Added a public routing swap tag (quantumlib#5844)

A RoutingSwapTag is a tag optionally applied on swaps that were inserted as part of a routed procedure. This serves to differentiate between swaps native to the original circuit and swaps added during routing.

cirq/__init__.py

@@ -287,6 +287,7 @@
     reset,
     reset_each,
     ResetChannel,
+    RoutingSwapTag,
     riswap,
     Rx,
     Ry,

cirq/json_resolver_cache.py

@@ -204,6 +204,7 @@ def _symmetricalqidpair(qids):
         'ResetChannel': cirq.ResetChannel,
         'Result': cirq.ResultDict,  # Keep support for Cirq < 0.14.
         'ResultDict': cirq.ResultDict,
+        'RoutingSwapTag': cirq.RoutingSwapTag,
         'Rx': cirq.Rx,
         'Ry': cirq.Ry,
         'Rz': cirq.Rz,

cirq/ops/__init__.py

@@ -188,7 +188,7 @@
     SwapPowGate,
 )
 
-from cirq.ops.tags import VirtualTag
+from cirq.ops.tags import RoutingSwapTag, VirtualTag
 
 from cirq.ops.three_qubit_gates import (
     CCNOT,

cirq/ops/tags.py

@@ -43,3 +43,26 @@ def _json_dict_(self) -> Dict[str, str]:
 
     def __hash__(self):
         return hash(VirtualTag)
+
+
+class RoutingSwapTag:
+    """A 'cirq.TaggedOperation' tag indicated that the operation is an inserted SWAP.
+
+    A RoutingSwapTag is meant to be used to distinguish SWAP operations that are inserted during
+    a routing procedure and SWAP operations that are part of the original circuit before routing.
+    """
+
+    def __eq__(self, other):
+        return isinstance(other, RoutingSwapTag)
+
+    def __str__(self) -> str:
+        return '<r>'
+
+    def __repr__(self) -> str:
+        return 'cirq.RoutingSwapTag()'
+
+    def _json_dict_(self) -> Dict[str, str]:
+        return {}
+
+    def __hash__(self):
+        return hash(RoutingSwapTag)

cirq/ops/tags_test.py

@@ -24,3 +24,15 @@ def test_virtual_tag():
 
     cirq.testing.assert_equivalent_repr(tag1)
     cirq.testing.assert_equivalent_repr(tag2)
+
+
+def test_routing_swap_tag():
+    tag1 = cirq.ops.RoutingSwapTag()
+    tag2 = cirq.ops.RoutingSwapTag()
+
+    assert tag1 == tag2
+    assert str(tag1) == str(tag2) == '<r>'
+    assert hash(tag1) == hash(tag2)
+
+    cirq.testing.assert_equivalent_repr(tag1)
+    cirq.testing.assert_equivalent_repr(tag2)

cirq/protocols/json_test_data/RoutingSwapTag.json

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

cirq/protocols/json_test_data/RoutingSwapTag.repr

@@ -0,0 +1 @@
+cirq.RoutingSwapTag()