CSwapGate¶

class
CSwapGate
(label=None, ctrl_state=None)[source]¶ Bases:
qiskit.circuit.controlledgate.ControlledGate
ControlledSWAP gate, also known as the Fredkin gate.
Circuit symbol:
q_0: ─X─ │ q_1: ─X─ │ q_2: ─■─
Matrix representation:
\[\begin{split}CSWAP\ q_0, q_1, q_2 = 0 \rangle \langle 0 \otimes I \otimes I + 1 \rangle \langle 1 \otimes SWAP = \begin{pmatrix} 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 \\ 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 \\ 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 \\ \end{pmatrix}\end{split}\]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_2. Thus a textbook matrix for this gate will be:
q_0: ─■─ │ q_1: ─X─ │ q_2: ─X─
\[\begin{split}CSWAP\ q_2, q_1, q_0 = 0 \rangle \langle 0 \otimes I \otimes I + 1 \rangle \langle 1 \otimes SWAP = \begin{pmatrix} 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 \\ \end{pmatrix}\end{split}\]In the computational basis, this gate swaps the states of the two target qubits if the control qubit is in the \(1\rangle\) state.
\[0, b, c\rangle \rightarrow 0, b, c\rangle 1, b, c\rangle \rightarrow 1, c, b\rangle\]Create new CSWAP gate.
Methods Defined Here
Return inverse CSwap gate (itself).
Attributes

ctrl_state
¶ Return the control state of the gate as a decimal integer.
 Return type
int

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.
 Return type
List

duration
¶ Get the duration.

label
¶ Return instruction label
 Return type
str

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.
 Return type
str

num_ctrl_qubits
¶ Get number of control qubits.
 Returns
The number of control qubits for the gate.
 Return type
int

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.
