Implements the LineInititialMapper strategy

cirq-core/cirq/__init__.py

@@ -348,6 +348,7 @@
     expand_composite,
     HardCodedInitialMapper,
     is_negligible_turn,
+    LineInitialMapper,
     MappingManager,
     map_moments,
     map_operations,

cirq-core/cirq/protocols/json_test_data/spec.py

@@ -86,6 +86,7 @@
         # Routing utilities
         'HardCodedInitialMapper',
         'MappingManager',
+        'LineInitialMapper',
         # global objects
         'CONTROL_TAG',
         'PAULI_BASIS',

cirq-core/cirq/transformers/__init__.py

@@ -44,7 +44,13 @@
     two_qubit_gate_product_tabulation,
 )
 
-from cirq.transformers.routing import AbstractInitialMapper, HardCodedInitialMapper, MappingManager
+
+from cirq.transformers.routing import (
+    AbstractInitialMapper,
+    HardCodedInitialMapper,
+    LineInitialMapper,
+    MappingManager,
+)
 
 from cirq.transformers.target_gatesets import (
     create_transformer_with_kwargs,

cirq-core/cirq/transformers/routing/__init__.py

@@ -16,3 +16,4 @@
 
 from cirq.transformers.routing.initial_mapper import AbstractInitialMapper, HardCodedInitialMapper
 from cirq.transformers.routing.mapping_manager import MappingManager
+from cirq.transformers.routing.line_initial_mapper import LineInitialMapper

