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Quantum Gate
{ Currently studying this, there may be errors. ~drummyfish }
Quantum (logic) gate is a quantum computing equivalent of a traditional logic gate. A quantum gate takes as an input N qubits and transforms their states to new states.
Quantum gates are represented by complex matrices that transform the qubit states (which can be seen as points in multidimensional space, see Bloch sphere). A gate operating on N qubits is represented by a 2^Nx2^N matrix. These matrices have to be unitary. Operations performed by quantum gates may be reversed, unlike those of classical logic gates.
We normally represent a single qubit state with a column vector |a> = a0 * |0> + a1 * |1> = [a0, a1] (look up bra-ket notation). Multiple qubit states are represented as a tensor product of the individual state, e.g. |a,b> = [a0 * b0, a0 * b1, a1 * b0, a1 * b1]. Applying a quantum gate G to such a qubit vector q is performed by simple matrix multiplication: G * v.
Basic gates
Here are some of the most common quantum gates.
Identity
Acts on 1 qubit, leaves the qubit state unchanged.
1 0
0 1
Pauli Gates
Act on 1 qubit. There are three types of Pauli gates: X, Y and Z, each one rotates the qubit about the respective axis by pi radians.
The X gate is:
0 1
1 0
The Y gate is:
0 -i
i 0
The Z gate is:
1 0
0 -1
NOT
The not gate is identical to the Pauli X gate. It acts on 1 qubit and switches the probabilities of measuring 0 vs 1.
CNOT
Controlled NOT, acts on 2 qubits. Performs NOT on the second qubit if the first qubit is |1>.
1 0 0 0
0 1 0 0
0 0 0 1
0 0 1 0
TODO