Qiskit: bind parameters to sub-circuit

Question

I am trying to implement a Hadamard test on a parametrized sub-circuit $V(\bf{a})$. I create an outer circuit and then append the parametrized $V(\bf{a})$ to it.

def outer_circuit(subcircuit, imaginary=False):

    qc = QuantumCircuit(4, 1)
    qc.append(subcircuit, qc.qubits)
    [...] # operations 
    
    return qc


def subcircuit():
    
    a = ParameterVector('a', 9)
    m, n = Parameter('m'), Parameter('n')
    qc = QuantumCircuit(4, name='V')

    [...] # random operations which use parameters

    return qc

When I finally contruct the bigger circuit and try to bind the parameters, I get the following error:

operator = subcircuit()

test = outer_circuit(operator)

test.bind_parameters({'a': np.random.rand(9)})

>>
CircuitError: 'Cannot bind parameters (a) not present in the circuit.'

How do I get overcome this problem?

Answer 1

When you append a sub-circuit to a larger circuit, the sub-circuit parameters are added to list of circuit parameters. You can easily check this as follows:

operator = subcircuit()
print(operator.parameters)

test = outer_circuit(operator)
print(test.parameters)

So, you can bind them as any parameters in the larger circuit.

The issue is that the keys of the parameter dictionary must be the Parameter instances not the Parameter names^{[see here]}. So, you need to call bind_parameters as follows:

test.bind_parameters({a: np.random.rand(9)})

Of course to do this the variable a needs to be in the scope when you call bind_parameters. The fastest way to do this in your case - although not recommended - is to make it global:

def subcircuit():
    global a
    a = ParameterVector('a', 9)
    [...]