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Fix behavior of MergeInteractions for tagged partial CZ gates #4288

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Merged
merged 2 commits into from
Jul 6, 2021
Merged

Fix behavior of MergeInteractions for tagged partial CZ gates #4288

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16234fa
Add test for the failing case with a tagged operation
cduck Jul 5, 2021
b13c256
Fix merge interactions for tagged operation
cduck Jul 5, 2021
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16 changes: 8 additions & 8 deletions cirq-core/cirq/optimizers/merge_interactions.py
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Original file line number Diff line number Diff line change
Expand Up @@ -55,16 +55,9 @@ def optimization_at(

switch_to_new = False
switch_to_new |= any(
len(old_op.qubits) == 2 and not isinstance(old_op.gate, ops.CZPowGate)
len(old_op.qubits) == 2 and not self._may_keep_old_op(old_op)
for old_op in old_operations
)
if not self.allow_partial_czs:
switch_to_new |= any(
isinstance(old_op, ops.GateOperation)
and isinstance(old_op.gate, ops.CZPowGate)
and old_op.gate.exponent != 1
for old_op in old_operations
)

# This point cannot be optimized using this method
if not switch_to_new and old_interaction_count <= 1:
Expand All @@ -89,6 +82,13 @@ def optimization_at(
new_operations=new_operations,
)

def _may_keep_old_op(self, old_op: 'cirq.Operation') -> bool:
"""Returns True if the old two-qubit operation may be left unchanged
without decomposition."""
if self.allow_partial_czs:
return isinstance(old_op.gate, ops.CZPowGate)
return isinstance(old_op.gate, ops.CZPowGate) and old_op.gate.exponent == 1

def _op_to_matrix(
self, op: ops.Operation, qubits: Tuple['cirq.Qid', ...]
) -> Optional[np.ndarray]:
Expand Down
10 changes: 10 additions & 0 deletions cirq-core/cirq/optimizers/merge_interactions_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -163,6 +163,16 @@ def test_optimizes_single_iswap():
assert len([1 for op in c.all_operations() if len(op.qubits) == 2]) == 2


def test_optimizes_tagged_partial_cz():
a, b = cirq.LineQubit.range(2)
c = cirq.Circuit((cirq.CZ ** 0.5)(a, b).with_tags('mytag'))
assert_optimization_not_broken(c)
cirq.MergeInteractions(allow_partial_czs=False).optimize_circuit(c)
assert (
len([1 for op in c.all_operations() if len(op.qubits) == 2]) == 2
), 'It should take 2 CZ gates to decompose a CZ**0.5 gate'


@pytest.mark.parametrize(
'circuit',
(
Expand Down