Add isolated qubit functionality for gridmetadata. (quantumlib#5001)

Adds isolated qubit property to GridDeviceMetadata. Also adds error checking for self loops. cc: @verult .

cirq-core/cirq/devices/grid_device_metadata.py

@@ -39,12 +39,14 @@ def __init__(
         qubit_pairs: Iterable[Tuple['cirq.Qid', 'cirq.Qid']],
         gateset: 'cirq.Gateset',
         gate_durations: Optional[Dict['cirq.GateFamily', 'cirq.Duration']] = None,
+        all_qubits: Optional[Iterable['cirq.Qid']] = None,
     ):
         """Create a GridDeviceMetadata object.
 
         Create a GridDevice which has a well defined set of couplable
         qubit pairs that have the same two qubit gates available in
-        both coupling directions.
+        both coupling directions. Gate times (if provided) are expected
+        to be uniform across all qubits on the device.
 
         Args:
             qubit_pairs: Iterable of pairs of `cirq.Qid`s representing
@@ -55,25 +57,51 @@ def __init__(
                 instances mapping to `cirq.Duration` instances for
                 gate timing metadata information. If provided,
                 must match all entries in gateset.
+            all_qubits: Optional iterable specifying all qubits
+                found on the device. If None, all_qubits will
+                be inferred from the entries in qubit_pairs.
 
         Raises:
             ValueError: if the union of GateFamily keys in gate_durations
                 is not identical to set of gate families in gateset.
+            ValueError: If qubit_pairs contains a self loop.
+            ValueError: if all_qubits is provided and is not a superset
+                of all the qubits found in qubit_pairs.
         """
-        qubit_pairs = list(qubit_pairs)
-        flat_pairs = [q for pair in qubit_pairs for q in pair]
+        sorted_pairs = sorted(list(qubit_pairs))
+        for a, b in sorted_pairs:
+            if a == b:
+                raise ValueError(f"Self loop encountered in qubit {a}")
+
         # Keep lexigraphically smaller tuples for undirected edges.
-        sorted_pairs = sorted(qubit_pairs)
-        pair_set = set()
+        edge_set = set()
+        node_set = set()
         for a, b in sorted_pairs:
-            if (b, a) not in pair_set:
-                pair_set.add((a, b))
+            node_set.add(a)
+            node_set.add(b)
+            if (b, a) not in edge_set:
+                edge_set.add((a, b))
+
+        if all_qubits is None:
+            all_qubits = node_set
+
+        all_qubits = frozenset(all_qubits)
+        for q in node_set:
+            if q not in all_qubits:
+                raise ValueError(
+                    f"Qubit {q} found in node_set and not in"
+                    " all_qubits. all_qubits must contain at least"
+                    " all the qubits found in all_qubits."
+                )
 
         connectivity = nx.Graph()
-        connectivity.add_edges_from(sorted(pair_set), directed=False)
-        super().__init__(flat_pairs, connectivity)
-        self._qubit_pairs = frozenset(pair_set)
+        connectivity.add_nodes_from(sorted(all_qubits))
+        connectivity.add_edges_from(sorted(edge_set), directed=False)
+        super().__init__(all_qubits, connectivity)
+
+        self._qubit_pairs = frozenset(edge_set)
         self._gateset = gateset
+        self._isolated_qubits = all_qubits.difference(node_set)
 
         if gate_durations is not None:
             working_gatefamilies = frozenset(gate_durations.keys())
@@ -93,6 +121,11 @@ def qubit_pairs(self) -> FrozenSet[Tuple['cirq.Qid', 'cirq.Qid']]:
         """Returns the set of all couple-able qubits on the device."""
         return self._qubit_pairs
 
+    @property
+    def isolated_qubits(self) -> FrozenSet['cirq.Qid']:
+        """Returns the set of all isolated qubits on the device (if appliable)."""
+        return self._isolated_qubits
+
     @property
     def gateset(self) -> 'cirq.Gateset':
         """Returns the `cirq.Gateset` of supported gates on this device."""
@@ -112,12 +145,14 @@ def _value_equality_values_(self):
             tuple(sorted(self._qubit_pairs)),
             self._gateset,
             tuple(duration_equality),
+            tuple(sorted(self.qubit_set)),
         )
 
     def __repr__(self) -> str:
         return (
             f'cirq.GridDeviceMetadata({repr(self._qubit_pairs)},'
-            f' {repr(self._gateset)}, {repr(self._gate_durations)})'
+            f' {repr(self._gateset)}, {repr(self._gate_durations)},'
+            f' {repr(self.qubit_set)})'
         )
 
     def _json_dict_(self):
@@ -129,8 +164,9 @@ def _json_dict_(self):
             'qubit_pairs': sorted(list(self._qubit_pairs)),
             'gateset': self._gateset,
             'gate_durations': duration_payload,
+            'all_qubits': sorted(list(self.qubit_set)),
         }
 
