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quantum-computingqiskit

How to build a complex controlled gate in the Qiskit?

I work on theory tasks in quantum computing, and make simple experiments with Qiskit. Unfortunately, I can't find a way how to make a complex control gates there, where control is in the quantum register.

I would like to have a "c_if" analogue, which can be chained and use quantum bits as a control. Smth like 

swap(q1, q2).c_if(q0,Zero).c_if(q3,One)

Is there such an operation in the qiskit? How could I emulate such an operation if it doesn't exist?

enter image description here

Solution

  • Check out Qiskit documentation for the MCXGate, know as the Multi-controlled-X Gate. This gate lets you define how many control qubits you would like to include (perhaps the majority of your quantum register) and define a control state.

    
from qiskit import * 

my_circuit = QuantumRegister(3,3)

my_circuit.append(circuit.library.MCXGate(2, ctrl_state='10'), [0,1,2])

    Check out the documentation here.

    There are also variations that will do Y gate Z gate or whatever you like depending if the circuit sees the correct control gate.