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Speed up execution time of merge_single_qubit_moments_to_phxz transformer by avoiding redundant calls to unitary protocol #6174

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merged 5 commits into from
Jul 11, 2023
Merged

Speed up execution time of merge_single_qubit_moments_to_phxz transformer by avoiding redundant calls to unitary protocol #6174

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832bd9f
Speed up execution time of merge_single_qubit_moments_to_phxz transfo…
tanujkhattar Jun 28, 2023
d2760e6
Fix mypy errors
tanujkhattar Jun 28, 2023
e58c96f
Merge branch 'master' into merge_single_qubit_moments_to_phxz_fast
tanujkhattar Jun 29, 2023
6464578
Merge branch 'master' into merge_single_qubit_moments_to_phxz_fast
tanujkhattar Jul 10, 2023
9823d4e
Merge branch 'master' into merge_single_qubit_moments_to_phxz_fast
tanujkhattar Jul 10, 2023
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21 changes: 17 additions & 4 deletions cirq-core/cirq/transformers/merge_single_qubit_gates.py
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Original file line number Diff line number Diff line change
Expand Up @@ -127,10 +127,23 @@ def merge_func(m1: 'cirq.Moment', m2: 'cirq.Moment') -> Optional['cirq.Moment']:
return None
ret_ops = []
for q in m1.qubits | m2.qubits:
mat = protocols.unitary(circuits.Circuit(m.operation_at(q) or [] for m in [m1, m2]))
gate = single_qubit_decompositions.single_qubit_matrix_to_phxz(mat, atol)
if gate:
ret_ops.append(gate(q))
op1, op2 = m1.operation_at(q), m2.operation_at(q)
if op1 and op2:
mat = protocols.unitary(op2) @ protocols.unitary(op1)
gate = single_qubit_decompositions.single_qubit_matrix_to_phxz(mat, atol)
if gate:
ret_ops.append(gate(q))
Comment on lines +134 to +135
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It seems that this can return an empty moment if this condition and another at line 145 are False.

Does the merge_moments function below handle such case and keeps the original moments unchanged?

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It seems merge_moments expects merge_func returns None when moments cannot be merged, please correct the return value below.

Ref:

Cirq/cirq-core/cirq/transformers/transformer_primitives.py

Lines 543 to 546 in 7b753f6

if merged_moment is None:
merged_moments.append(current_moment)
else:
merged_moments[-1] = merged_moment

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single_qubit_matrix_to_phxz returns None when the matrix corresponds to an identity gate (it doesn't represent a failure to construct a PhasedXZGate).

If all merged operations correspond to identity, it is perfectly reasonable to return an empty moment corresponding to the "merged" moment from the merge_func and thus replace two moments containing single qubit gates with a single empty moment in the circuit.

Note that we already check whether the two moments m1 and m2 are merge-able or not in the can_merge_moment condition at the beginning of merge_func. Thus, no additional changes are needed here and the behavior is as intended.

else:
op = op1 or op2
assert op is not None
if isinstance(op.gate, ops.PhasedXZGate):
ret_ops.append(op)
else:
gate = single_qubit_decompositions.single_qubit_matrix_to_phxz(
protocols.unitary(op), atol
)
if gate:
ret_ops.append(gate(q))
return circuits.Moment(ret_ops)

return transformer_primitives.merge_moments(
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