Add default decomposition for cirq.TwoQubitDiagonalGate (#5084)

* Add default decomposition for cirq.TwoQubitDiagonalGate

* Update pyquil tests

cirq-core/cirq/circuits/quil_output_test.py

@@ -434,14 +434,15 @@ def test_equivalent_unitaries():
 CPHASE(pi/2) 0 1
 """
 
-QUIL_DIAGONAL_DEFGATE_PROGRAM = """
-DEFGATE USERGATE1:
-    1.0, 0.0, 0.0, 0.0
-    0.0, 1.0, 0.0, 0.0
-    0.0, 0.0, 1.0, 0.0
-    0.0, 0.0, 0.0, 1.0
-
-USERGATE1 0 1
+QUIL_DIAGONAL_DECOMPOSE_PROGRAM = """
+RZ(0) 0
+RZ(0) 1
+CPHASE(0) 0 1
+X 0
+X 1
+CPHASE(0) 0 1
+X 0
+X 1
 """
 
 
@@ -463,13 +464,13 @@ def test_two_qubit_diagonal_gate_quil_output():
     # Qubit ordering differs between pyQuil and Cirq.
     cirq_unitary = cirq.Circuit(cirq.SWAP(q0, q1), operations, cirq.SWAP(q0, q1)).unitary()
     assert np.allclose(pyquil_unitary, cirq_unitary)
-    # Also test non-CPHASE case.
+    # Also test non-CPHASE case, which decomposes into X/RZ/CPhase
     operations = [
         cirq.TwoQubitDiagonalGate([0, 0, 0, 0])(q0, q1),
     ]
     output = cirq.QuilOutput(operations, (q0, q1))
     program = pyquil.Program(str(output))
-    assert f"\n{program.out()}" == QUIL_DIAGONAL_DEFGATE_PROGRAM
+    assert f"\n{program.out()}" == QUIL_DIAGONAL_DECOMPOSE_PROGRAM
 
 
 def test_parseable_defgate_output():

cirq-core/cirq/ops/two_qubit_diagonal_gate.py

@@ -23,7 +23,7 @@
 
 from cirq import protocols, value
 from cirq._compat import proper_repr
-from cirq.ops import raw_types
+from cirq.ops import raw_types, common_gates
 
 if TYPE_CHECKING:
     import cirq
@@ -72,6 +72,16 @@ def _unitary_(self) -> Optional[np.ndarray]:
             return None
         return np.diag([np.exp(1j * angle) for angle in self._diag_angles_radians])
 
+    def _decompose_(self, qubits: Sequence['cirq.Qid']) -> 'cirq.OP_TREE':
+        x0, x1, x2, x3 = self._diag_angles_radians
+        q0, q1 = qubits
+        yield common_gates.ZPowGate(exponent=x2 / np.pi).on(q0)
+        yield common_gates.ZPowGate(exponent=x1 / np.pi).on(q1)
+        yield common_gates.CZPowGate(exponent=(x3 - (x1 + x2)) / np.pi).on(q0, q1)
+        yield common_gates.XPowGate().on_each(q0, q1)
+        yield common_gates.CZPowGate(exponent=x0 / np.pi).on(q0, q1)
+        yield common_gates.XPowGate().on_each(q0, q1)
+
     def _apply_unitary_(self, args: 'protocols.ApplyUnitaryArgs') -> np.ndarray:
         if self._is_parameterized_():
             return NotImplemented

cirq-core/cirq/ops/two_qubit_diagonal_gate_test.py

@@ -34,6 +34,22 @@ def test_consistent_protocols(gate):
     cirq.testing.assert_implements_consistent_protocols(gate)
 
 
+def test_parameterized_decompose():
+    angles = sympy.symbols('x0, x1, x2, x3')
+    parameterized_op = cirq.TwoQubitDiagonalGate(angles).on(*cirq.LineQubit.range(2))
+    decomposed_circuit = cirq.Circuit(cirq.decompose(parameterized_op))
+    for resolver in (
+        cirq.Linspace('x0', -2, 2, 6)
+        * cirq.Linspace('x1', -2, 2, 6)
+        * cirq.Linspace('x2', -2, 2, 6)
+        * cirq.Linspace('x3', -2, 2, 6)
+    ):
+        np.testing.assert_allclose(
+            cirq.unitary(cirq.resolve_parameters(parameterized_op, resolver)),
+            cirq.unitary(cirq.resolve_parameters(decomposed_circuit, resolver)),
+        )
+
+
 def test_unitary():
     diagonal_angles = [2, 3, 5, 7]
     assert cirq.has_unitary(cirq.TwoQubitDiagonalGate(diagonal_angles))