Move Coupler to cirq_gooogle.ops (quantumlib#6697)

* Move Coupler to cirq_gooogle.ops

- Noticed when adding code that there were circular
imports due to Couplers api codes needing code from devices.
- Moved this to ops directory instead, which avoids the circular
import.
- Note that, although GridQubit is in cirq.devices, LineQubit and
NamedQubit are in cirq.ops, so this also seems to be a good place
to put this class.

Author: dstrain115

cirq-google/cirq_google/__init__.py

@@ -20,7 +20,6 @@
 from cirq_google._version import __version__
 
 from cirq_google.devices import (
-    Coupler,
     GoogleNoiseProperties,
     GridDevice,
     NoiseModelFromGoogleNoiseProperties,
@@ -56,6 +55,7 @@
 
 from cirq_google.ops import (
     CalibrationTag,
+    Coupler,
     FSimGateFamily,
     FSimViaModelTag,
     InternalGate,

cirq-google/cirq_google/api/v2/program.py

@@ -12,12 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import re
-from typing import TYPE_CHECKING
 
 import cirq
 
-if TYPE_CHECKING:
-    import cirq
+from cirq_google.ops.coupler import Coupler
 
 GRID_QUBIT_ID_PATTERN = r'^q?(-?\d+)_(-?\d+)$'
 
@@ -35,9 +33,6 @@ def qubit_to_proto_id(q: cirq.Qid) -> str:
     For `cirq_google.Coupler`s, this id is `c_{qubit0}_{qubit1}` where
     qubit0 and qubit1 are the ids for the two Qid in the Coupler.
     """
-    # Avoid circular import
-    from cirq_google.devices.coupler import Coupler
-
     if isinstance(q, cirq.GridQubit):
         return f'{q.row}_{q.col}'
     elif isinstance(q, cirq.NamedQubit):
@@ -76,9 +71,6 @@ def qubit_from_proto_id(proto_id: str) -> cirq.Qid:
     Returns:
         A `cirq.Qid` corresponding to the proto id.
     """
-    # Avoid circular import
-    from cirq_google.devices.coupler import Coupler
-
     qubit_field = proto_id.split('_')
     num_coords = len(qubit_field)
     if proto_id[:2] == 'c_':

cirq-google/cirq_google/devices/__init__.py

@@ -21,6 +21,4 @@
 
 from cirq_google.devices.known_devices import Sycamore, Sycamore23
 
-from cirq_google.devices.coupler import Coupler
-
 from cirq_google.devices.grid_device import GridDevice

cirq-google/cirq_google/devices/grid_device.py

@@ -37,7 +37,6 @@
 from cirq_google import transformers
 from cirq_google.api import v2
 from cirq_google.devices import known_devices
-from cirq_google.devices.coupler import Coupler
 from cirq_google.experimental import ops as experimental_ops
 
 
@@ -623,7 +622,7 @@ def validate_operation(self, operation: cirq.Operation) -> None:
             raise ValueError(f'Operation {operation} contains a gate which is not supported.')
 
         for q in operation.qubits:
-            if isinstance(q, Coupler):
+            if isinstance(q, ops.Coupler):
                 if any(qc not in self._metadata.qubit_set for qc in q.qubits):
                     raise ValueError(f'Qubits on coupler not on device: {q.qubits}.')
                 if frozenset(q.qubits) not in self._metadata.qubit_pairs:

cirq-google/cirq_google/devices/grid_device_test.py

@@ -21,7 +21,8 @@
 import cirq
 import cirq_google
 from cirq_google.api import v2
-from cirq_google.devices import Coupler, grid_device
+from cirq_google.devices import grid_device
+from cirq_google.ops import Coupler
 
 
 GRID_HEIGHT = 5

cirq-google/cirq_google/ops/__init__.py

@@ -18,6 +18,8 @@
 
 from cirq_google.ops.calibration_tag import CalibrationTag
 
+from cirq_google.ops.coupler import Coupler
+
 from cirq_google.ops.fsim_gate_family import FSimGateFamily
 
 from cirq_google.ops.fsim_via_model_tag import FSimViaModelTag