Increase coverage of clifford protocols to parity_gates (#6338)

quantumlib · Nov 11, 2023 · 0b90ed6 · 0b90ed6
1 parent 11ae0bd
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Showing 2 changed files with 85 additions and 18 deletions.
diff --git a/cirq-core/cirq/ops/parity_gates.py b/cirq-core/cirq/ops/parity_gates.py
@@ -14,15 +14,23 @@
 
 """Quantum gates that phase with respect to product-of-pauli observables."""
 
-from typing import Any, Dict, List, Optional, Tuple, Union, TYPE_CHECKING
+from typing import Any, Dict, List, Optional, Tuple, Union, TYPE_CHECKING, Sequence
 from typing_extensions import Self
 
 import numpy as np
 
 from cirq import protocols, value
 from cirq._compat import proper_repr
 from cirq._doc import document
-from cirq.ops import gate_features, eigen_gate, common_gates, pauli_gates
+from cirq.ops import (
+    gate_features,
+    eigen_gate,
+    common_gates,
+    pauli_gates,
+    clifford_gate,
+    pauli_interaction_gate,
+)
+
 
 if TYPE_CHECKING:
     import cirq
@@ -87,25 +95,29 @@ def _trace_distance_bound_(self) -> Optional[float]:
         return abs(np.sin(self._exponent * 0.5 * np.pi))
 
     def _decompose_into_clifford_with_qubits_(self, qubits):
-        from cirq.ops.clifford_gate import SingleQubitCliffordGate
-        from cirq.ops.pauli_interaction_gate import PauliInteractionGate
-
         if self.exponent % 2 == 0:
             return []
         if self.exponent % 2 == 0.5:
             return [
-                PauliInteractionGate(pauli_gates.X, False, pauli_gates.X, False).on(*qubits),
-                SingleQubitCliffordGate.X_sqrt.on_each(*qubits),
+                pauli_interaction_gate.PauliInteractionGate(
+                    pauli_gates.X, False, pauli_gates.X, False
+                ).on(*qubits),
+                clifford_gate.SingleQubitCliffordGate.X_sqrt.on_each(*qubits),
             ]
         if self.exponent % 2 == 1:
-            return [SingleQubitCliffordGate.X.on_each(*qubits)]
+            return [clifford_gate.SingleQubitCliffordGate.X.on_each(*qubits)]
         if self.exponent % 2 == 1.5:
             return [
-                PauliInteractionGate(pauli_gates.X, False, pauli_gates.X, False).on(*qubits),
-                SingleQubitCliffordGate.X_nsqrt.on_each(*qubits),
+                pauli_interaction_gate.PauliInteractionGate(
+                    pauli_gates.X, False, pauli_gates.X, False
+                ).on(*qubits),
+                clifford_gate.SingleQubitCliffordGate.X_nsqrt.on_each(*qubits),
             ]
         return NotImplemented
 
+    def _has_stabilizer_effect_(self) -> bool:
+        return self.exponent % 2 in (0, 0.5, 1, 1.5)
+
     def _decompose_(self, qubits: Tuple['cirq.Qid', ...]) -> 'cirq.OP_TREE':
         yield common_gates.YPowGate(exponent=-0.5).on_each(*qubits)
         yield ZZPowGate(exponent=self.exponent, global_shift=self.global_shift)(*qubits)
@@ -192,25 +204,29 @@ def _trace_distance_bound_(self) -> Optional[float]:
         return abs(np.sin(self._exponent * 0.5 * np.pi))
 
     def _decompose_into_clifford_with_qubits_(self, qubits):
-        from cirq.ops.clifford_gate import SingleQubitCliffordGate
-        from cirq.ops.pauli_interaction_gate import PauliInteractionGate
-
         if self.exponent % 2 == 0:
             return []
         if self.exponent % 2 == 0.5:
             return [
-                PauliInteractionGate(pauli_gates.Y, False, pauli_gates.Y, False).on(*qubits),
-                SingleQubitCliffordGate.Y_sqrt.on_each(*qubits),
+                pauli_interaction_gate.PauliInteractionGate(
+                    pauli_gates.Y, False, pauli_gates.Y, False
+                ).on(*qubits),
+                clifford_gate.SingleQubitCliffordGate.Y_sqrt.on_each(*qubits),
             ]
         if self.exponent % 2 == 1:
-            return [SingleQubitCliffordGate.Y.on_each(*qubits)]
+            return [clifford_gate.SingleQubitCliffordGate.Y.on_each(*qubits)]
         if self.exponent % 2 == 1.5:
             return [
-                PauliInteractionGate(pauli_gates.Y, False, pauli_gates.Y, False).on(*qubits),
-                SingleQubitCliffordGate.Y_nsqrt.on_each(*qubits),
+                pauli_interaction_gate.PauliInteractionGate(
+                    pauli_gates.Y, False, pauli_gates.Y, False
+                ).on(*qubits),
+                clifford_gate.SingleQubitCliffordGate.Y_nsqrt.on_each(*qubits),
             ]
         return NotImplemented
 