     @classmethod
-    def _from_json_dict_(cls, qubit_pairs, gateset, gate_durations, **kwargs):
-        return cls(qubit_pairs, gateset, dict(gate_durations))
+    def _from_json_dict_(cls, qubit_pairs, gateset, gate_durations, all_qubits, **kwargs):
+        return cls(qubit_pairs, gateset, dict(gate_durations), all_qubits)

cirq-core/cirq/devices/grid_device_metadata_test.py

@@ -21,10 +21,9 @@
 def test_griddevice_metadata():
     qubits = cirq.GridQubit.rect(2, 3)
     qubit_pairs = [(a, b) for a in qubits for b in qubits if a != b and a.is_adjacent(b)]
-
+    isolated_qubits = [cirq.GridQubit(9, 9), cirq.GridQubit(10, 10)]
     gateset = cirq.Gateset(cirq.XPowGate, cirq.YPowGate, cirq.ZPowGate, cirq.CZ)
-    metadata = cirq.GridDeviceMetadata(qubit_pairs, gateset)
-
+    metadata = cirq.GridDeviceMetadata(qubit_pairs, gateset, all_qubits=qubits + isolated_qubits)
     expected_pairings = frozenset(
         {
             (cirq.GridQubit(0, 0), cirq.GridQubit(0, 1)),
@@ -36,14 +35,16 @@ def test_griddevice_metadata():
             (cirq.GridQubit(0, 0), cirq.GridQubit(1, 0)),
         }
     )
-    assert metadata.qubit_set == frozenset(qubits)
+    assert metadata.qubit_set == frozenset(qubits + isolated_qubits)
     assert metadata.qubit_pairs == expected_pairings
     assert metadata.gateset == gateset
     expected_graph = nx.Graph()
+    expected_graph.add_nodes_from(sorted(list(qubits + isolated_qubits)))
     expected_graph.add_edges_from(sorted(list(expected_pairings)), directed=False)
     assert metadata.nx_graph.edges() == expected_graph.edges()
     assert metadata.nx_graph.nodes() == expected_graph.nodes()
     assert metadata.gate_durations is None
+    assert metadata.isolated_qubits == frozenset(isolated_qubits)
 
 
 def test_griddevice_metadata_bad_durations():
@@ -58,6 +59,24 @@ def test_griddevice_metadata_bad_durations():
         cirq.GridDeviceMetadata([qubits], gateset, gate_durations=invalid_duration)
 
 
+def test_griddevice_metadata_bad_isolated():
+    qubits = cirq.GridQubit.rect(2, 3)
+    qubit_pairs = [(a, b) for a in qubits for b in qubits if a != b and a.is_adjacent(b)]
+    fewer_qubits = [cirq.GridQubit(0, 0)]
+    gateset = cirq.Gateset(cirq.XPowGate, cirq.YPowGate, cirq.ZPowGate, cirq.CZ)
+    with pytest.raises(ValueError, match='node_set'):
+        _ = cirq.GridDeviceMetadata(qubit_pairs, gateset, all_qubits=fewer_qubits)
+
+
+def test_griddevice_self_loop():
+    bad_pairs = [
+        (cirq.GridQubit(0, 0), cirq.GridQubit(0, 0)),
+        (cirq.GridQubit(1, 0), cirq.GridQubit(1, 1)),
+    ]
+    with pytest.raises(ValueError, match='Self loop'):
+        _ = cirq.GridDeviceMetadata(bad_pairs, cirq.Gateset(cirq.XPowGate))
+
+
 def test_griddevice_json_load():
     qubits = cirq.GridQubit.rect(2, 3)
     qubit_pairs = [(a, b) for a in qubits for b in qubits if a != b and a.is_adjacent(b)]
@@ -67,7 +86,10 @@ def test_griddevice_json_load():
         cirq.GateFamily(cirq.YPowGate): cirq.Duration(picos=2),
         cirq.GateFamily(cirq.ZPowGate): cirq.Duration(picos=3),
     }
-    metadata = cirq.GridDeviceMetadata(qubit_pairs, gateset, gate_durations=duration)
+    isolated_qubits = [cirq.GridQubit(9, 9), cirq.GridQubit(10, 10)]
+    metadata = cirq.GridDeviceMetadata(
+        qubit_pairs, gateset, gate_durations=duration, all_qubits=qubits + isolated_qubits
+    )
     rep_str = cirq.to_json(metadata)
     assert metadata == cirq.read_json(json_text=rep_str)
 
