Three Qubit Gates¶

Three-qubit gates¶

There are two commonly-used three-qubit gates. For three qubits, the basis vectors are ordered as

$$\left|000\right\rangle, \left|001\right\rangle, \left|010\right\rangle, \left|011\right\rangle, \left|100\right\rangle, \left|101\right\rangle, \left|110\right\rangle, \left|111\right\rangle,$$

which, as bitstrings, represent the integers $0,1,2,\cdots, 7$. Again, Qiskit uses a representation in which the first qubit is on the right-most side of the tensor product and the third qubit is on the left-most side:

$$\left|abc\right\rangle : \underset{\text{qubit 2}}{\left|a\right\rangle}\otimes \underset{\text{qubit 1}}{\left|b\right\rangle}\otimes \underset{\text{qubit 0}}{\left|c\right\rangle}.$$

Toffoli gate ($ccx$ gate)¶

The Toffoli gate flips the third qubit if the first two qubits (LSB) are both $\left|1\right\rangle$:

$$\left|abc\right\rangle \rightarrow \left|bc\oplus a\right\rangle \otimes \left|b\right\rangle \otimes \left|c\right\rangle.$$

In matrix form, the Toffoli gate is $$ C_{CX} = \begin{pmatrix} 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0\\ 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0\\ 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1\\ 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0\\ 0 & 0 & 0 & 0 & 0 & 1 & 0 & 0\\ 0 & 0 & 0 & 0 & 0 & 0 & 1 & 0\\ 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 \end{pmatrix}. $$

Controlled swap gate (Fredkin Gate)¶

The Fredkin gate, or the controlled swap gate, exchanges the second and third qubits if the first qubit (LSB) is $\left|1\right\rangle$:

$$ \left|abc\right\rangle \rightarrow \begin{cases} \left|bac\right\rangle~~\text{if}~c=1 \cr \left|abc\right\rangle~~\text{if}~c=0 \end{cases}.$$

In matrix form, the Fredkin gate is

$$ C_{\mathrm{SWAP}} = \begin{pmatrix} 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0\\ 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0\\ 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 0 & 0 & 1 & 0 & 0\\ 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0\\ 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 0 & 0 & 0 & 1 & 0\\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 \end{pmatrix}. $$

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