How to use the “compose” function in Qiskit

The “compose” function is used to join two circuits q_1 and q_2.

Let qc_1:

And qc_2:

To join 1 and 2, just use the following code:

qc = qc_1.compose(qc_2)

An alternative method, already deprecated, is to use the summation:

qc = qc_1 + qc_2

Compose has the advantage of being more general. It is possible to change the “wiring” of the circuit:

qc = qc_1.compose(qc_2, qubits =[0,2,1,3])

It is also possible to join a circuit with fewer qubits to a circuit with more qubits, indicating how they are wired together. If it is the opposite, joining a circuit with more qubits to another one with less, will give an error — the trick is to invert the composition of the circuit and use the option Front = True.

Qiskit Documentation:

https://qiskit.org/documentation/stubs/qiskit.circuit.QuantumCircuit.html#qiskit.circuit.QuantumCircuit.compose

Example code:

https://github.com/asgunzi/qiskit_compose