BarrierBeforeFinalMeasurements fails if there are loose qubits or clbits

[jakelishman]

Environment

• Qiskit Terra version: 0.22
• Python version: 3.10
• Operating system: macOS

What is happening?

The BarrierBeforeFinalMeasurements pass fails if any of the proceeding operations involve loose qubits or clbits (i.e. those not in registers).

How can we reproduce the issue?

Loose qubit:

from qiskit.circuit import QuantumCircuit, Qubit, Clbit
from qiskit.transpiler.passes import BarrierBeforeFinalMeasurements

qc = QuantumCircuit([Qubit(), Clbit()])
qc.h(0)
qc.measure(0, 0)
qc.x(0).c_if(0, False)
qc.measure(0, 0)

BarrierBeforeFinalMeasurements()(qc)

gives

---------------------------------------------------------------------------
DAGCircuitError                           Traceback (most recent call last)
<ipython-input-2-bca4f6f26fdc> in <module>
      8 qc.measure(0, 0)
      9
---> 10 BarrierBeforeFinalMeasurements()(qc)

~/code/qiskit/terra/qiskit/transpiler/basepasses.py in __call__(self, circuit, property_set)
    119             self.property_set = property_set_
    120
--> 121         result = self.run(circuit_to_dag(circuit))
    122
    123         result_circuit = circuit

~/code/qiskit/terra/qiskit/transpiler/passes/utils/barrier_before_final_measurements.py in run(self, dag)
     62         final_qubits = dag.qubits
     63
---> 64         barrier_layer.apply_operation_back(Barrier(len(final_qubits)), list(final_qubits), [])
     65
     66         # Preserve order of final ops collected earlier from the original DAG.

~/code/qiskit/terra/qiskit/dagcircuit/dagcircuit.py in apply_operation_back(self, op, qargs, cargs)
    619
    620         self._check_condition(op.name, getattr(op, "condition", None))
--> 621         self._check_bits(qargs, self.output_map)
    622         self._check_bits(all_cbits, self.output_map)
    623

~/code/qiskit/terra/qiskit/dagcircuit/dagcircuit.py in _check_bits(self, args, amap)
    504         for wire in args:
    505             if wire not in amap:
--> 506                 raise DAGCircuitError(f"(qu)bit {wire} not found in {amap}")
    507
    508     @staticmethod

DAGCircuitError: '(qu)bit <qiskit.circuit.quantumregister.Qubit object at 0x107e04840> not found in OrderedDict()'

Similarly, if I put the qubit in a register and use only the loose clbit, such as by constructing the circuit with

from qiskit.circuit import QuantumCircuit, Qubit, Clbit, QuantumRegister
from qiskit.transpiler.passes import BarrierBeforeFinalMeasurements

qc = QuantumCircuit(QuantumRegister(1), [Clbit()])
qc.h(0)
qc.measure(0, 0)
qc.x(0).c_if(0, False)
qc.measure(0, 0)

BarrierBeforeFinalMeasurements()(qc)

I get a similar error on the clbit:

---------------------------------------------------------------------------
DAGCircuitError                           Traceback (most recent call last)
<ipython-input-4-61050a9d8f53> in <module>
      8 qc.measure(0, 0)
      9
---> 10 BarrierBeforeFinalMeasurements()(qc)

~/code/qiskit/terra/qiskit/transpiler/basepasses.py in __call__(self, circuit, property_set)
    119             self.property_set = property_set_
    120
--> 121         result = self.run(circuit_to_dag(circuit))
    122
    123         result_circuit = circuit

~/code/qiskit/terra/qiskit/transpiler/passes/utils/barrier_before_final_measurements.py in run(self, dag)
     71         # Move final ops to the new layer and append the new layer to the DAG.
     72         for final_node in ordered_final_nodes:
---> 73             barrier_layer.apply_operation_back(final_node.op, final_node.qargs, final_node.cargs)
     74
     75         for final_op in final_ops:

~/code/qiskit/terra/qiskit/dagcircuit/dagcircuit.py in apply_operation_back(self, op, qargs, cargs)
    620         self._check_condition(op.name, getattr(op, "condition", None))
    621         self._check_bits(qargs, self.output_map)
--> 622         self._check_bits(all_cbits, self.output_map)
    623
    624         node_index = self._add_op_node(op, qargs, cargs)

~/code/qiskit/terra/qiskit/dagcircuit/dagcircuit.py in _check_bits(self, args, amap)
    504         for wire in args:
    505             if wire not in amap:
--> 506                 raise DAGCircuitError(f"(qu)bit {wire} not found in {amap}")
    507
    508     @staticmethod

DAGCircuitError: "(qu)bit <qiskit.circuit.classicalregister.Clbit object at 0x1150450c0> not found in OrderedDict([(Qubit(QuantumRegister(1, 'q1'), 0), DAGOutNode(wire=Qubit(QuantumRegister(1, 'q1'), 0)))])"

What should happen?

The clbit error is the only one that particularly matters, since BarrierBeforeFinalMeasurements is run as part of routing where the qubit registers have already been canonicalised. We shouldn't be getting an error in the internal structures.

Any suggestions?

The pass creates a new internal DAG for the barrier layer like this: https://github.com/Qiskit/qiskit-terra/blob/fca8db6b86cba106ed0449a99cd10223899b6145/qiskit/transpiler/passes/utils/barrier_before_final_measurements.py#L53-L58

which doesn't include loose clbits. We don't want to use DAGCircuit.copy_empty_like because that would also copy things like metadata and global phase, but we do need to add the loose bits.