cirq-core/cirq/transformers/routing/line_initial_mapper.py

@@ -0,0 +1,192 @@
+# Copyright 2022 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.
+
+"""Concrete implementation of AbstractInitialMapper that places lines of qubits onto the device."""
+
+from typing import Dict, List, Set, TYPE_CHECKING
+import networkx as nx
+
+from cirq.transformers.routing import AbstractInitialMapper
+from cirq import protocols
+
+if TYPE_CHECKING:
+    import cirq
+
+
+class LineInitialMapper(AbstractInitialMapper):
+    """Places logical qubits in the circuit onto physical qubits on the device."""
+
+    def __init__(self, device_graph: nx.Graph) -> None:
+        """Initializes a LineInitialMapper.
+
+        Args:
+            device_graph: device graph
+        """
+        self.device_graph = device_graph
+        self.mapped_physicals: Set['cirq.Qid'] = set()
+        self.partners: Dict['cirq.Qid', 'cirq.Qid'] = {}
+
+    def _make_circuit_graph(self, circuit: 'cirq.AbstractCircuit') -> List[List['cirq.Qid']]:
+        """Creates a (potentially incomplete) qubit connectivity graph of the circuit.
+
+        Iterates over moments in the circuit from left to right and adds edges between logical
+        qubits if the logical qubit pair l1 and l2
+            (1) have degree < 2,
+            (2) are involved in a 2-qubit operation in the current moment, and
+            (3) adding such an edge will not produce a cycle in the graph.
+
+        Args:
+            circuit: the input circuit with logical qubits
+
+        Returns:
+            The (potentially incomplete) qubit connectivity graph of the circuit, which is
+                guaranteed to be a forest of line graphs.
+        """
+        circuit_graph: List[List['cirq.Qid']] = [[q] for q in sorted(circuit.all_qubits())]
+        component_id: Dict['cirq.Qid', int] = {q[0]: i for i, q in enumerate(circuit_graph)}
+
+        def degree_lt_two(q: 'cirq.Qid'):
+            return any(circuit_graph[component_id[q]][i] == q for i in [-1, 0])
+
+        for op in circuit.all_operations():
+            if protocols.num_qubits(op) != 2:
+                continue
+
+            q0, q1 = op.qubits
+            c0, c1 = component_id[q0], component_id[q1]
+
+            # Keep track of partners for mapping isolated qubits later.
+            if q0 not in self.partners:
+                self.partners[q0] = q1
+            if q1 not in self.partners:
+                self.partners[q1] = q0
+
+            if not (degree_lt_two(q0) and degree_lt_two(q1) and c0 != c1):
+                continue
+
+            # Make sure c0/q0 are for the largest component.
+            if len(circuit_graph[c0]) < len(circuit_graph[c1]):
+                c0, c1, q0, q1 = c1, c0, q1, q0
+
+            # copy smaller component into larger one.
+            if circuit_graph[c0][0] == q0:
+                if circuit_graph[c1][0] == q1:
+                    for q in circuit_graph[c1]:
+                        circuit_graph[c0].insert(0, q)
+                        component_id[q] = c0
+                else:
+                    for q in reversed(circuit_graph[c1]):
+                        circuit_graph[c0].insert(0, q)
+                        component_id[q] = c0
+            else:
+                if circuit_graph[c1][0] == q1:
+                    for q in circuit_graph[c1]:
+                        circuit_graph[c0].append(q)
+                        component_id[q] = c0
+                else:
+                    for q in reversed(circuit_graph[c1]):
+                        circuit_graph[c0].append(q)
+                        component_id[q] = c0
+
+        return sorted([circuit_graph[c] for c in set(component_id.values())], key=len, reverse=True)
+
+    def initial_mapping(self, circuit: 'cirq.AbstractCircuit') -> Dict['cirq.Qid', 'cirq.Qid']:
+        """Maps disjoint lines of logical qubits onto lines of physical qubits.
+
+        Starting from the center physical qubit on the device, attempts to map disjoint lines of
+        logical qubits given by the circuit graph onto one long line of physical qubits on the
+        device, greedily maximizing each physical qubit's degree.
+        If this mapping cannot be completed as one long line of qubits in the circuit graph mapped
+        to qubits in the device graph, the line can be split as several line segments and then we:
+            (i)   Map first line segment.
+            (ii)  Find another high degree vertex in G near the center.
+            (iii) Map the second line segment
+            (iv)  etc.
+        A line is split by mapping the next logical qubit to the nearest available physical qubit
+        to the center of the device graph.
+
+        Args:
+            circuit: the input circuit with logical qubits
+
+        Returns:
+            a dictionary that maps logical qubits in the circuit (keys) to physical qubits on the
+            device (values).
+        """
+        qubit_map: Dict['cirq.Qid', 'cirq.Qid'] = {}
+        circuit_graph = self._make_circuit_graph(circuit)
+        physical_center = nx.center(self.device_graph)[0]
+
+        def next_physical(current_physical: 'cirq.Qid') -> 'cirq.Qid':
+            # Greedily map to highest degree neighbor that that is available
+            neighbors = sorted(self.device_graph.neighbors(current_physical))
+            sorted_neighbors = sorted(
+                neighbors,
+                key=lambda x: self.device_graph.degree(x),
+                reverse=True,
+            )
+            for neighbor in sorted_neighbors:
+                if neighbor not in self.mapped_physicals:
+                    return neighbor
+            # If cannot map onto one long line of physical qubits, then break down into multiple
+            # small lines by finding nearest available qubit to the physical center
+            return self._closest_unmapped_qubit(physical_center)
+
+        pq = physical_center
+        first_isolated_idx = len(circuit_graph)
+        for idx, logical_line in enumerate(circuit_graph):
+            if len(logical_line) == 1:
+                first_isolated_idx = idx
+                break
+
+            for lq in logical_line:
+                self.mapped_physicals.add(pq)
+                qubit_map[lq] = pq
+                # Edge case: if mapping n qubits on an n-qubit device should not call next_physical
+                # when finished mapping the last logical qubit else will raise an error
+                if len(circuit.all_qubits()) != len(self.mapped_physicals):
+                    pq = next_physical(pq)
+
+        for i in range(first_isolated_idx, len(circuit_graph)):
+            lq = circuit_graph[i][0]
+            partner = qubit_map[self.partners[lq]] if lq in self.partners else physical_center
+            pq = self._closest_unmapped_qubit(partner)
+            self.mapped_physicals.add(pq)
+            qubit_map[lq] = pq
+
+        return qubit_map
+
+    def _closest_unmapped_qubit(self, source: 'cirq.Qid') -> 'cirq.Qid':
+        """Finds the closest available neighbor to a physical qubit 'source' on the device.
+
+        Args:
+            source: a physical qubit on the device.
+
+        Returns:
+            the closest available physical qubit to 'source'.
+
+        Raises:
+            ValueError: if there are no available qubits left on the device.
+        """
+        for _, successors in nx.bfs_successors(self.device_graph, source):
+            for successor in successors:
+                if successor not in self.mapped_physicals:
+                    return successor
+        raise ValueError("No available physical qubits left on the device.")
+
+    def __eq__(self, other) -> bool:
+        return nx.utils.graphs_equal(self.device_graph, other.device_graph)
+
+    def __repr__(self):
+        graph_type = type(self.device_graph).__name__
+        return f'cirq.LineInitialMapper(nx.{graph_type}({dict(self.device_graph.adjacency())}))'

