# CXGate#

class qiskit.circuit.library.CXGate(label=None, ctrl_state=None)[source]#

Bases: ControlledGate

Controlled-X gate.

Can be applied to a QuantumCircuit with the cx() and cnot() methods.

Circuit symbol:

q_0: ──■──
┌─┴─┐
q_1: ┤ X ├
└───┘


Matrix representation:

$\begin{split}CX\ q_0, q_1 = I \otimes |0\rangle\langle0| + X \otimes |1\rangle\langle1| = \begin{pmatrix} 1 & 0 & 0 & 0 \\ 0 & 0 & 0 & 1 \\ 0 & 0 & 1 & 0 \\ 0 & 1 & 0 & 0 \end{pmatrix}\end{split}$

Note

In Qiskit’s convention, higher qubit indices are more significant (little endian convention). In many textbooks, controlled gates are presented with the assumption of more significant qubits as control, which in our case would be q_1. Thus a textbook matrix for this gate will be:

     ┌───┐
q_0: ┤ X ├
└─┬─┘
q_1: ──■──

$\begin{split}CX\ q_1, q_0 = |0 \rangle\langle 0| \otimes I + |1 \rangle\langle 1| \otimes X = \begin{pmatrix} 1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 0 & 1 \\ 0 & 0 & 1 & 0 \end{pmatrix}\end{split}$

In the computational basis, this gate flips the target qubit if the control qubit is in the $$|1\rangle$$ state. In this sense it is similar to a classical XOR gate.

$|a, b\rangle \rightarrow |a, a \oplus b\rangle$

Create new CX gate.

Attributes

condition_bits#

Get Clbits in condition.

ctrl_state#

Return the control state of the gate as a decimal integer.

decompositions#

Get the decompositions of the instruction from the SessionEquivalenceLibrary.

definition#

Return definition in terms of other basic gates. If the gate has open controls, as determined from self.ctrl_state, the returned definition is conjugated with X without changing the internal _definition.

duration#

Get the duration.

label#

Return instruction label

name#

Get name of gate. If the gate has open controls the gate name will become:

<original_name_o<ctrl_state>

where <original_name> is the gate name for the default case of closed control qubits and <ctrl_state> is the integer value of the control state for the gate.

num_clbits#

Return the number of clbits.

num_ctrl_qubits#

Get number of control qubits.

Returns:

The number of control qubits for the gate.

Return type:

int

num_qubits#

Return the number of qubits.

params#

Get parameters from base_gate.

Returns:

List of gate parameters.

Return type:

list

Raises:

CircuitError – Controlled gate does not define a base gate

unit#

Get the time unit of duration.

Methods

control(num_ctrl_qubits=1, label=None, ctrl_state=None)[source]#

Return a controlled-X gate with more control lines.

Parameters:
• num_ctrl_qubits (int) – number of control qubits.

• label (str or None) – An optional label for the gate [Default: None]

• ctrl_state (int or str or None) – control state expressed as integer, string (e.g. ‘110’), or None. If None, use all 1s.

Returns:

controlled version of this gate.

Return type:

ControlledGate

inverse()[source]#

Return inverted CX gate (itself).