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Operation U1 followed by U2 is equivalent to<br />

U3 = U2U1<br />

If U1 acts on Hilbert space HA and U2 acts on Hilbert space HB, then<br />

U3 = U2 ⊗ U1<br />

acts on the composite Hilbert space HAB = HA ⊗ HB<br />

Some elemental quantum gates are introduced as follows:<br />

• NOT gate<br />

|0>+¡|1> X<br />

|1>+¡|0><br />

|0>+¡|1> Z<br />

|0> -¡|1><br />

|0>+¡|1><br />

H<br />

0 + 1 0 − 1<br />

α + β<br />

2 2<br />

Figure 2.2: Circuit representations of single qubit logic gates<br />

The quantum NOT gate acts linearly. It takes the state α | 0〉 + β | 1〉 to the corre-<br />

sponding state α | 1〉 + β | 0〉 in which the vectors of | 0〉 and | 1〉 have been flipped.<br />

The quantum NOT gate can be represented as:<br />

⎛<br />

X =<br />

⎜<br />

⎝<br />

16<br />

0 1<br />

1 0<br />

⎞<br />

⎟<br />

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