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The most common quantum gates operate on spaces of one or two qubits. This means that as matrices, quantum gates can be described by 2 × 2 or 4 × 4 matrices with Orthonormal rows.

Remark. "Quantum logic" can refer either to the performance of quantum logic gates or to a foundational formalism for Quantum Mechanics called Quantum Logic based on a modification of some of the rules of Propositional Logic .


EXAMPLES


Hadamard gate. This gate operates on a single qubit. It is represented by the Hadamard matrix:

: H = rac{1}{\sqrt{2}} \begin{bmatrix} 1 & 1 \ 1 & -1 \end{bmatrix}

Since the rows of the matrix are orthogonal, ''H'' is indeed a unitary matrix.

Phase shifter gates. Gates in this class operate on a single qubit. They are represented by 2 × 2 matrices of the form

: R( heta) = \begin{bmatrix} 1 & 0\ 0 & e^{2 \pi i heta} \end{bmatrix}

where θ is the ''phase shift''.

Controlled gates. Suppose ''U'' is a gate that operates on single qubits with matrix representation

: U = \begin{bmatrix} x_{00} & x_{01} \ x_{10} & x_{11} \end{bmatrix}

The ''controlled-U gate'' is a gate that operates on two qubits in such a way that the first qubit serves as a control.

  :<math> 1 0 Angle \mapsto 1 Angle U 0 Angle 1 angle \left(x_{00} 0 angle + x_{01} 1 angle ight) </math>
  :<math> 1 1 Angle \mapsto 1 Angle U 1 Angle 1 angle \left(x_{10} 0 angle + x_{11} 1 angle ight) </math>
  :<math> \operatorname{D( Heta)}: i,j,k Angle Ightarrow \begin{cases} I \cos( Heta) i,j,k Angle + \sin( Heta) i,j,1-k Angle & \mbox{for }i j=1 \ i,j,k angle & \mbox{otherwise}\end{cases}</math>