def _position_in_cmap(self, j: int, k: int,
layout: Layout) -> Tuple[int, ...]:
"""A helper function to track the movement of virtual qubits through the swaps.
Args:
j: The index of decision variable j (i.e. virtual qubit).
k: The index of decision variable k (i.e. virtual qubit).
layout: The current layout that takes into account previous swap gates.
Returns:
The position in the coupling map of the virtual qubits j and k as a tuple.
"""
bit0 = self._bit_indices[layout.get_physical_bits()[j]]
bit1 = self._bit_indices[layout.get_physical_bits()[k]]
return bit0, bit1
class StochasticSwap(TransformationPass):
"""
Maps a DAGCircuit onto a `coupling_map` adding swap gates.
Uses a randomized algorithm.
"""
def __init__(self,
coupling_map,
initial_layout=None,
trials=20,
seed=None):
"""
Map a DAGCircuit onto a `coupling_map` using swap gates.
If initial_layout is not None, we assume the input circuit
has been layed out before running this pass, and that
the layout process yields a DAG, coupling map, and layout
with the following properties:
1. All three have the same number of qubits
2. The layout a bijection from the DAG qubits to the coupling map
For this mapping pass, it may also be necessary that
3. The coupling map is a connected graph
If these are not satisfied, the behavior is undefined.
Args:
coupling_map (CouplingMap): Directed graph representing a coupling
map.
initial_layout (Layout): initial layout of qubits in mapping
trials (int): maximum number of iterations to attempt
seed (int): seed for random number generator
"""
super().__init__()
self.coupling_map = coupling_map
self.initial_layout = initial_layout
self.input_layout = None
self.trials = trials
self.seed = seed
self.qregs = None
self.rng = None
self.requires.append(BarrierBeforeFinalMeasurements())
def run(self, dag):
"""
Run the StochasticSwap pass on `dag`.
Args:
dag (DAGCircuit): DAG to map.
Returns:
DAGCircuit: A mapped DAG.
Raises:
TranspilerError: if the coupling map or the layout are not
compatible with the DAG
"""
if self.initial_layout is None:
if self.property_set["layout"]:
self.initial_layout = self.property_set["layout"]
else:
self.initial_layout = Layout.generate_trivial_layout(
*dag.qregs.values())
if len(dag.qubits()) != len(self.initial_layout):
raise TranspilerError(
'The layout does not match the amount of qubits in the DAG')
if len(self.coupling_map.physical_qubits) != len(self.initial_layout):
raise TranspilerError(
"Mappers require to have the layout to be the same size as the coupling map"
)
self.input_layout = self.initial_layout.copy()
self.qregs = dag.qregs
if self.seed is None:
self.seed = np.random.randint(0, np.iinfo(np.int32).max)
self.rng = np.random.RandomState(self.seed)
logger.debug("StochasticSwap RandomState seeded with seed=%s",
self.seed)
new_dag = self._mapper(dag, self.coupling_map, trials=self.trials)
# self.property_set["layout"] = self.initial_layout
return new_dag
def _layer_permutation(self, layer_partition, layout, qubit_subset,
coupling, trials):
"""Find a swap circuit that implements a permutation for this layer.
The goal is to swap qubits such that qubits in the same two-qubit gates
are adjacent.
Based on S. Bravyi's algorithm.
layer_partition (list): The layer_partition is a list of (qu)bit
lists and each qubit is a tuple (qreg, index).
layout (Layout): The layout is a Layout object mapping virtual
qubits in the input circuit to physical qubits in the coupling
graph. It reflects the current positions of the data.
qubit_subset (list): The qubit_subset is the set of qubits in
the coupling graph that we have chosen to map into, as tuples
(Register, index).
coupling (CouplingMap): Directed graph representing a coupling map.
This coupling map should be one that was provided to the
stochastic mapper.
trials (int): Number of attempts the randomized algorithm makes.
Returns:
Tuple: success_flag, best_circuit, best_depth, best_layout, trivial_flag
If success_flag is True, then best_circuit contains a DAGCircuit with
the swap circuit, best_depth contains the depth of the swap circuit,
and best_layout contains the new positions of the data qubits after the
swap circuit has been applied. The trivial_flag is set if the layer
has no multi-qubit gates.
