Qiskit error: "Circuit circuit-91 contains invalid instructions"

Question

I am following the examples given in this qiskit article about the Grover's algorithm.

I am trying to use this method that is given:

def diffuser(nqubits):
    qc = QuantumCircuit(nqubits)
    # Apply transformation |s> -> |00..0> (H-gates)
    for qubit in range(nqubits):
        qc.h(qubit)
    # Apply transformation |00..0> -> |11..1> (X-gates)
    for qubit in range(nqubits):
        qc.x(qubit)
    # Do multi-controlled-Z gate
    qc.h(nqubits-1)
    qc.mct(list(range(nqubits-1)), nqubits-1)  # multi-controlled-toffoli
    qc.h(nqubits-1)
    # Apply transformation |11..1> -> |00..0>
    for qubit in range(nqubits):
        qc.x(qubit)
    # Apply transformation |00..0> -> |s>
    for qubit in range(nqubits):
        qc.h(qubit)
    # We will return the diffuser as a gate
    U_s = qc.to_gate()
    U_s.name = "U$_s$"
    return U_s

With this code:

backend = Aer.get_backend("statevector_simulator")

circuit = QuantumCircuit(psi) #psi is my quantumregister variable

# code to implement Hadamard transform and oracle

circuit.append(diffuser(3), psi) #psi is a 3 qubit quantum register
job = backend.run(c)

I think the issue is with the circuit.append() method; it only occurs when I uncomment that line.

The error I get is below:

Simulation failed and returned the following error message:
ERROR:  [Experiment 0] Circuit circuit-91 contains invalid instructions {"gates": {U$_s$}} for "statevector" method.

When I add this line of code to the end of the file:

v = job.result().get_statevector(c).measure()[0]

to get the output of the code, I end up with this error:

qiskit.exceptions.QiskitError: 'Data for experiment "circuit-91" could not be found.'

Does anybody know why this is happening?

Answer 1

You have to transpile your circuit before running it. So, before calling backend.run you have to add the following:

from qiskit import transpile

circuit = transpile(circuit, backend=backend)

This is because your circuit contains a composite gate (the one created by diffuser() method) which in turn contains another composite gate (mct) . Qiskit transpiler will unroll these gates to one- and two-qubit gates, then translate these gates into gates from the target backend's basis gate set.