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Fix idle qubit detection to ignore barriers #639

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Fix idle qubit detection to ignore barriers #639

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44f3189
Fix idle qubit detection to ignore barriers
raunakkumarsingh Jul 16, 2024
6613701
added test for barrier ignored
raunakkumarsingh Jul 18, 2024
aec527c
correct code lint
raunakkumarsingh Jul 19, 2024
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2 changes: 2 additions & 0 deletions circuit_knitting/utils/transforms.py
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Original file line number Diff line number Diff line change
Expand Up @@ -172,6 +172,8 @@ def _partition_labels_from_circuit(
# Determine which qubit wires are idle/unused
idle_wires = set(range(circuit.num_qubits))
for instruction in circuit.data:
if instruction.operation.name == "barrier":
continue
for q1 in instruction.qubits:
q1_id = circuit.find_bit(q1).index
idle_wires.discard(q1_id)
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42 changes: 42 additions & 0 deletions test/utils/test_transforms.py
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Expand Up @@ -262,6 +262,48 @@ def test_separate_circuit(self):
[("A", 0), ("B", 0), ("C", 0)], separated_circuits.qubit_map
)

with self.subTest("barriers are ignored"):
qreg = QuantumRegister(3)
circuit = QuantumCircuit(qreg)
circuit.h(0)
circuit.x(0)
circuit.barrier(0)
circuit.x(1)
circuit.h(1)
circuit.y(2)
circuit.h(2)

separated_circuits = separate_circuit(circuit, partition_labels="ABC")
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Actually, I believe this test does not actually check that the relevant bug is fixed.

The idle wire detection code only runs when partition_labels is not provided (or is None). In this case, the partition labels are determined automatically from the connectivity of the circuit. #594 improved on this code so that idle wires are removed from the circuit during separation, but the problem is that any qubit with a barrier (e.g., if a barrier is placed over all qubits, even the idle ones) will be falsely detected as an active qubit. Your change should indeed fix #621 but the test does not ensure that it is fixed. The test is actually almost testing the check desired in #626, but not quite.


compare1 = QuantumCircuit(1)
compare1.h(0)
compare1.x(0)
compare2 = QuantumCircuit(1)
compare2.x(0)
compare2.h(0)
compare3 = QuantumCircuit(1)
compare3.y(0)
compare3.h(0)

for i, operation in enumerate(compare1.data):
self.assertEqual(
operation.operation.name,
separated_circuits.subcircuits["A"].data[i].operation.name,
)
for i, operation in enumerate(compare2.data):
self.assertEqual(
operation.operation.name,
separated_circuits.subcircuits["B"].data[i].operation.name,
)
for i, operation in enumerate(compare3.data):
self.assertEqual(
operation.operation.name,
separated_circuits.subcircuits["C"].data[i].operation.name,
)
self.assertEqual(
[("A", 0), ("B", 0), ("C", 0)], separated_circuits.qubit_map
)

with self.subTest("Test bit mapping with partition labels"):
# Prepare a HWEA and add some measurements to clbits in a random order
circuit = prepare_hwea()
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