Qiskit ERROR: Circuit contains invalid instructions

Question

Recently I encountered a weird qiskit error which produces a pretty useless traceback. To replicate this error, I create a very simple circuit given below:

When I run it on a simulator, I get an error whose traceback is at the bottom.

Could anyone point out why this is happening?

The circuit above is produced with the following code:

# Importing standard Qiskit libraries
from qiskit import QuantumCircuit, execute, Aer, QuantumRegister, ClassicalRegister
from qiskit.compiler import transpile, assemble
from qiskit.tools.jupyter import *
from qiskit.visualization import *

def create_controled_gate():
    h_circuit = QuantumCircuit(1, name='H')
    h_circuit.h(0)
    gate = h_circuit.to_gate().control()
    return gate

circuit = QuantumCircuit(2)
controlled_gate = create_controled_gate()
circuit.append(controlled_gate, [0,1])

cr = ClassicalRegister(2,'creg')
circuit.add_register(cr)
circuit.measure(range(2), range(2))
circuit.draw()

aer_sim = Aer.get_backend('aer_simulator')
qobj = assemble(circuit, shots=1000)
job = aer_sim.run(qobj)
hist = job.result().get_counts()
print(hist)

QiskitError                               Traceback (most recent call last)
Input In [5], in <module>
      2 qobj = assemble(circuit, shots=1000)
      3 job = aer_sim.run(qobj)
----> 4 hist = job.result().get_counts()
      5 print(hist)

File ~/.pyenv/versions/3.8.7/lib/python3.8/site-packages/qiskit/result/result.py:269, in Result.get_counts(self, experiment)
    267 dict_list = []
    268 for key in exp_keys:
--> 269     exp = self._get_experiment(key)
    270     try:
    271         header = exp.header.to_dict()

File ~/.pyenv/versions/3.8.7/lib/python3.8/site-packages/qiskit/result/result.py:397, in Result._get_experiment(self, key)
    395 result_status = getattr(self, "status", "Result was not successful")
    396 exp_status = getattr(exp, "status", "Experiment was not successful")
--> 397 raise QiskitError(result_status, ", ", exp_status)

QiskitError: 'ERROR:  [Experiment 0] Circuit circuit-85 contains invalid instructions {"gates": {cH}} for "statevector" method. ,  ERROR: Circuit circuit-85 contains invalid instructions {"gates": {cH}} for "statevector" method.'

I use qiskit 0.22.2.

P.S. I'm not trying to implement a controlled H gate, but rather understand why this error occurs and how I can avoid it in more complicated scenarios.

Answer 1

Controlled-H is not one of the basis gates of AerSimulator. You can get this list of gates using:

aer_sim.configuration().basis_gates

So, you need to transpile the circuit into the supported set of gates before running it:

from qiskit import transpile

circuit = transpile(circuit, backend=aer_sim)
job = aer_sim.run(circuit)

Notes:

• There is no need to call assemble(). Actually, passing an assembled qobj directly to the run() method of the Aer simulator backends has been deprecated in favor of passing transpiled circuits directly.

• Using execute() function also works because transpile() is called internally.

Answer 2

I've had better luck using execute() rather than run(). I changed the last five lines to:

from qiskit import execute

aer_sim = Aer.get_backend('aer_simulator')
job = aer_sim.execute(circuit, backend, shots=1000)
hist = job.result().get_counts()
print(hist)

and it worked fine.

I wished I knew what was actually causing your problem. I suspect you've found a bug. execute has all the bells and whistles I need, and seems to be more robust.