rotations 2 qubits 1 parameter
Controlled $x$ Rotation
Rotates the target about the $x$ axis by $\theta$ when the control qubit is in the $|1\rangle$ state.
The controlled-$R_x$ gate applies the single-qubit rotation $R_x(\theta)$ to the target, conditioned on the control.
$$ \mathrm{C}R_x(\theta) = \begin{bmatrix} 1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & \cos\frac{\theta}{2} & -i\sin\frac{\theta}{2} \\ 0 & 0 & -i\sin\frac{\theta}{2} & \cos\frac{\theta}{2} \end{bmatrix} $$
Special values
| $\theta$ | Gate |
|---|---|
| $0$ | Identity |
| $\pi$ | CNOT, up to a phase of $-i$ on the control's $\lvert 1\rangle$ subspace |
| $4\pi$ | Identity (the period is $4\pi$, not $2\pi$) |
Properties
- Inverse: $\mathrm{C}R_x(\theta)^\dagger = \mathrm{C}R_x(-\theta)$.
- Related to $\mathrm{C}R_z$ by a basis change on the target: $\mathrm{C}R_x(\theta) = (I \otimes H) \, \mathrm{C}R_z(\theta) \, (I \otimes H)$.
- Note the $4\pi$ periodicity: $\mathrm{C}R_x(2\pi) = Z \otimes I$, not the identity, since $R_x(2\pi) = -I$ and the sign is only applied when the control is set — where it acts as a $Z$ on the control.
Usage
- A standard entangling block in hardware-efficient variational ansätze.
The unitary, drawn
Every entry of the matrix as a disc. The area is the transition probability $|U_{jk}|^2$; the colour and the hand are the phase. Rows are outputs $\langle j|$, columns inputs $|k\rangle$, in the computational basis. Each parameter has a slider; play runs through the family along the path of the Quirk link ($\theta = 4\pi t$).
Phase
Disc area $= |U_{jk}|^2$. Hover an entry for its value, or take the JSON.
Weyl chamber
Where this gate sits among all two-qubit gates, up to single-qubit rotations before and after.
Coordinates:
not entangling entangling perfect entangler
$(a, b, c)$ means the gate equals $e^{i(a\,XX + b\,YY + c\,ZZ)}$ up to single-qubit gates, reduced to the chamber $\pi/4 \ge a \ge b \ge |c|$ (the convention of the canonical gate, Qiskit and Cirq). The $a = \pi/4$ face is a mirror: $(\pi/4, b, c)$ and $(\pi/4, b, -c)$ are the same gate. Drag the figure to rotate it, double-click to reset.
SDK support
How to call this gate in each framework. Hover a ⓘ for a note; the support matrix compares every gate at once.
| SDK | Name |
|---|---|
| Qiskit | qiskit.circuit.library.CRXGate |
| PennyLane | pennylane.CRX |
| Cirq | — ⓘ |
| Q# | — ⓘ |
| PyQuil | — ⓘ |
| Braket | — ⓘ |
| BQSKit | bqskit.ir.gates.CRXGate |
| Qibo | qibo.gates.CRX |
| pytket | pytket.circuit.OpType.CRx ⓘ |
| Stim | — |
| OpenQASM | stdgates.inc: crx |