Raises:
TranspilerError: if anything went wrong.
"""
return _layer_permutation(layer_partition, self.initial_layout, layout,
qubit_subset, coupling, trials, self.qregs,
self.rng)
def _layer_update(self, i, first_layer, best_layout, best_depth,
best_circuit, layer_list):
"""Provide a DAGCircuit for a new mapped layer.
i (int) = layer number
first_layer (bool) = True if this is the first layer in the
circuit with any multi-qubit gates
best_layout (Layout) = layout returned from _layer_permutation
best_depth (int) = depth returned from _layer_permutation
best_circuit (DAGCircuit) = swap circuit returned
from _layer_permutation
layer_list (list) = list of DAGCircuit objects for each layer,
output of DAGCircuit layers() method
Return a DAGCircuit object to append to the output DAGCircuit
that the _mapper method is building.
"""
layout = best_layout
logger.debug("layer_update: layout = %s", pformat(layout))
logger.debug("layer_update: self.initial_layout = %s",
pformat(self.initial_layout))
dagcircuit_output = DAGCircuit()
for register in layout.get_virtual_bits().keys():
if register[0] not in dagcircuit_output.qregs.values():
dagcircuit_output.add_qreg(register[0])
# If this is the first layer with multi-qubit gates,
# output all layers up to this point and ignore any
# swap gates. Set the initial layout.
if first_layer:
logger.debug("layer_update: first multi-qubit gate layer")
# Output all layers up to this point
for j in range(i + 1):
# Make qubit edge map and extend by classical bits
edge_map = layout.combine_into_edge_map(self.initial_layout)
for bit in dagcircuit_output.clbits():
edge_map[bit] = bit
dagcircuit_output.compose_back(layer_list[j]["graph"],
edge_map)
# Otherwise, we output the current layer and the associated swap gates.
else:
# Output any swaps
if best_depth > 0:
logger.debug(
"layer_update: there are swaps in this layer, "
"depth %d", best_depth)
dagcircuit_output.extend_back(best_circuit)
else:
logger.debug("layer_update: there are no swaps in this layer")
# Make qubit edge map and extend by classical bits
edge_map = layout.combine_into_edge_map(self.initial_layout)
for bit in dagcircuit_output.clbits():
edge_map[bit] = bit
# Output this layer
dagcircuit_output.compose_back(layer_list[i]["graph"], edge_map)
return dagcircuit_output
def _mapper(self, circuit_graph, coupling_graph, trials=20):
"""Map a DAGCircuit onto a CouplingMap using swap gates.
Use self.initial_layout for the initial layout.
Args:
circuit_graph (DAGCircuit): input DAG circuit
coupling_graph (CouplingMap): coupling graph to map onto
trials (int): number of trials.
Returns:
DAGCircuit: object containing a circuit equivalent to
circuit_graph that respects couplings in coupling_graph
Layout: a layout object mapping qubits of circuit_graph into
qubits of coupling_graph. The layout may differ from the
initial_layout if the first layer of gates cannot be
executed on the initial_layout, since in this case
it is more efficient to modify the layout instead of swapping
Dict: a final-layer qubit permutation
Raises:
TranspilerError: if there was any error during the mapping
or with the parameters.