@@ -86,17 +108,22 @@ def test_griddevice_metadata_equality():
         cirq.GateFamily(cirq.YPowGate): cirq.Duration(picos=13),
         cirq.GateFamily(cirq.ZPowGate): cirq.Duration(picos=12),
     }
+    isolated_qubits = [cirq.GridQubit(9, 9)]
     metadata = cirq.GridDeviceMetadata(qubit_pairs, gateset, gate_durations=duration)
     metadata2 = cirq.GridDeviceMetadata(qubit_pairs[:2], gateset, gate_durations=duration)
     metadata3 = cirq.GridDeviceMetadata(qubit_pairs, gateset, gate_durations=None)
     metadata4 = cirq.GridDeviceMetadata(qubit_pairs, gateset, gate_durations=duration2)
     metadata5 = cirq.GridDeviceMetadata(reversed(qubit_pairs), gateset, gate_durations=duration)
+    metadata6 = cirq.GridDeviceMetadata(
+        qubit_pairs, gateset, gate_durations=duration, all_qubits=qubits + isolated_qubits
+    )
 
     eq = cirq.testing.EqualsTester()
     eq.add_equality_group(metadata)
     eq.add_equality_group(metadata2)
     eq.add_equality_group(metadata3)
     eq.add_equality_group(metadata4)
+    eq.add_equality_group(metadata6)
 
     assert metadata == metadata5
 
@@ -110,5 +137,8 @@ def test_repr():
         cirq.GateFamily(cirq.YPowGate): cirq.Duration(picos=3),
         cirq.GateFamily(cirq.ZPowGate): cirq.Duration(picos=2),
     }
-    metadata = cirq.GridDeviceMetadata(qubit_pairs, gateset, gate_durations=duration)
+    isolated_qubits = [cirq.GridQubit(9, 9)]
+    metadata = cirq.GridDeviceMetadata(
+        qubit_pairs, gateset, gate_durations=duration, all_qubits=qubits + isolated_qubits
+    )
     cirq.testing.assert_equivalent_repr(metadata)

cirq-core/cirq/protocols/json_test_data/GridDeviceMetadata.json

@@ -155,5 +155,47 @@
         "picos": 1
       }
     ]
+  ],
+  "all_qubits": [
+    {
+      "cirq_type": "GridQubit",
+      "row": 0,
+      "col": 0
+    },
+    {
+      "cirq_type": "GridQubit",
+      "row": 0,
+      "col": 1
+    },
+    {
+      "cirq_type": "GridQubit",
+      "row": 0,
+      "col": 2
+    },
+    {
+      "cirq_type": "GridQubit",
+      "row": 1,
+      "col": 0
+    },
+    {
+      "cirq_type": "GridQubit",
+      "row": 1,
+      "col": 1
+    },
+    {
+      "cirq_type": "GridQubit",
+      "row": 1,
+      "col": 2
+    },
+    {
+      "cirq_type": "GridQubit",
+      "row": 9,
+      "col": 9
+    },
+    {
+      "cirq_type": "GridQubit",
+      "row": 10,
+      "col": 10
+    }
   ]
-}
+}

cirq-core/cirq/protocols/json_test_data/GridDeviceMetadata.repr

@@ -1 +1 @@
-cirq.GridDeviceMetadata(frozenset({(cirq.GridQubit(0, 2), cirq.GridQubit(1, 2)), (cirq.GridQubit(0, 0), cirq.GridQubit(1, 0)), (cirq.GridQubit(0, 1), cirq.GridQubit(1, 1)), (cirq.GridQubit(0, 0), cirq.GridQubit(0, 1)), (cirq.GridQubit(0, 1), cirq.GridQubit(0, 2)), (cirq.GridQubit(1, 1), cirq.GridQubit(1, 2)), (cirq.GridQubit(1, 0), cirq.GridQubit(1, 1))}), cirq.Gateset(cirq.ops.common_gates.XPowGate, cirq.ops.common_gates.YPowGate, cirq.ops.common_gates.ZPowGate), {cirq.GateFamily(cirq.ops.common_gates.XPowGate): cirq.Duration(nanos=1), cirq.GateFamily(cirq.ops.common_gates.YPowGate): cirq.Duration(picos=1), cirq.GateFamily(cirq.ops.common_gates.ZPowGate): cirq.Duration(picos=1)})
+cirq.GridDeviceMetadata(frozenset({(cirq.GridQubit(0, 2), cirq.GridQubit(1, 2)), (cirq.GridQubit(0, 0), cirq.GridQubit(1, 0)), (cirq.GridQubit(0, 1), cirq.GridQubit(1, 1)), (cirq.GridQubit(0, 0), cirq.GridQubit(0, 1)), (cirq.GridQubit(0, 1), cirq.GridQubit(0, 2)), (cirq.GridQubit(1, 1), cirq.GridQubit(1, 2)), (cirq.GridQubit(1, 0), cirq.GridQubit(1, 1))}), cirq.Gateset(cirq.ops.common_gates.XPowGate, cirq.ops.common_gates.YPowGate, cirq.ops.common_gates.ZPowGate), {cirq.GateFamily(cirq.ops.common_gates.XPowGate): cirq.Duration(nanos=1), cirq.GateFamily(cirq.ops.common_gates.YPowGate): cirq.Duration(picos=1), cirq.GateFamily(cirq.ops.common_gates.ZPowGate): cirq.Duration(picos=1)}, cirq.GridQubit.rect(2, 3) + [cirq.GridQubit(9, 9), cirq.GridQubit(10, 10)])