Add serialization for Coupler Qids (quantumlib#6682)

* Add serialization for Coupler Qids

- Adds serialization for coupler Qids to change them into a qubit
string and back.

Author: dstrain115

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

@@ -31,13 +31,21 @@ def qubit_to_proto_id(q: cirq.Qid) -> str:
     For `cirq.NamedQubit`s this id is the name.
 
     For `cirq.LineQubit`s this is string of the `x` attribute.
+
+    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):
         return q.name
     elif isinstance(q, cirq.LineQubit):
         return f'{q.x}'
+    elif isinstance(q, Coupler):
+        return f'c_{qubit_to_proto_id(q.qubit0)}_{qubit_to_proto_id(q.qubit1)}'
     else:
         raise ValueError(f'Qubits of type {type(q)} do not support proto id')
 
@@ -49,6 +57,15 @@ def qubit_from_proto_id(proto_id: str) -> cirq.Qid:
 
     Proto IDs of the form {int} are parsed as LineQubits.
 
+    Proto IDs of the form c_{int}_{int} are parsed as Couplers
+    between two LineQubits.
+
+    Proto IDs of the form c_{int}_{int}_{int}_{int} are parsed as Couplers
+    between two GridQubit.
+
+    Proto IDs of the form c_{name}_{name} are parsed as Couplers
+    between two NamedQubits.
+
     All other proto IDs are parsed as NamedQubits. Note that this will happily
     accept any string; for circuits which explicitly use Grid or LineQubits,
     prefer one of the specialized methods below.
@@ -59,8 +76,29 @@ def qubit_from_proto_id(proto_id: str) -> cirq.Qid:
     Returns:
         A `cirq.Qid` corresponding to the proto id.
     """
-    num_coords = len(proto_id.split('_'))
-    if num_coords == 2:
+    # 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_':
+        if num_coords == 5:
+            # 2 grid qubits: c_2_1_4_3
+            grid_qubit0_str = qubit_field[1] + '_' + qubit_field[2]
+            grid_qubit1_str = qubit_field[3] + '_' + qubit_field[4]
+            try:
+                grid_qubit0 = grid_qubit_from_proto_id(grid_qubit0_str)
+                grid_qubit1 = grid_qubit_from_proto_id(grid_qubit1_str)
+                return Coupler(grid_qubit0, grid_qubit1)
+            except ValueError:
+                pass  # Not valid grid qubits.
+        elif num_coords == 3:
+            # 2 line qubits: c_2_4
+            # Or two named qubits: c_qubita_qubitb
+            line_qubit0 = qubit_from_proto_id(qubit_field[1])
+            line_qubit1 = qubit_from_proto_id(qubit_field[2])
+            return Coupler(line_qubit0, line_qubit1)
+    elif num_coords == 2:
         try:
             grid_q = grid_qubit_from_proto_id(proto_id)
             return grid_q

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

@@ -15,6 +15,7 @@
 import pytest
 
 import cirq
+import cirq_google as cg
 import cirq_google.api.v2 as v2
 
 
@@ -26,6 +27,14 @@ def test_qubit_to_proto_id():
     assert v2.qubit_to_proto_id(cirq.LineQubit(10)) == '10'
     assert v2.qubit_to_proto_id(cirq.LineQubit(-1)) == '-1'
     assert v2.qubit_to_proto_id(cirq.NamedQubit('named')) == 'named'
+    grid_coupler = cg.Coupler(cirq.GridQubit(2, 1), cirq.GridQubit(4, 3))
+    assert v2.qubit_to_proto_id(grid_coupler) == 'c_2_1_4_3'
+    grid_coupler = cg.Coupler(cirq.GridQubit(4, 3), cirq.GridQubit(2, 1))
+    assert v2.qubit_to_proto_id(grid_coupler) == 'c_2_1_4_3'
+    line_coupler = cg.Coupler(cirq.LineQubit(4), cirq.LineQubit(5))
+    assert v2.qubit_to_proto_id(line_coupler) == 'c_4_5'
+    named_coupler = cg.Coupler(cirq.NamedQubit('named1'), cirq.NamedQubit('named2'))
+    assert v2.qubit_to_proto_id(named_coupler) == 'c_named1_named2'
 
 
 def test_to_proto_id_unsupport_qid():
@@ -92,3 +101,12 @@ def test_generic_qubit_from_proto_id():
     # All non-int-parseable names are converted to NamedQubits.
     assert v2.qubit_from_proto_id('a') == cirq.NamedQubit('a')
     assert v2.qubit_from_proto_id('1_b') == cirq.NamedQubit('1_b')
+
+    # Test Coupler Qids
+    grid_coupler = cg.Coupler(cirq.GridQubit(2, 1), cirq.GridQubit(4, 3))
+    assert v2.qubit_from_proto_id('c_2_1_4_3') == grid_coupler
+    line_coupler = cg.Coupler(cirq.LineQubit(4), cirq.LineQubit(5))
+    assert v2.qubit_from_proto_id('c_4_5') == line_coupler
+    named_coupler = cg.Coupler(cirq.NamedQubit('named1'), cirq.NamedQubit('named2'))
+    assert v2.qubit_from_proto_id('c_named1_named2') == named_coupler
+    assert v2.qubit_from_proto_id('c_a_b_c_d') == cirq.NamedQubit('c_a_b_c_d')