`backend.num_qubits` is incorrect (or broken) for simulation backends

[richrines1]

Describe the bug

• for "ibmq_qasm_simulator" and "simulator_statevector", backend.num_qubits returns 32 but submitting a circuit with more than 31 qubits results in a "Circuits do not match the target definition" error
• for "simulator_mps", backend.num_qubits returns 100 but submitting a circuit with more than 63 qubits results in the same error
• for "simulator_stabilizer" and "simulator_extended_stabilizer", backend.num_qubits itself throws an error

Steps to reproduce

service = qiskit_ibm_runtime.QiskitRuntimeService()

def test_backend(backend_name, num_qubits):
    backend = service.backend(backend_name)
    print(f"{backend_name} (backend has {backend.num_qubits} qubits):")

    circuit = qiskit.QuantumCircuit(num_qubits, 1)
    circuit.measure(0, 0)

    job = qiskit_ibm_runtime.Sampler(backend).run(circuit, shots=1)
    job.wait_for_final_state()

    print(f"{circuit.num_qubits}-qubit circuit: {job.status()}\n")

test_backend("ibmq_qasm_simulator", 31)
test_backend("ibmq_qasm_simulator", 32)

test_backend("simulator_statevector", 31)
test_backend("simulator_statevector", 32)

test_backend("simulator_mps", 63)
test_backend("simulator_mps", 64)

prints:

ibmq_qasm_simulator (backend has 32 qubits):
31-qubit circuit: JobStatus.DONE

ibmq_qasm_simulator (backend has 32 qubits):
32-qubit circuit: JobStatus.ERROR

simulator_statevector (backend has 32 qubits):
31-qubit circuit: JobStatus.DONE

simulator_statevector (backend has 32 qubits):
32-qubit circuit: JobStatus.ERROR

simulator_mps (backend has 100 qubits):
63-qubit circuit: JobStatus.DONE

simulator_mps (backend has 100 qubits):
64-qubit circuit: JobStatus.ERROR

(where for each failing job, job.error_message() returns "...Circuits do not match the target definition (non-ISA circuits)...")

meanwhile the following two lines fail immediately with a TranspilerError:

service.backend("simulator_stabilizer").num_qubits  # TranspilerError: "The number of qubits for Instruction(name='cx', num_qubits=2, num_clbits=0, params=[]) does not match the number of qubits in the properties dictionary: (0,)"

service.backend("simulator_extended_stabilizer").num_qubits  # TranspilerError: "The number of qubits for Instruction(name='cx', num_qubits=2, num_clbits=0, params=[]) does not match the number of qubits in the properties dictionary: (0,)"

Expected behavior

i would expect to be able to submit a circuit with (op to) backend.num_qubits qubits

Additional Information

• qiskit-ibm-runtime version: 0.21.0
• Python version: 3.11.8
• Operating system: Ubuntu 22.04.1

[alejomonbar]

@richrines1 did you find a solution to this bug?

[ElePT]

I think it's relevant to note that cloud simulators have been retired as of May 15th, you can refer to this migration guide to see why have cloud simulators been retired as well as how to migrate to local simulators.

[jyu00]

Closing this issue since as @ElePT mentioned, cloud simulators have been retired.

[richrines1]

thanks @ElePT @jyu00

cloud simulators seem to still be available via the ibm cloud qiskit runtime plans, for which this bug persists - are these similarly headed for retirement?

[jyu00]

You are correct that simulators are still available via the lite plan in the IBM Cloud channel.

[vtomole]

Hi @jyu00 ,

Will the IBM cloud simulators always be available?

[vtomole]

Ping @jyu00. This message might have slipped by you.

[jyu00]

@vtomole I'm not in a position to make an official statement. However, given cloud simulator support was dropped from IBM Quantum Platform due to their limitations, I would not be surprised if it's also dropped from IBM Cloud channel in the near future.

[vtomole]

Thanks @jyu00 :)