Simplify decomposition of controlled eigengates with global phase

Author: codrut3

cirq-core/cirq/ops/classically_controlled_operation_test.py

@@ -473,7 +473,8 @@ def test_decompose():
     op = cirq.H(q0).with_classical_controls('a')
     assert cirq.decompose(op) == [
         (cirq.Y(q0) ** 0.5).with_classical_controls('a'),
-        cirq.XPowGate(exponent=1.0, global_shift=-0.25).on(q0).with_classical_controls('a'),
+        cirq.X(q0).with_classical_controls('a'),
+        cirq.global_phase_operation(1j**-0.5).with_classical_controls('a'),
     ]
 
 

cirq-core/cirq/ops/common_gates.py

@@ -352,6 +352,12 @@ def __init__(self, *, rads: value.TParamVal):
     def _with_exponent(self, exponent: value.TParamVal) -> 'Rx':
         return Rx(rads=exponent * _pi(exponent))
 
+    def _decompose_(self, qubits: Tuple['cirq.Qid', ...]) -> NotImplementedType:
+        """Returns:
+        NotImplemented, to signify the gate doesn't decompose further.
+        """
+        return NotImplemented
+
     def _circuit_diagram_info_(
         self, args: 'cirq.CircuitDiagramInfoArgs'
     ) -> Union[str, 'protocols.CircuitDiagramInfo']:
@@ -537,6 +543,12 @@ def __init__(self, *, rads: value.TParamVal):
     def _with_exponent(self, exponent: value.TParamVal) -> 'Ry':
         return Ry(rads=exponent * _pi(exponent))
 
+    def _decompose_(self, qubits: Tuple['cirq.Qid', ...]) -> NotImplementedType:
+        """Returns:
+        NotImplemented, to signify the gate doesn't decompose further.
+        """
+        return NotImplemented
+
     def _circuit_diagram_info_(
         self, args: 'cirq.CircuitDiagramInfoArgs'
     ) -> Union[str, 'protocols.CircuitDiagramInfo']:
@@ -882,6 +894,12 @@ def __init__(self, *, rads: value.TParamVal):
     def _with_exponent(self, exponent: value.TParamVal) -> 'Rz':
         return Rz(rads=exponent * _pi(exponent))
 
+    def _decompose_(self, qubits: Tuple['cirq.Qid', ...]) -> NotImplementedType:
+        """Returns:
+        NotImplemented, to signify the gate doesn't decompose further.
+        """
+        return NotImplemented
+
     def _circuit_diagram_info_(
         self, args: 'cirq.CircuitDiagramInfoArgs'
     ) -> Union[str, 'protocols.CircuitDiagramInfo']:

cirq-core/cirq/ops/controlled_gate.py

@@ -33,6 +33,7 @@
     control_values as cv,
     controlled_operation as cop,
     diagonal_gate as dg,
+    eigen_gate,
     global_phase_op as gp,
     op_tree,
     raw_types,
@@ -159,6 +160,12 @@ def _decompose_with_context_(
         self, qubits: Tuple['cirq.Qid', ...], context: Optional['cirq.DecompositionContext'] = None
     ) -> Union[None, NotImplementedType, 'cirq.OP_TREE']:
         control_qubits = list(qubits[: self.num_controls()])
+        # If the subgate is an EigenGate with non-zero phase, try to decompose it
+        # into a phase-free gate and a global phase gate.
+        if isinstance(self.sub_gate, eigen_gate.EigenGate) and self.sub_gate.global_shift != 0:
+            result = self._decompose_sub_gate_with_controls(qubits, context)
+            if result is not NotImplemented:
+                return result
         if (
             protocols.has_unitary(self.sub_gate)
             and protocols.num_qubits(self.sub_gate) == 1
@@ -219,6 +226,11 @@ def _decompose_with_context_(
                     control_qid_shape=self.control_qid_shape,
                 ).on(*control_qubits)
                 return [result, controlled_phase_op]
+        return self._decompose_sub_gate_with_controls(qubits, context)
+
+    def _decompose_sub_gate_with_controls(
+        self, qubits: Tuple['cirq.Qid', ...], context: Optional['cirq.DecompositionContext'] = None
+    ) -> Union[None, NotImplementedType, 'cirq.OP_TREE']:
         result = protocols.decompose_once_with_qubits(
             self.sub_gate,
             qubits[self.num_controls() :],

cirq-core/cirq/ops/controlled_gate_test.py

@@ -494,6 +494,22 @@ def _test_controlled_gate_is_consistent(
         np.testing.assert_allclose(cirq.unitary(cgate), cirq.unitary(circuit), atol=1e-13)
 
