The probabilities of finding the spin system in the state j + ↑〉,
j*↑〉 and j0↑〉 are, respectively, jA0j2 ð ¼ jBj2), jB0j2 ð ¼ jAj2) and jC0j2
ð ¼ jCj2Þ (see Eq. (8)). The control qubit is up. In Fig. 3, the red solid
line represents now jA0j2 and the blue solid line jB0j2. At τ C 96 ns,
the probability of finding the spin system in the state j*↑〉, jB0j2, is
practically equal to the unity (0.9995), i.e., the target qubit has
flipped. The third line of the truth table reflects this result. One can
easily complete this table by applying the pulse sequence on the
other four input states. The pulse sequence does not affect the last
two input states j0ms〉. Moreover, the probabilities jCj2 and jC0j2
are close to zero when the pulse sequence is applied onto another
input. The first four lines constitute the effective truth table of the
gate. Here, it gives the probability of finding the spin system in a
specific output state when it is prepared in some input state.
Its usual graphical representation is also given. The amplitudes
of probability could exhibit phase factors. They do not affect
the logical operation of the gate and may be removed [29]. To
summarize the functioning of the logic gate, the first pulse creates
a superposition of three states of the spin system (Eqs. (4) and (8)).
Each of them acquires a dynamic phase during the free precession.
The second pulse fixes the state of the target qubit. The later is
modified if and only if the control qubit is up.