cirq-core/cirq/transformers/routing/line_initial_mapper_test.py

@@ -0,0 +1,108 @@
+# Copyright 2022 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 networkx as nx
+import pytest
+
+import cirq
+
+
+def construct_small_circuit():
+    return cirq.Circuit(
+        [
+            cirq.Moment(cirq.CNOT(cirq.NamedQubit('1'), cirq.NamedQubit('3'))),
+            cirq.Moment(cirq.CNOT(cirq.NamedQubit('2'), cirq.NamedQubit('3'))),
+            cirq.Moment(
+                cirq.CNOT(cirq.NamedQubit('4'), cirq.NamedQubit('3')), cirq.X(cirq.NamedQubit('5'))
+            ),
+        ]
+    )
+
+
+def construct_step_circuit(k: int):
+    q = cirq.LineQubit.range(k)
+    return cirq.Circuit([cirq.CNOT(q[i], q[i + 1]) for i in range(k - 1)])
+
+
+def test_line_breaking_on_grid_device():
+    # tests
+    #   -if strategy is able to map into several small lines if fails to map onto one long line
+    #   -if # of physical qubits <= # of logical qubits then strategy should succeed
+
+    step_circuit = construct_step_circuit(49)
+    device = cirq.testing.construct_grid_device(7, 7)
+    device_graph = device.metadata.nx_graph
+    mapper = cirq.LineInitialMapper(device_graph)
+    mapping = mapper.initial_mapping(step_circuit)
+
+    # all qubits in the input circuit are placed on the device
+    assert set(mapping.keys()) == set(step_circuit.all_qubits())
+
+    # the induced graph of the device on the physical qubits in the map is connected
+    assert nx.is_connected(nx.induced_subgraph(device_graph, mapping.values()))
+
+    step_circuit = construct_step_circuit(50)
+    with pytest.raises(ValueError, match="No available physical qubits left on the device"):
+        mapper.initial_mapping(step_circuit)
+
+
+def test_small_circuit_on_grid_device():
+    circuit = construct_small_circuit()
+
+    device_graph = cirq.testing.construct_grid_device(7, 7).metadata.nx_graph
+    mapper = cirq.LineInitialMapper(device_graph)
+    mapping = mapper.initial_mapping(circuit)
+
+    assert nx.center(device_graph)[0] == cirq.GridQubit(3, 3)
+    mapped_circuit = circuit.transform_qubits(mapping)
+    diagram = """(2, 2): ───@───────────
+           │
+(2, 3): ───┼───────X───
+           │
+(3, 2): ───X───X───X───
+               │   │
+(3, 3): ───────@───┼───
+                   │
+(4, 2): ───────────@───"""
+    cirq.testing.assert_has_diagram(mapped_circuit, diagram)
+
+
+@pytest.mark.parametrize(
+    "qubits, n_moments, op_density, random_state",
+    [
+        (5 * size, 20 * size, density, seed)
+        for size in range(1, 3)
+        for seed in range(10)
+        for density in [0.4, 0.5, 0.6]
+    ],
+)
+def test_random_circuits_grid_device(
+    qubits: int, n_moments: int, op_density: float, random_state: int
+):
+    c_orig = cirq.testing.random_circuit(
+        qubits=qubits, n_moments=n_moments, op_density=op_density, random_state=random_state
+    )
+    device = cirq.testing.construct_grid_device(7, 7)
+    device_graph = device.metadata.nx_graph
+    mapper = cirq.LineInitialMapper(device_graph)
+    mapping = mapper.initial_mapping(c_orig)
+
+    assert set(mapping.keys()) == set(c_orig.all_qubits())
+    assert nx.is_connected(nx.induced_subgraph(device_graph, mapping.values()))
+
+
+def test_repr():
+    device_graph = cirq.testing.construct_grid_device(7, 7).metadata.nx_graph
+    mapper = cirq.LineInitialMapper(device_graph)
+    cirq.testing.assert_equivalent_repr(mapper, setup_code='import cirq\nimport networkx as nx')