Add PhysicalZTag (#2753)

* Add PhysicalZTag

- Simple class in cirq.google.ops that can denote physical Z gates.
- Just includes the class definition so far, no functionality.

cirq/google/__init__.py

@@ -54,7 +54,8 @@
     LinePlacementStrategy,
 )
 
-from cirq.google.ops.sycamore_gate import (
+from cirq.google.ops import (
+    PhysicalZTag,
     SycamoreGate,
     SYC,
 )

cirq/google/ops/__init__.py

@@ -16,3 +16,6 @@
     SycamoreGate,
     SYC,
 )
+
+from cirq.google.ops.physical_z_tag import (
+    PhysicalZTag,)

cirq/google/ops/physical_z_tag.py

@@ -0,0 +1,42 @@
+# Copyright 2020 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.
+"""A class that can be used to denote a physical Z gate."""
+from cirq import protocols
+
+
+class PhysicalZTag:
+    """Class to add as a tag onto an Operation to denote a Physical Z operation.
+
+    By default, all Z rotations on Google devices are considered to be virtual.
+    When performing the Z operation, the device will update its internal phase
+    tracking mechanisms, essentially commuting it forwards through the circuit
+    until it hits a non-commuting operation (Such as a sqrt(iSwap)).
+
+    When applied to a Z rotation operation, this tag indicates to the hardware
+    that an actual physical operation should be done instead.  This class can
+    only be applied to instances of `cirq.ZPowGate`.  If applied to other gates
+    (such as PhasedXZGate), this class will have no effect.
+    """
+
+    def __str__(self):
+        return 'PhysicalZTag()'
+
+    def __repr__(self):
+        return 'cirq.google.PhysicalZTag()'
+
+    def _json_dict_(self):
+        return protocols.obj_to_dict_helper(self, [])
+
+    def __eq__(self, other):
+        return isinstance(other, PhysicalZTag)

cirq/google/ops/physical_z_tag_test.py

@@ -0,0 +1,25 @@
+# Copyright 2020 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_equality():
+    assert cirq.google.PhysicalZTag() == cirq.google.PhysicalZTag()
+
+
+def test_syc_str_repr():
+    assert str(cirq.google.PhysicalZTag()) == 'PhysicalZTag()'
+    assert repr(cirq.google.PhysicalZTag()) == 'cirq.google.PhysicalZTag()'
+    cirq.testing.assert_equivalent_repr(cirq.google.PhysicalZTag(),
+                                        setup_code=('import cirq\n'))

cirq/protocols/json_serialization.py

@@ -117,6 +117,7 @@ def two_qubit_matrix_gate(matrix):
                 'PhasedISwapPowGate': cirq.PhasedISwapPowGate,
                 'PhasedXPowGate': cirq.PhasedXPowGate,
                 'PhasedXZGate': cirq.PhasedXZGate,
+                'PhysicalZTag': cirq.google.PhysicalZTag,
                 'QuantumFourierTransformGate': cirq.QuantumFourierTransformGate,
                 'ResetChannel': cirq.ResetChannel,
                 'SingleQubitMatrixGate': single_qubit_matrix_gate,

cirq/protocols/json_test_data/PhysicalZTag.json

@@ -0,0 +1,3 @@
+{
+      "cirq_type": "PhysicalZTag"
+}

cirq/protocols/json_test_data/PhysicalZTag.repr

@@ -0,0 +1 @@
+cirq.google.PhysicalZTag()

cirq/protocols/json_test_data/TaggedOperation.json

@@ -17,6 +17,8 @@
   "tags":
   [
     "tag1",
-    "tag2"
+    {
+      "cirq_type": "PhysicalZTag"
+    }
   ]
 }

cirq/protocols/json_test_data/TaggedOperation.repr

@@ -1 +1 @@
-cirq.TaggedOperation(cirq.X.on(cirq.NamedQubit('q1')), 'tag1', 'tag2')
+cirq.TaggedOperation(cirq.X.on(cirq.NamedQubit('q1')), 'tag1', cirq.google.PhysicalZTag())

docs/api.rst

@@ -502,6 +502,7 @@ Functionality specific to quantum hardware and services from Google.
     cirq.google.GreedySequenceSearchStrategy
     cirq.google.JobConfig
     cirq.google.LinePlacementStrategy
+    cirq.google.PhysicalZTag
     cirq.google.ProtoVersion
     cirq.google.QuantumEngineSampler
     cirq.google.SerializableDevice