Add control_keys protocol (simplified) (#4610)

Use control keys in circuit construction to block controlled gates "earliest" append method from falling back to before the measurement.

Parts 5&7 of https://tinyurl.com/cirq-feedforward. (I grouped them together in a single PR so that circuit.append could be a POC that control_keys protocol design works as intended).

Replaces/simplifies #4490 since we will handle "extern" control keys in subcircuits as a separate PR.

cirq-core/cirq/__init__.py

@@ -503,6 +503,7 @@
     CircuitDiagramInfo,
     CircuitDiagramInfoArgs,
     commutes,
+    control_keys,
     decompose,
     decompose_once,
     decompose_once_with_qubits,
@@ -550,6 +551,7 @@
     SupportsConsistentApplyUnitary,
     SupportsCircuitDiagramInfo,
     SupportsCommutes,
+    SupportsControlKey,
     SupportsDecompose,
     SupportsDecomposeWithQubits,
     SupportsEqualUpToGlobalPhase,

cirq-core/cirq/circuits/circuit.py

@@ -1868,9 +1868,14 @@ def transform_qubits(
     def _prev_moment_available(self, op: 'cirq.Operation', end_moment_index: int) -> Optional[int]:
         last_available = end_moment_index
         k = end_moment_index
+        op_control_keys = protocols.control_keys(op)
+        op_qubits = op.qubits
         while k > 0:
             k -= 1
-            if not self._can_commute_past(k, op):
+            moment = self._moments[k]
+            if moment.operates_on(op_qubits) or (
+                op_control_keys & protocols.measurement_key_objs(moment)
+            ):
                 return last_available
             if self._can_add_op_at(k, op):
                 last_available = k
@@ -1924,9 +1929,6 @@ def _can_add_op_at(self, moment_index: int, operation: 'cirq.Operation') -> bool
             return True
         return self._device.can_add_operation_into_moment(operation, self._moments[moment_index])
 
-    def _can_commute_past(self, moment_index: int, operation: 'cirq.Operation') -> bool:
-        return not self._moments[moment_index].operates_on(operation.qubits)
-
     def insert(
         self,
         index: int,

cirq-core/cirq/circuits/circuit_test.py

@@ -83,6 +83,21 @@ def validate_moment(self, moment):
 moment_and_op_type_validating_device = _MomentAndOpTypeValidatingDeviceType()
 
 
+class ControlOp(cirq.Operation):
+    def __init__(self, keys):
+        self._keys = keys
+
+    def with_qubits(self, *new_qids):
+        pass  # coverage: ignore
+
+    @property
+    def qubits(self):
+        return []  # coverage: ignore
+
+    def _control_keys_(self):
+        return self._keys
+
+
 def test_alignment():
     assert repr(cirq.Alignment.LEFT) == 'cirq.Alignment.LEFT'
     assert repr(cirq.Alignment.RIGHT) == 'cirq.Alignment.RIGHT'
@@ -224,6 +239,21 @@ def test_append_single():
     assert c == cirq.Circuit([cirq.Moment([cirq.X(a)])])
 
 
+def test_append_control_key():
+    q = cirq.LineQubit(0)
+
+    c = cirq.Circuit()
+    c.append(cirq.measure(q, key='a'))
+    c.append(ControlOp([cirq.MeasurementKey('a')]))
+    assert len(c) == 2
+
+    c = cirq.Circuit()
+    c.append(cirq.measure(q, key='a'))
+    c.append(ControlOp([cirq.MeasurementKey('b')]))
+    c.append(ControlOp([cirq.MeasurementKey('b')]))
+    assert len(c) == 1
+
+
 def test_append_multiple():
     a = cirq.NamedQubit('a')
     b = cirq.NamedQubit('b')

cirq-core/cirq/protocols/__init__.py

@@ -49,6 +49,10 @@
     definitely_commutes,
     SupportsCommutes,
 )
+from cirq.protocols.control_key_protocol import (
+    control_keys,
+    SupportsControlKey,
+)
 from cirq.protocols.circuit_diagram_info_protocol import (
     circuit_diagram_info,
     CircuitDiagramInfo,

cirq-core/cirq/protocols/control_key_protocol.py

@@ -0,0 +1,60 @@
+# Copyright 2021 The Cirq Developers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Protocol for object that have control keys."""
+
+from typing import AbstractSet, Any, Iterable, TYPE_CHECKING
+
+from typing_extensions import Protocol
+
+from cirq._doc import doc_private
+
+if TYPE_CHECKING:
+    import cirq
+
+
+class SupportsControlKey(Protocol):
+    """An object that is a has a classical control key or keys.
+
+    Control keys are used in referencing the results of a measurement.
+
+    Users should implement `_control_keys_` returning an iterable of
+    `MeasurementKey`.
+    """
+
+    @doc_private
+    def _control_keys_(self) -> Iterable['cirq.MeasurementKey']:
+        """Return the keys for controls referenced by the receiving object.
+
+        Returns:
+            The measurement keys the value is controlled by. If the value is not
+            classically controlled, the result is the empty tuple.
+        """
+
+
+def control_keys(val: Any) -> AbstractSet['cirq.MeasurementKey']:
+    """Gets the keys that the value is classically controlled by.
+
+    Args:
+        val: The object that may be classically controlled.
+
+    Returns:
+        The measurement keys the value is controlled by. If the value is not
+        classically controlled, the result is the empty tuple.
+    """
+    getter = getattr(val, '_control_keys_', None)
+    result = NotImplemented if getter is None else getter()
+    if result is not NotImplemented and result is not None:
+        return set(result)
+
+    return set()

cirq-core/cirq/protocols/control_key_protocol_test.py

@@ -0,0 +1,29 @@
+# Copyright 2021 The Cirq Developers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import cirq
+
+
+def test_control_key():
+    class Named:
+        def _control_keys_(self):
+            return [cirq.MeasurementKey('key')]
+
+    class NoImpl:
+        def _control_keys_(self):
+            return NotImplemented
+
+    assert cirq.control_keys(Named()) == {cirq.MeasurementKey('key')}
+    assert not cirq.control_keys(NoImpl())
+    assert not cirq.control_keys(5)

cirq-core/cirq/protocols/json_test_data/spec.py

@@ -138,6 +138,7 @@
         'SupportsCircuitDiagramInfo',
         'SupportsCommutes',
         'SupportsConsistentApplyUnitary',
+        'SupportsControlKey',
         'SupportsDecompose',
         'SupportsDecomposeWithQubits',
         'SupportsEqualUpToGlobalPhase',