BarrierBeforeFinalMeasurements fails if there are loose qubits or clbits #8923
Description
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.