Sync _has_stabilizer_effect_ implementations and use that for Cliffor…

…d checks (#3656)

Brings has_stabilizer_effect protocol in line with actual implementations so that it can be used in all Clifford eligibility checks instead of a patchwork of custom conditions.

#2423

cirq/ops/common_gates.py

@@ -96,7 +96,7 @@ def _act_on_(self, args: Any):
         from cirq.sim import clifford
 
         if isinstance(args, clifford.ActOnCliffordTableauArgs):
-            if protocols.is_parameterized(self) or self.exponent % 0.5 != 0:
+            if not protocols.has_stabilizer_effect(self):
                 return NotImplemented
             tableau = args.tableau
             q = args.axes[0]
@@ -112,7 +112,7 @@ def _act_on_(self, args: Any):
             return True
 
         if isinstance(args, clifford.ActOnStabilizerCHFormArgs):
-            if protocols.is_parameterized(self) or self.exponent % 0.5 != 0:
+            if not protocols.has_stabilizer_effect(self):
                 return NotImplemented
             _act_with_gates(args, H, ZPowGate(exponent=self._exponent), H)
             # Adjust the global phase based on the global_shift parameter.
@@ -251,7 +251,7 @@ def _phase_by_(self, phase_turns, qubit_index):
     def _has_stabilizer_effect_(self) -> Optional[bool]:
         if self._is_parameterized_():
             return None
-        return self.exponent % 1 == 0
+        return self.exponent % 0.5 == 0
 
     def __str__(self) -> str:
         if self._global_shift == -0.5:
@@ -319,7 +319,7 @@ def _act_on_(self, args: Any):
         from cirq.sim import clifford
 
         if isinstance(args, clifford.ActOnCliffordTableauArgs):
-            if protocols.is_parameterized(self) or self.exponent % 0.5 != 0:
+            if not protocols.has_stabilizer_effect(self):
                 return NotImplemented
             tableau = args.tableau
             q = args.axes[0]
@@ -341,7 +341,7 @@ def _act_on_(self, args: Any):
             return True
 
         if isinstance(args, clifford.ActOnStabilizerCHFormArgs):
-            if protocols.is_parameterized(self) or self.exponent % 0.5 != 0:
+            if not protocols.has_stabilizer_effect(self):
                 return NotImplemented
             effective_exponent = self._exponent % 2
             state = args.state
@@ -442,7 +442,7 @@ def _phase_by_(self, phase_turns, qubit_index):
     def _has_stabilizer_effect_(self) -> Optional[bool]:
         if self._is_parameterized_():
             return None
-        return self.exponent % 1 == 0
+        return self.exponent % 0.5 == 0
 
     def __str__(self) -> str:
         if self._global_shift == -0.5:
@@ -508,7 +508,7 @@ def _act_on_(self, args: Any):
         from cirq.sim import clifford
 
         if isinstance(args, clifford.ActOnCliffordTableauArgs):
-            if protocols.is_parameterized(self) or self.exponent % 0.5 != 0:
+            if not protocols.has_stabilizer_effect(self):
                 return NotImplemented
             tableau = args.tableau
             q = args.axes[0]
@@ -524,7 +524,7 @@ def _act_on_(self, args: Any):
             return True
 
         if isinstance(args, clifford.ActOnStabilizerCHFormArgs):
-            if protocols.is_parameterized(self) or self.exponent % 0.5 != 0:
+            if not protocols.has_stabilizer_effect(self):
                 return NotImplemented
             q = args.axes[0]
             effective_exponent = self._exponent % 2
@@ -795,7 +795,7 @@ def _act_on_(self, args: Any):
         from cirq.sim import clifford
 
         if isinstance(args, clifford.ActOnCliffordTableauArgs):
-            if protocols.is_parameterized(self) or self.exponent % 1 != 0:
+            if not protocols.has_stabilizer_effect(self):
                 return NotImplemented
             tableau = args.tableau
             q = args.axes[0]
@@ -808,7 +808,7 @@ def _act_on_(self, args: Any):
             return True
 
         if isinstance(args, clifford.ActOnStabilizerCHFormArgs):
-            if protocols.is_parameterized(self) or self.exponent % 1 != 0:
+            if not protocols.has_stabilizer_effect(self):
                 return NotImplemented
             q = args.axes[0]
             state = args.state
@@ -958,7 +958,7 @@ def _act_on_(self, args: Any):
         from cirq.sim import clifford
 
