system collapsing into the state |01), and leaving |0) to Alice as the only positive measurement for the first qubit. In each case, the measurement of the first qubit conditions the measurement of the second, because of the effect of state collapse that occurs in between. 18.3 Bell state measurement Table 18.2 Outcome of Bell measurement with EPR-Bell states as input (normalization factor being overlooked). Input state /xy) Pre-measurement state Cbit output m, m' 1Boo) = |00) + |11) 1001 = |00) + |11) |B10) = |00) - |11) 1B11) = |01) - |10) Figure 18.5 Principle of Bell measurement or measurement in Bell states. From any physical perception of our "classical world," the effect of state entanglement seems rather nonintuitive. We may illustrate it through a figurative experiment. Assume that we place two coins in two separate boxes. In each box, the position (or states) of the coins can be either heads or tails, but we have no awareness of which one. In the classical world, opening a box tells us only that the coin is heads or tails, but we cannot know of the oth