How to use the “compose” function in Qiskit
The “compose” function is used to join two circuits q_1 and q_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.