Qiskit transpiler error: Unable to translate

Question

This code worked in a previous version of ibmq_simulator and, I think, it works with qiskit_aer, but it doesn't work here and I don't understand the error message. The issue is there is no "path" from the input gates to the target basis, apparently. I don't know why - the input gates are only x with a condition.

Here's the code

import numpy as np
import matplotlib.pyplot as plt
import random
from qiskit import QuantumCircuit, QuantumRegister,  ClassicalRegister, execute
import qiskit
from qiskit_ibm_provider import IBMProvider

def create_registers():
    alice_q = QuantumRegister(1, 'alice (q)')
    peter_alice_q =         QuantumRegister(1, 'peter/alice (q)')
    peter_bob_q = QuantumRegister(1, 'peter/bob (q)')
    bob_c = ClassicalRegister(3, 'bob (c)')
    circ = QuantumCircuit(alice_q, peter_alice_q,
                          peter_bob_q, bob_c)
    return circ

circ = create_registers()
circ.x(2).c_if(1, 1) 
circ.measure(2, 2)

provider = IBMProvider()
device = provider.get_backend('ibmq_qasm_simulator')
shots = 1000
circ=qiskit.compiler.transpile(circ, basis_gates=['x', 'if_else'])
job = execute(circ, backend=device, shots=shots)

and here's the error message: raise TranspilerError( qiskit.transpiler.exceptions.TranspilerError: "Unable to translate the operations in the circuit: ['x', 'if_else', 'measure'] to the backend's (or manually specified) target basis: ['cu2', 'mcp', 'ry', 'rz', 'measure', 'rzz', 'ryy', 'tdg', 'mcz', 'mcswap', 'cz', 'rxx', 'cp', 'kraus', 'cx', 't', 'z', 'h', 'rx', 'u3', 'sdg', 'mcry', 'multiplexer', 'delay', 'u1', 'roerror', 'u', 'mcsx', 'mcy', 'diagonal', 'x', 'mcrz', 'ccx', 'mcrx', 'p', 'cswap', 'initialize', 'r', 'sx', 'mcr', 'mcu1', 'y', 'mcu3', 'rzx', 'csx', 's', 'id', 'mcx', 'barrier', 'swap', 'u2', 'cy', 'cu3', 'unitary', 'snapshot', 'mcu2', 'cu1']. This likely means the target basis is not universal or there are additional equivalence rules needed in the EquivalenceLibrary being used. For more details on this error see: https://qiskit.org/documentation/stubs/qiskit.transpiler.passes.BasisTranslator.html#translation_errors"

So either the target basis is not universal - and that seems unlikely - or there are equivalence rules needed. I don't know why an equivalence rule would be needed for such a simple circuit.

Furthermore, I see that the backed that works - aer - and the one that doesn't work, seem to use different equivalence libraries. How do I control which equivalence library I'm using?

Answer 1

The backend ibmq_qasm_simulator does not support dynamic circuits.

You can transpile your circuit as follows:

circ=qiskit.compiler.transpile(circ, backend=device, optimization_level=2)

note that, you should transpile the circuit with optimization_level greater than or equal to 2 to avoid the error you are facing. However, when you try to run the circuit:

job = device.run(circ, shots=shots)

you will get

UserWarning: The backend ibmq_qasm_simulator does not support dynamic circuits.