def add_physical_qubit(self, physical_qubit):
"""Add a physical qubit to the coupling graph as a node.
physical_qubit (int): An integer representing a physical qubit.
Raises:
CouplingError: if trying to add duplicate qubit
"""
if not isinstance(physical_qubit, int):
raise CouplingError("Physical qubits should be integers.")
if physical_qubit in self.physical_qubits:
raise CouplingError(
"The physical qubit %s is already in the coupling graph" % physical_qubit)
self.graph.add_node(physical_qubit)
self._dist_matrix = None # invalidate
self._qubit_list = None # invalidate
def _compute_distance_matrix(self):
"""Compute the full distance matrix on pairs of nodes.
The distance map self._dist_matrix is computed from the graph using
all_pairs_shortest_path_length.
"""
if not self.is_connected():
raise CouplingError("coupling graph not connected")
self._dist_matrix = rx.digraph_distance_matrix(self.graph, as_undirected=True)
def distance(self, physical_qubit1, physical_qubit2):
"""Returns the undirected distance between physical_qubit1 and physical_qubit2.
Args:
physical_qubit1 (int): A physical qubit
physical_qubit2 (int): Another physical qubit
Returns:
int: The undirected distance
Raises:
CouplingError: if the qubits do not exist in the CouplingMap
"""
if physical_qubit1 >= self.size():
raise CouplingError("%s not in coupling graph" % physical_qubit1)
if physical_qubit2 >= self.size():
raise CouplingError("%s not in coupling graph" % physical_qubit2)
self.compute_distance_matrix()
return int(self._dist_matrix[physical_qubit1, physical_qubit2])
def distance(self, physical_qubit1, physical_qubit2):
"""Returns the undirected distance between physical_qubit1 and physical_qubit2.
Args:
physical_qubit1 (int): A physical qubit
physical_qubit2 (int): Another physical qubit
Returns:
int: The undirected distance
Raises:
CouplingError: if the qubits do not exist in the CouplingMap
"""
if physical_qubit1 not in self.physical_qubits:
raise CouplingError("%s not in coupling graph" % (physical_qubit1,))
if physical_qubit2 not in self.physical_qubits:
raise CouplingError("%s not in coupling graph" % (physical_qubit2,))
if self._dist_matrix is None:
self._compute_distance_matrix()
return self._dist_matrix[physical_qubit1, physical_qubit2]
def compute_distance_matrix(self):
"""Compute the full distance matrix on pairs of nodes.
The distance map self._dist_matrix is computed from the graph using
all_pairs_shortest_path_length. This is normally handled internally
by the :attr:`~qiskit.transpiler.CouplingMap.distance_matrix`
attribute or the :meth:`~qiskit.transpiler.CouplingMap.distance` method
but can be called if you're accessing the distance matrix outside of
those or want to pre-generate it.
"""
if self._dist_matrix is None:
if not self.is_connected():
raise CouplingError("coupling graph not connected")
self._dist_matrix = rx.digraph_distance_matrix(self.graph,
as_undirected=True)
def _compute_distance_matrix(self):
"""Compute the full distance matrix on pairs of nodes.
The distance map self._dist_matrix is computed from the graph using
all_pairs_shortest_path_length.
"""
if not self.is_connected():
raise CouplingError("coupling graph not connected")
lengths = nx.all_pairs_shortest_path_length(self.graph.to_undirected(as_view=True))
lengths = dict(lengths)
size = len(lengths)
cmap = np.zeros((size, size))
for idx in range(size):
cmap[idx, np.fromiter(lengths[idx].keys(), dtype=int)] = np.fromiter(
lengths[idx].values(), dtype=int)
self._dist_matrix = cmap
def shortest_undirected_path(self, physical_qubit1, physical_qubit2):
"""Returns the shortest undirected path between physical_qubit1 and physical_qubit2.
Args:
physical_qubit1 (int): A physical qubit
physical_qubit2 (int): Another physical qubit
Returns:
List: The shortest undirected path
Raises:
CouplingError: When there is no path between physical_qubit1, physical_qubit2.
"""
try:
return nx.shortest_path(self.graph.to_undirected(as_view=True), source=physical_qubit1,
target=physical_qubit2)
except nx.exception.NetworkXNoPath:
raise CouplingError(
"Nodes %s and %s are not connected" % (str(physical_qubit1), str(physical_qubit2)))
def shortest_undirected_path(self, physical_qubit1, physical_qubit2):
"""Returns the shortest undirected path between physical_qubit1 and physical_qubit2.
Args:
physical_qubit1 (int): A physical qubit
physical_qubit2 (int): Another physical qubit
Returns:
List: The shortest undirected path
Raises:
CouplingError: When there is no path between physical_qubit1, physical_qubit2.
"""
paths = rx.digraph_dijkstra_shortest_paths(self.graph,
source=physical_qubit1,
target=physical_qubit2,
as_undirected=True)
if not paths:
raise CouplingError("Nodes %s and %s are not connected" %
(str(physical_qubit1), str(physical_qubit2)))
return paths[physical_qubit2]
def reduce(self, mapping):
"""Returns a reduced coupling map that
corresponds to the subgraph of qubits
selected in the mapping.
Args:
mapping (list): A mapping of reduced qubits to device
qubits.
Returns:
CouplingMap: A reduced coupling_map for the selected qubits.
Raises:
CouplingError: Reduced coupling map must be connected.
"""
from scipy.sparse import coo_matrix, csgraph
reduced_qubits = len(mapping)
inv_map = [None] * (max(mapping) + 1)
for idx, val in enumerate(mapping):
inv_map[val] = idx
reduced_cmap = []
for edge in self.get_edges():
if edge[0] in mapping and edge[1] in mapping:
reduced_cmap.append([inv_map[edge[0]], inv_map[edge[1]]])
# Verify coupling_map is connected
rows = np.array([edge[0] for edge in reduced_cmap], dtype=int)
cols = np.array([edge[1] for edge in reduced_cmap], dtype=int)
data = np.ones_like(rows)
mat = coo_matrix((data, (rows, cols)),
shape=(reduced_qubits, reduced_qubits)).tocsr()
if csgraph.connected_components(mat)[0] != 1:
raise CouplingError("coupling_map must be connected.")
return CouplingMap(reduced_cmap)
