Make a controlled gate from a QuantumCircuit that uses append in Qiskit

Question

Here is a minimal example for what I want to do:

from qiskit import QuantumCircuit

qc1 = QuantumCircuit(1, name='qc1') 
qc1.x(0)

qc2 = QuantumCircuit(1, name='qc2') 
qc2.h(0)
qc2.append(qc1, [0])
qc2.z(0)

c_qc2 = qc2.to_gate().control(1)

This gives the following error:

qiskit.exceptions.QiskitError: 'One or more instructions cannot be converted to a gate. "qc1" is not a gate instruction'

I'm aware that in this example I can get the desired controlled gate of qc2 by manually adding a control qubit to each gate in qc2. However in practice qc2 would be complex and doing so would be tiresome.

I also tried qc2.append(qc1.to_instruction(), [0]) which didn't solve the problem. Any help is appreciated. Thanks!

Answer 1

Edit: Miss understood your question earlier. As commented by @Egrettal you can do the following.

from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit

qc1 = QuantumCircuit(1, name='qc1') 
qc1.x(0)
print(qc1)


qc2 = QuantumCircuit(1, name='qc2') 
qc2.h(0)
qc2.append(qc1.to_gate(), [0])
qc2.z(0)
print(qc2)


xs_gate = qc2.to_gate()
cxs_gate = xs_gate.control()
circuit = QuantumCircuit(2)
circuit.append(cxs_gate, [0,1])
print('\n New circuit with controlled:\n',circuit)
print('\n Decomposed new circuit:\n', circuit.decompose())

Which will output:

     ┌───┐
q_0: ┤ X ├
     └───┘


     ┌───┐┌─────┐┌───┐
q_0: ┤ H ├┤ qc1 ├┤ Z ├
     └───┘└─────┘└───┘



 New circuit with controlled:
             
q_0: ───■───
     ┌──┴──┐
q_1: ┤ qc2 ├
     └─────┘

 Decomposed new circuit:
     ┌────────┐                                                     »
q_0: ┤ P(π/2) ├──■───────────────────■───────────────────────────■──»
     ├────────┤┌─┴─┐┌─────────────┐┌─┴─┐┌──────────┐┌─────────┐┌─┴─┐»
q_1: ┤ P(π/2) ├┤ X ├┤ U(0,0,-π/2) ├┤ X ├┤ U(0,0,0) ├┤ RY(π/4) ├┤ X ├»
     └────────┘└───┘└─────────────┘└───┘└──────────┘└─────────┘└───┘»
«                      ┌──────┐                                       »
«q_0: ──────────────■──┤ P(0) ├──■────────────────■────────────────■──»
«     ┌──────────┐┌─┴─┐├──────┤┌─┴─┐┌──────────┐┌─┴─┐┌──────────┐┌─┴─┐»
«q_1: ┤ RY(-π/4) ├┤ X ├┤ P(0) ├┤ X ├┤ U(0,0,0) ├┤ X ├┤ U(0,0,0) ├┤ X ├»
«     └──────────┘└───┘└──────┘└───┘└──────────┘└───┘└──────────┘└───┘»
«     ┌────────┐                                     
«q_0: ┤ P(π/2) ├──■───────────────────■──────────────
«     ├────────┤┌─┴─┐┌─────────────┐┌─┴─┐┌──────────┐
«q_1: ┤ P(π/2) ├┤ X ├┤ U(0,0,-π/2) ├┤ X ├┤ U(0,0,0) ├
«     └────────┘└───┘└─────────────┘└───┘└──────────┘