         if isinstance(args, clifford.ActOnCliffordTableauArgs):
-            if protocols.is_parameterized(self) or self.exponent % 1 != 0:
+            if not protocols.has_stabilizer_effect(self):
                 return NotImplemented
             tableau = args.tableau
             q1 = args.axes[0]
@@ -981,7 +981,7 @@ def _act_on_(self, args: Any):
             return True
 
         if isinstance(args, clifford.ActOnStabilizerCHFormArgs):
-            if protocols.is_parameterized(self) or self.exponent % 1 != 0:
+            if not protocols.has_stabilizer_effect(self):
                 return NotImplemented
             q1 = args.axes[0]
             q2 = args.axes[1]
@@ -1181,7 +1181,7 @@ def _act_on_(self, args: Any):
         from cirq.sim import clifford
 
         if isinstance(args, clifford.ActOnCliffordTableauArgs):
-            if protocols.is_parameterized(self) or self.exponent % 1 != 0:
+            if not protocols.has_stabilizer_effect(self):
                 return NotImplemented
             tableau = args.tableau
             q1 = args.axes[0]
@@ -1197,7 +1197,7 @@ def _act_on_(self, args: Any):
             return True
 
         if isinstance(args, clifford.ActOnStabilizerCHFormArgs):
-            if protocols.is_parameterized(self) or self.exponent % 1 != 0:
+            if not protocols.has_stabilizer_effect(self):
                 return NotImplemented
             q1 = args.axes[0]
             q2 = args.axes[1]

cirq/ops/common_gates_test.py

@@ -917,32 +917,8 @@ def test_parameterized_cphase():
     assert cirq.cphase(sympy.pi / 2) == cirq.CZ ** 0.5
 
 
-def test_x_stabilizer():
-    gate = cirq.X
-    assert cirq.has_stabilizer_effect(gate)
-    assert not cirq.has_stabilizer_effect(gate ** 0.5)
-    assert cirq.has_stabilizer_effect(gate ** 0)
-    assert not cirq.has_stabilizer_effect(gate ** -0.5)
-    assert cirq.has_stabilizer_effect(gate ** 4)
-    assert not cirq.has_stabilizer_effect(gate ** 1.2)
-    foo = sympy.Symbol('foo')
-    assert not cirq.has_stabilizer_effect(gate ** foo)
-
-
-def test_y_stabilizer():
-    gate = cirq.Y
-    assert cirq.has_stabilizer_effect(gate)
-    assert not cirq.has_stabilizer_effect(gate ** 0.5)
-    assert cirq.has_stabilizer_effect(gate ** 0)
-    assert not cirq.has_stabilizer_effect(gate ** -0.5)
-    assert cirq.has_stabilizer_effect(gate ** 4)
-    assert not cirq.has_stabilizer_effect(gate ** 1.2)
-    foo = sympy.Symbol('foo')
-    assert not cirq.has_stabilizer_effect(gate ** foo)
-
-
-def test_z_stabilizer():
-    gate = cirq.Z
+@pytest.mark.parametrize('gate', [cirq.X, cirq.Y, cirq.Z])
+def test_x_y_z_stabilizer(gate):
     assert cirq.has_stabilizer_effect(gate)
     assert cirq.has_stabilizer_effect(gate ** 0.5)
     assert cirq.has_stabilizer_effect(gate ** 0)
@@ -965,20 +941,8 @@ def test_h_stabilizer():
     assert not cirq.has_stabilizer_effect(gate ** foo)
 
 
-def test_cz_stabilizer():
-    gate = cirq.CZ
-    assert cirq.has_stabilizer_effect(gate)
-    assert not cirq.has_stabilizer_effect(gate ** 0.5)
-    assert cirq.has_stabilizer_effect(gate ** 0)
-    assert not cirq.has_stabilizer_effect(gate ** -0.5)
-    assert cirq.has_stabilizer_effect(gate ** 4)
-    assert not cirq.has_stabilizer_effect(gate ** 1.2)
-    foo = sympy.Symbol('foo')
-    assert not cirq.has_stabilizer_effect(gate ** foo)
-
-
-def test_cnot_stabilizer():
-    gate = cirq.CNOT
+@pytest.mark.parametrize('gate', [cirq.CX, cirq.CZ])
+def test_cx_cz_stabilizer(gate):
     assert cirq.has_stabilizer_effect(gate)
     assert not cirq.has_stabilizer_effect(gate ** 0.5)
     assert cirq.has_stabilizer_effect(gate ** 0)

cirq/ops/global_phase_op.py

@@ -57,6 +57,9 @@ def _apply_unitary_(self, args) -> np.ndarray:
         args.target_tensor *= self.coefficient
         return args.target_tensor
 