+    def _has_stabilizer_effect_(self) -> bool:
+        return self.exponent % 2 in (0, 0.5, 1, 1.5)
+
     def _decompose_(self, qubits: Tuple['cirq.Qid', ...]) -> 'cirq.OP_TREE':
         yield common_gates.XPowGate(exponent=0.5).on_each(*qubits)
         yield ZZPowGate(exponent=self.exponent, global_shift=self.global_shift)(*qubits)
@@ -265,6 +281,34 @@ def _decompose_(self, qubits):
             exponent=-2 * self.exponent, global_shift=-self.global_shift / 2
         )(qubits[0], qubits[1])
 
+    def _decompose_into_clifford_with_qubits_(
+        self, qubits: Sequence['cirq.Qid']
+    ) -> Sequence[Union['cirq.Operation', Sequence['cirq.Operation']]]:
+        if not self._has_stabilizer_effect_():
+            return NotImplemented
+        if self.exponent % 2 == 0:
+            return []
+        if self.exponent % 2 == 1:
+            return clifford_gate.SingleQubitCliffordGate.Z.on_each(*qubits)
+
+        if self.exponent % 2 == 0.5:
+            return [
+                pauli_interaction_gate.PauliInteractionGate(
+                    pauli_gates.Z, False, pauli_gates.Z, False
+                ).on(*qubits),
+                clifford_gate.SingleQubitCliffordGate.Z_sqrt.on_each(*qubits),
+            ]
+        else:
+            return [
+                pauli_interaction_gate.PauliInteractionGate(
+                    pauli_gates.Z, False, pauli_gates.Z, False
+                ).on(*qubits),
+                clifford_gate.SingleQubitCliffordGate.Z_nsqrt.on_each(*qubits),
+            ]
+
+    def _has_stabilizer_effect_(self) -> bool:
+        return self.exponent % 2 in (0, 0.5, 1, 1.5)
+
     def _eigen_components(self) -> List[Tuple[float, np.ndarray]]:
         return [(0, np.diag([1, 0, 0, 1])), (1, np.diag([0, 1, 1, 0]))]
 

diff --git a/cirq-core/cirq/ops/parity_gates_test.py b/cirq-core/cirq/ops/parity_gates_test.py
@@ -332,3 +332,26 @@ def custom_resolver(cirq_type: str):
         json_text=cirq.to_json(cirq.ms(np.pi / 2)), resolvers=[custom_resolver]
     ) == cirq.ms(np.pi / 2)
     assert custom_resolver('X') is None
+
+
+@pytest.mark.parametrize('gate_cls', (cirq.XXPowGate, cirq.YYPowGate, cirq.ZZPowGate))
+@pytest.mark.parametrize(
+    'exponent,is_clifford',
+    ((0, True), (0.5, True), (0.75, False), (1, True), (1.5, True), (-1.5, True)),
+)
+def test_clifford_protocols(gate_cls: type[cirq.EigenGate], exponent: float, is_clifford: bool):
+    gate = gate_cls(exponent=exponent)
+    assert hasattr(gate, '_decompose_into_clifford_with_qubits_')
+    if is_clifford:
+        clifford_decomposition = cirq.Circuit(
+            gate._decompose_into_clifford_with_qubits_(cirq.LineQubit.range(2))
+        )
+        assert cirq.has_stabilizer_effect(gate)
+        assert cirq.has_stabilizer_effect(clifford_decomposition)
+        if exponent == 0:
+            assert clifford_decomposition == cirq.Circuit()
+        else:
+            np.testing.assert_allclose(cirq.unitary(gate), cirq.unitary(clifford_decomposition))
+    else:
+        assert not cirq.has_stabilizer_effect(gate)
+        assert gate._decompose_into_clifford_with_qubits_(cirq.LineQubit.range(2)) is NotImplemented