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Fix global phase computation in matrix gate decomposition #7482

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Jul 22, 2025
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Fix global phase computation in matrix gate decomposition #7482

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aee1ca5
Test for misordered qubits in matrix gate decomposition
daxfohl Jul 11, 2025
a1c68c1
Fix misordered qubits in matrix gate decomposition
daxfohl Jul 11, 2025
9b915e9
lint
daxfohl Jul 11, 2025
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6 changes: 3 additions & 3 deletions cirq-core/cirq/ops/matrix_gates.py
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Original file line number Diff line number Diff line change
Expand Up @@ -22,7 +22,7 @@

from cirq import _import, linalg, protocols
from cirq._compat import proper_repr
from cirq.ops import global_phase_op, identity, phased_x_z_gate, raw_types
from cirq.ops import global_phase_op, phased_x_z_gate, raw_types

if TYPE_CHECKING:
import cirq
Expand Down Expand Up @@ -170,8 +170,8 @@ def _decompose_(self, qubits: tuple[cirq.Qid, ...]) -> cirq.OP_TREE:
return NotImplemented
# The above algorithms ignore phase, but phase is important to maintain if the gate is
# controlled. Here, we add it back in with a global phase op.
ident = identity.IdentityGate(qid_shape=self._qid_shape).on(*qubits) # Preserve qid order
u = protocols.unitary(Circuit(ident, *decomposed)).reshape(self._matrix.shape)
circuit = Circuit(*decomposed)
u = circuit.unitary(qubit_order=qubits, qubits_that_should_be_present=qubits)
phase_delta = linalg.phase_delta(u, self._matrix)
# Phase delta is on the complex unit circle, so if real(phase_delta) >= 1, that means
# no phase delta. (>1 is rounding error).
Expand Down
13 changes: 13 additions & 0 deletions cirq-core/cirq/ops/matrix_gates_test.py
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Expand Up @@ -413,3 +413,16 @@ def test_matrixgate_name_serialization():
gate_after_serialization3 = cirq.read_json(json_text=cirq.to_json(gate3))
assert gate3._name == ''
assert gate_after_serialization3._name == ''


def test_decompose_when_qubits_not_in_ascending_order():
# Previous code for preserving global phase would misorder qubits
q0, q1 = cirq.LineQubit.range(2)
circuit1 = cirq.Circuit()
matrix = cirq.testing.random_unitary(4, random_state=0)
circuit1.append(cirq.MatrixGate(matrix).on(q1, q0))
u1 = cirq.unitary(circuit1)
decomposed = cirq.decompose(circuit1)
circuit2 = cirq.Circuit(decomposed)
u2 = cirq.unitary(circuit2)
np.testing.assert_allclose(u1, u2, atol=1e-14)