 
+@pytest.mark.parametrize(
+    'sub_gate, expected_decomposition',
+    [
+        (cirq.XPowGate(global_shift=0.22), [cirq.Y**-0.5, cirq.CZ, cirq.Y**0.5, cirq.Z**0.22]),
+        (cirq.ZPowGate(exponent=1.2, global_shift=0.3), [cirq.CZ**1.2, cirq.Z**0.36]),
+    ],
+)
+def test_decompose_takes_out_global_phase(
+    sub_gate: cirq.Gate, expected_decomposition: Sequence[cirq.Gate]
+):
+    cgate = cirq.ControlledGate(sub_gate, num_controls=1)
+    qubits = cirq.LineQubit.range(cgate.num_qubits())
+    dec = cirq.decompose(cgate.on(*qubits))
+    assert [op.gate for op in dec] == expected_decomposition
+
+
 def test_pow_inverse():
     assert cirq.inverse(CRestricted, None) is None
     assert cirq.pow(CRestricted, 1.5, None) is None

cirq-core/cirq/ops/eigen_gate.py

@@ -34,7 +34,7 @@
 import sympy
 
 from cirq import protocols, value
-from cirq.ops import raw_types
+from cirq.ops import global_phase_op, raw_types
 
 if TYPE_CHECKING:
     import cirq
@@ -375,6 +375,24 @@ def _json_dict_(self) -> Dict[str, Any]:
     def _measurement_key_objs_(self):
         return frozenset()
 
+    def _decompose_(
+        self, qubits: Tuple['cirq.Qid', ...]
+    ) -> Union[NotImplementedType, 'cirq.OP_TREE']:
+        """Attempts to decompose the gate into a phase-free gate and a global phase gate.
+
+        Returns:
+            NotImplemented, if global phase or exponent are 0. Otherwise a phase-free gate
+            applied to the qubits followed by a global phase gate.
+        """
+        if self.global_shift == 0 or self.exponent == 0:
+            return NotImplemented
+        self_without_phase = self._with_exponent(self.exponent)
+        # This doesn't work for gates that fix global_shift, such as Rx. These gates must define
+        # their own _decompose_ method.
+        self_without_phase._global_shift = 0
+        global_phase = 1j ** (2 * self.global_shift * self.exponent)
+        return [self_without_phase.on(*qubits), global_phase_op.GlobalPhaseGate(global_phase)()]
+
 
 def _lcm(vals: Iterable[int]) -> int:
     t = 1

cirq-core/cirq/ops/eigen_gate_test.py

@@ -20,6 +20,7 @@
 
 import cirq
 from cirq import value
+from cirq.ops import global_phase_op
 from cirq.testing import assert_has_consistent_trace_distance_bound
 
 
@@ -421,3 +422,43 @@ def _with_exponent(self, exponent):
 )
 def test_equal_up_to_global_phase(gate1, gate2, eq_up_to_global_phase):
     assert cirq.equal_up_to_global_phase(gate1, gate2) == eq_up_to_global_phase
+
+
+@pytest.mark.parametrize(
+    'gate',
+    [
+        cirq.Z,
+        cirq.Z**2,
+        cirq.XPowGate(global_shift=0.0),
+        cirq.rx(0),
+        cirq.ry(0),
+        cirq.rz(0),
+        cirq.CZPowGate(exponent=0.0, global_shift=0.25),
+    ],
+)
+def test_decompose_once_returns_not_implemented(gate: cirq.Gate):
+    qubits = cirq.LineQubit.range(gate.num_qubits())
+    assert cirq.decompose_once(gate.on(*qubits), default=NotImplemented) == NotImplemented
+
+
+@pytest.mark.parametrize(
+    'gate, expected_decomposition',
+    [
+        (cirq.X, [cirq.X]),
+        (cirq.ZPowGate(global_shift=0.5), [cirq.Z, global_phase_op.GlobalPhaseGate(1j)]),
+        (
+            cirq.ZPowGate(global_shift=0.5) ** sympy.Symbol('e'),
+            [
+                cirq.Z ** sympy.Symbol('e'),
+                global_phase_op.GlobalPhaseGate(1j ** (1.0 * sympy.Symbol('e'))),
+            ],
+        ),
+        (cirq.rx(np.pi / 2), [cirq.rx(np.pi / 2)]),
+        (cirq.ry(np.pi / 2), [cirq.ry(np.pi / 2)]),
+        (cirq.rz(np.pi / 2), [cirq.rz(np.pi / 2)]),
+    ],
+)
+def test_decompose_takes_out_global_phase(gate: cirq.Gate, expected_decomposition: List[cirq.Gate]):
+    qubits = cirq.LineQubit.range(gate.num_qubits())
+    dec = cirq.decompose(gate.on(*qubits))
+    assert [op.gate for op in dec] == expected_decomposition