+    def _has_stabilizer_effect_(self) -> bool:
+        return True
+
     def _act_on_(self, args: Any):
         from cirq.sim import clifford
 

cirq/ops/global_phase_op_test.py

@@ -23,6 +23,7 @@ def test_init():
     assert op.coefficient == 1j
     assert op.qubits == ()
     assert op.with_qubits() is op
+    assert cirq.has_stabilizer_effect(op)
 
     with pytest.raises(ValueError, match='not unitary'):
         _ = cirq.GlobalPhaseOperation(2)

cirq/ops/measurement_gate.py

@@ -217,6 +217,9 @@ def _from_json_dict_(cls, num_qubits, key, invert_mask, qid_shape=None, **kwargs
             qid_shape=None if qid_shape is None else tuple(qid_shape),
         )
 
+    def _has_stabilizer_effect_(self) -> Optional[bool]:
+        return True
+
     def _act_on_(self, args: Any) -> bool:
         from cirq import sim
 

cirq/ops/measurement_gate_test.py

@@ -34,6 +34,11 @@ def test_measure_init(num_qubits):
         cirq.MeasurementGate()
 
 
+@pytest.mark.parametrize('num_qubits', [1, 2, 4])
+def test_has_stabilizer_effect(num_qubits):
+    assert cirq.has_stabilizer_effect(cirq.MeasurementGate(num_qubits))
+
+
 def test_measurement_eq():
     eq = cirq.testing.EqualsTester()
     eq.make_equality_group(

cirq/protocols/has_stabilizer_effect_protocol.py

@@ -69,9 +69,9 @@ def _strat_has_stabilizer_effect_from_unitary(val: Any) -> Optional[bool]:
     Returns whether unitary of `val` normalizes the Pauli group. Works only for
     2x2 unitaries.
     """
-    if not protocols.has_unitary(val):
+    # Do not try this strategy if there is no unitary or if the number of
+    # qubits is not 1 since that would be expensive.
+    if not protocols.has_unitary(val) or protocols.num_qubits(val) != 1:
         return None
     unitary = protocols.unitary(val)
-    if unitary.shape == (2, 2):
-        return SingleQubitCliffordGate.from_unitary(unitary) is not None
-    return None
+    return SingleQubitCliffordGate.from_unitary(unitary) is not None

cirq/protocols/has_stabilizer_effect_protocol_test.py

@@ -41,13 +41,16 @@ def _has_stabilizer_effect_(self):
         return True
 
 
+q = cirq.LineQubit(0)
+
+
 class EmptyOp(cirq.Operation):
     """A trivial operation."""
 
     @property
     def qubits(self):
         # coverage: ignore
-        return ()
+        return (q,)
 
     def with_qubits(self, *new_qubits):
         # coverage: ignore

cirq/sim/clifford/clifford_simulator.py

@@ -32,13 +32,11 @@
 from typing import Any, Dict, List, Iterator, Sequence
 
 import numpy as np
-from cirq.ops.global_phase_op import GlobalPhaseOperation
 
 import cirq
 from cirq import circuits, study, ops, protocols, value
-from cirq.ops.clifford_gate import SingleQubitCliffordGate
 from cirq.ops.dense_pauli_string import DensePauliString
-from cirq.protocols import act_on, unitary
+from cirq.protocols import act_on
 from cirq.sim import clifford, simulator
 from cirq._compat import deprecated, deprecated_parameter
 from cirq.sim.simulator import check_all_resolved
@@ -60,17 +58,7 @@ def __init__(self, seed: 'cirq.RANDOM_STATE_OR_SEED_LIKE' = None):
     def is_supported_operation(op: 'cirq.Operation') -> bool:
         """Checks whether given operation can be simulated by this simulator."""
         # TODO: support more general Pauli measurements
-        if isinstance(op.gate, cirq.MeasurementGate):
-            return True
-        if isinstance(op, GlobalPhaseOperation):
-            return True
-        if not protocols.has_unitary(op):
-            return False
-        if len(op.qubits) == 1:
-            u = unitary(op)
-            return SingleQubitCliffordGate.from_unitary(u) is not None
-        else:
-            return op.gate in [cirq.CNOT, cirq.CZ]
+        return protocols.has_stabilizer_effect(op)
 
     def _base_iterator(
         self, circuit: circuits.Circuit, qubit_order: ops.QubitOrderOrList, initial_state: int