I gate (or identity gate) is a single-qubit quantum gate that leaves the state of a qubit unchanged. It is represented by the $2 \times 2$ identity matrix and drawn as a plain wire in quantum circuit diagrams.

$$I = \begin{pmatrix}1 & 0\\ 0 & 1\end{pmatrix}$$

The identity gate is trivial on its own but appears in multi-qubit notation to indicate that a qubit is carried through a time step without any operation being applied. For example, applying $H$ to the first qubit and nothing to the second is written as $H \otimes I$. It is also one of the four Pauli gates.

• I gate (Qiskit)
• I gate (cuStateVec)
• I gate (CUDA-Q)