cirq-core/cirq/ops/parity_gates_test.py

@@ -14,6 +14,8 @@
 
 """Tests for `parity_gates.py`."""
 
+from typing import List
+
 import numpy as np
 import pytest
 import sympy
@@ -348,3 +350,60 @@ def test_clifford_protocols(gate_cls: type[cirq.EigenGate], exponent: float, is_
     else:
         assert not cirq.has_stabilizer_effect(gate)
         assert gate._decompose_into_clifford_with_qubits_(cirq.LineQubit.range(2)) is NotImplemented
+
+
+@pytest.mark.parametrize(
+    'gate, expected_decomposition',
+    [
+        (
+            cirq.XXPowGate(),
+            [
+                (cirq.Y**-0.5).on(cirq.LineQubit(0)),
+                (cirq.Y**-0.5).on(cirq.LineQubit(1)),
+                cirq.Z(cirq.LineQubit(0)),
+                cirq.Z(cirq.LineQubit(1)),
+                (cirq.CZ**-2.0).on(cirq.LineQubit(0), cirq.LineQubit(1)),
+                (cirq.Y**0.5).on(cirq.LineQubit(0)),
+                (cirq.Y**0.5).on(cirq.LineQubit(1)),
+            ],
+        ),
+        (
+            cirq.YYPowGate(),
+            [
+                (cirq.X**0.5).on(cirq.LineQubit(0)),
+                (cirq.X**0.5).on(cirq.LineQubit(1)),
+                cirq.Z(cirq.LineQubit(0)),
+                cirq.Z(cirq.LineQubit(1)),
+                (cirq.CZ**-2.0).on(cirq.LineQubit(0), cirq.LineQubit(1)),
+                (cirq.X**-0.5).on(cirq.LineQubit(0)),
+                (cirq.X**-0.5).on(cirq.LineQubit(1)),
+            ],
+        ),
+        (
+            cirq.ZZPowGate(),
+            [
+                cirq.Z(cirq.LineQubit(0)),
+                cirq.Z(cirq.LineQubit(1)),
+                (cirq.CZ**-2.0).on(cirq.LineQubit(0), cirq.LineQubit(1)),
+            ],
+        ),
+        (
+            cirq.MSGate(rads=0),
+            [
+                (cirq.Y**-0.5).on(cirq.LineQubit(0)),
+                (cirq.Y**-0.5).on(cirq.LineQubit(1)),
+                (cirq.Z**0.0).on(cirq.LineQubit(0)),
+                (cirq.Z**0.0).on(cirq.LineQubit(1)),
+                cirq.CZPowGate(exponent=-0.0, global_shift=0.25).on(
+                    cirq.LineQubit(0), cirq.LineQubit(1)
+                ),
+                (cirq.Y**0.5).on(cirq.LineQubit(0)),
+                (cirq.Y**0.5).on(cirq.LineQubit(1)),
+            ],
+        ),
+    ],
+)
+def test_gate_decomposition(gate: cirq.Gate, expected_decomposition: List[cirq.Gate]):
+    qubits = cirq.LineQubit.range(gate.num_qubits())
+    dec = cirq.decompose(gate.on(*qubits))
+    assert [op for op in dec] == expected_decomposition