"""
# Schedule the input circuit by calling layers()
layerlist = list(circuit_graph.layers())
logger.debug("schedule:")
for i, v in enumerate(layerlist):
logger.debug(" %d: %s", i, v["partition"])
if self.initial_layout is not None:
qubit_subset = self.initial_layout.get_virtual_bits().keys()
else:
# Supply a default layout for this dag
self.initial_layout = Layout()
physical_qubit = 0
for qreg in circuit_graph.qregs.values():
for index in range(qreg.size):
self.initial_layout[(qreg, index)] = physical_qubit
physical_qubit += 1
qubit_subset = self.initial_layout.get_virtual_bits().keys()
# Restrict the coupling map to the image of the layout
coupling_graph = coupling_graph.subgraph(
self.initial_layout.get_physical_bits().keys())
if coupling_graph.size() < len(self.initial_layout):
raise TranspilerError(
"Coupling map too small for default layout")
self.input_layout = self.initial_layout.copy()
# Find swap circuit to preceed to each layer of input circuit
layout = self.initial_layout.copy()
# Construct an empty DAGCircuit with the same set of
# qregs and cregs as the input circuit
dagcircuit_output = DAGCircuit()
dagcircuit_output.name = circuit_graph.name
for qreg in circuit_graph.qregs.values():
dagcircuit_output.add_qreg(qreg)
for creg in circuit_graph.cregs.values():
dagcircuit_output.add_creg(creg)
# Make a trivial wire mapping between the subcircuits
# returned by _layer_update and the circuit we build
identity_wire_map = {}
for qubit in circuit_graph.qubits():
identity_wire_map[qubit] = qubit
for bit in circuit_graph.clbits():
identity_wire_map[bit] = bit
first_layer = True # True until first layer is output
logger.debug("initial_layout = %s", layout)
# Iterate over layers
for i, layer in enumerate(layerlist):
# Attempt to find a permutation for this layer
success_flag, best_circuit, best_depth, best_layout, trivial_flag \
= self._layer_permutation(layer["partition"], layout,
qubit_subset, coupling_graph,
trials)
logger.debug("mapper: layer %d", i)
logger.debug(
"mapper: success_flag=%s,best_depth=%s,trivial_flag=%s",
success_flag, str(best_depth), trivial_flag)
# If this fails, try one gate at a time in this layer
if not success_flag:
logger.debug(
"mapper: failed, layer %d, "
"retrying sequentially", i)
serial_layerlist = list(layer["graph"].serial_layers())
# Go through each gate in the layer
for j, serial_layer in enumerate(serial_layerlist):
success_flag, best_circuit, best_depth, best_layout, trivial_flag = \
self._layer_permutation(
serial_layer["partition"],
layout, qubit_subset,
coupling_graph,
trials)
logger.debug("mapper: layer %d, sublayer %d", i, j)
logger.debug(
"mapper: success_flag=%s,best_depth=%s,"
"trivial_flag=%s", success_flag, str(best_depth),
trivial_flag)
# Give up if we fail again
if not success_flag:
raise TranspilerError("swap mapper failed: " +
"layer %d, sublayer %d" % (i, j))
# If this layer is only single-qubit gates,
# and we have yet to see multi-qubit gates,
# continue to the next inner iteration
if trivial_flag and first_layer:
logger.debug("mapper: skip to next sublayer")
continue
if first_layer:
self.initial_layout = layout
# Update the record of qubit positions
# for each inner iteration
layout = best_layout
# Update the DAG
dagcircuit_output.extend_back(
self._layer_update(j, first_layer, best_layout,
best_depth, best_circuit,
serial_layerlist),
identity_wire_map)
if first_layer:
first_layer = False
else:
# Update the record of qubit positions for each iteration
layout = best_layout
if first_layer:
self.initial_layout = layout
# Update the DAG
dagcircuit_output.extend_back(
self._layer_update(i, first_layer, best_layout, best_depth,
best_circuit, layerlist),
identity_wire_map)
if first_layer:
first_layer = False
# This is the final edgemap. We might use it to correctly replace
# any measurements that needed to be removed earlier.
logger.debug("mapper: self.initial_layout = %s",
pformat(self.initial_layout))
logger.debug("mapper: layout = %s", pformat(layout))
last_edgemap = layout.combine_into_edge_map(self.initial_layout)
logger.debug("mapper: last_edgemap = %s", pformat(last_edgemap))
# If first_layer is still set, the circuit only has single-qubit gates
# so we can use the initial layout to output the entire circuit
# This code is dead due to changes to first_layer above.
if first_layer:
logger.debug("mapper: first_layer flag still set")
layout = self.initial_layout
for i, layer in enumerate(layerlist):
edge_map = layout.combine_into_edge_map(self.initial_layout)
dagcircuit_output.compose_back(layer["graph"], edge_map)
return dagcircuit_output