cirq-core/cirq/ops/raw_types_test.py

@@ -647,7 +647,7 @@ def test_tagged_operation_forwards_protocols():
     np.testing.assert_equal(cirq.unitary(tagged_h), cirq.unitary(h))
     assert cirq.has_unitary(tagged_h)
     assert cirq.decompose(tagged_h) == cirq.decompose(h)
-    assert [*tagged_h._decompose_()] == cirq.decompose(h)
+    assert [*tagged_h._decompose_()] == cirq.decompose_once(h)
     assert cirq.pauli_expansion(tagged_h) == cirq.pauli_expansion(h)
     assert cirq.equal_up_to_global_phase(h, tagged_h)
     assert np.isclose(cirq.kraus(h), cirq.kraus(tagged_h)).all()

cirq-core/cirq/transformers/merge_single_qubit_gates.py

@@ -116,7 +116,7 @@ def merge_single_qubit_moments_to_phxz(
 
     def can_merge_moment(m: 'cirq.Moment'):
         return all(
-            protocols.num_qubits(op) == 1
+            (protocols.num_qubits(op) == 1 or protocols.num_qubits(op) == 0)
             and protocols.has_unitary(op)
             and tags_to_ignore.isdisjoint(op.tags)
             for op in m
@@ -144,6 +144,10 @@ def merge_func(m1: 'cirq.Moment', m2: 'cirq.Moment') -> Optional['cirq.Moment']:
                     )
                     if gate:
                         ret_ops.append(gate(q))
+        # Transfer global phase
+        for op in m1.operations + m2.operations:
+            if protocols.num_qubits(op) == 0:
+                ret_ops.append(op)
         return circuits.Moment(ret_ops)
 
     return transformer_primitives.merge_moments(

cirq-core/cirq/transformers/merge_single_qubit_gates_test.py

@@ -221,13 +221,45 @@ def test_merge_single_qubit_moments_to_phxz_deep():
     )
 
 
-def test_merge_single_qubit_moments_to_phxz_global_phase():
+def test_merge_single_qubit_gates_to_phxz_global_phase():
     c = cirq.Circuit(cirq.GlobalPhaseGate(1j).on())
     c2 = cirq.merge_single_qubit_gates_to_phxz(c)
     assert c == c2
 
 
-def test_merge_single_qubit_moments_to_phased_x_and_z_global_phase():
+def test_merge_single_qubit_gates_to_phased_x_and_z_global_phase():
     c = cirq.Circuit(cirq.GlobalPhaseGate(1j).on())
     c2 = cirq.merge_single_qubit_gates_to_phased_x_and_z(c)
     assert c == c2
+
+
+def test_merge_single_qubit_moments_to_phxz_with_global_phase_in_first_moment():
+    q0 = cirq.LineQubit(0)
+    c_orig = cirq.Circuit(
+        cirq.Moment(cirq.Y(q0) ** 0.5, cirq.GlobalPhaseGate(1j**0.5).on()), cirq.Moment(cirq.X(q0))
+    )
+    c_expected = cirq.Circuit(
+        cirq.Moment(
+            cirq.PhasedXZGate(axis_phase_exponent=-0.5, x_exponent=0.5, z_exponent=-1.0).on(q0),
+            cirq.GlobalPhaseGate(1j**0.5).on(),
+        )
+    )
+    context = cirq.TransformerContext(tags_to_ignore=["ignore"])
+    c_new = cirq.merge_single_qubit_moments_to_phxz(c_orig, context=context)
+    assert c_new == c_expected
+
+
+def test_merge_single_qubit_moments_to_phxz_with_global_phase_in_second_moment():
+    q0 = cirq.LineQubit(0)
+    c_orig = cirq.Circuit(
+        cirq.Moment(cirq.Y(q0) ** 0.5), cirq.Moment(cirq.X(q0), cirq.GlobalPhaseGate(1j**0.5).on())
+    )
+    c_expected = cirq.Circuit(
+        cirq.Moment(
+            cirq.PhasedXZGate(axis_phase_exponent=-0.5, x_exponent=0.5, z_exponent=-1.0).on(q0),
+            cirq.GlobalPhaseGate(1j**0.5).on(),
+        )
+    )
+    context = cirq.TransformerContext(tags_to_ignore=["ignore"])
+    c_new = cirq.merge_single_qubit_moments_to_phxz(c_orig, context=context)
+    assert c_new == c_expected