Source code for qiskit.transpiler.passes.routing.stochastic_swap
# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2018.
#
# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.
"""Map a DAGCircuit onto a `coupling_map` adding swap gates."""
import logging
from math import inf
from collections import OrderedDict
import numpy as np
from qiskit.circuit.quantumregister import QuantumRegister
from qiskit.transpiler.basepasses import TransformationPass
from qiskit.transpiler.exceptions import TranspilerError
from qiskit.dagcircuit import DAGCircuit
from qiskit.circuit.library.standard_gates import SwapGate
from qiskit.transpiler.layout import Layout
# pylint: disable=no-name-in-module
from .cython.stochastic_swap.utils import nlayout_from_layout
# pylint: disable=no-name-in-module
from .cython.stochastic_swap.swap_trial import swap_trial
logger = logging.getLogger(__name__)
[docs]class StochasticSwap(TransformationPass):
"""Map a DAGCircuit onto a `coupling_map` adding swap gates.
Uses a randomized algorithm.
Notes:
1. Measurements may occur and be followed by swaps that result in repeated
measurement of the same qubit. Near-term experiments cannot implement
these circuits, so some care is required when using this mapper
with experimental backend targets.
2. We do not use the fact that the input state is zero to simplify
the circuit.
"""
def __init__(self, coupling_map, trials=20, seed=None, fake_run=False):
"""StochasticSwap initializer.
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.
trials (int): maximum number of iterations to attempt
seed (int): seed for random number generator
fake_run (bool): if true, it only pretend to do routing, i.e., no
swap is effectively added.
"""
super().__init__()
self.coupling_map = coupling_map
self.trials = trials
self.seed = seed
self.fake_run = fake_run
self.qregs = None
self.rng = None
self.trivial_layout = None
self._qubit_indices = None
[docs] 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 len(dag.qregs) != 1 or dag.qregs.get("q", None) is None:
raise TranspilerError("StochasticSwap runs on physical circuits only")
if len(dag.qubits) > len(self.coupling_map.physical_qubits):
raise TranspilerError("The layout does not match the amount of qubits in the DAG")
canonical_register = dag.qregs["q"]
self.trivial_layout = Layout.generate_trivial_layout(canonical_register)
self._qubit_indices = {bit: idx for idx, bit in enumerate(dag.qubits)}
self.qregs = dag.qregs
if self.seed is None:
self.seed = np.random.randint(0, np.iinfo(np.int32).max)
self.rng = np.random.default_rng(self.seed)
logger.debug("StochasticSwap default_rng seeded with seed=%s", self.seed)
new_dag = self._mapper(dag, self.coupling_map, trials=self.trials)
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.
Args:
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
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.
Raises:
TranspilerError: if anything went wrong.
"""
logger.debug("layer_permutation: layer_partition = %s", layer_partition)
logger.debug("layer_permutation: layout = %s", layout.get_virtual_bits())
logger.debug("layer_permutation: qubit_subset = %s", qubit_subset)
logger.debug("layer_permutation: trials = %s", trials)
# The input dag is on a flat canonical register
# TODO: cleanup the code that is general for multiple qregs below
canonical_register = QuantumRegister(len(layout), "q")
qregs = OrderedDict({canonical_register.name: canonical_register})
gates = [] # list of lists of tuples [[(register, index), ...], ...]
for gate_args in layer_partition:
if len(gate_args) > 2:
raise TranspilerError("Layer contains > 2-qubit gates")
if len(gate_args) == 2:
gates.append(tuple(gate_args))
logger.debug("layer_permutation: gates = %s", gates)
# Can we already apply the gates? If so, there is no work to do.
dist = sum(coupling.distance(layout[g[0]], layout[g[1]]) for g in gates)
logger.debug("layer_permutation: distance = %s", dist)
if dist == len(gates):
logger.debug("layer_permutation: nothing to do")
circ = DAGCircuit()
circ.add_qreg(canonical_register)
return True, circ, 0, layout
# Begin loop over trials of randomized algorithm
num_qubits = len(layout)
best_depth = inf # initialize best depth
best_edges = None # best edges found
best_circuit = None # initialize best swap circuit
best_layout = None # initialize best final layout
cdist2 = coupling._dist_matrix ** 2
# Scaling matrix
scale = np.zeros((num_qubits, num_qubits))
int_qubit_subset = np.fromiter(
(self._qubit_indices[bit] for bit in qubit_subset),
dtype=np.int32,
count=len(qubit_subset),
)
int_gates = np.fromiter(
(self._qubit_indices[bit] for gate in gates for bit in gate),
dtype=np.int32,
count=2 * len(gates),
)
int_layout = nlayout_from_layout(layout, self._qubit_indices, num_qubits, coupling.size())
trial_circuit = DAGCircuit() # SWAP circuit for slice of swaps in this trial
trial_circuit.add_qubits(layout.get_virtual_bits())
edges = np.asarray(coupling.get_edges(), dtype=np.int32).ravel()
cdist = coupling._dist_matrix
for trial in range(trials):
logger.debug("layer_permutation: trial %s", trial)
# This is one Trial --------------------------------------
dist, optim_edges, trial_layout, depth_step = swap_trial(
num_qubits,
int_layout,
int_qubit_subset,
int_gates,
cdist2,
cdist,
edges,
scale,
self.rng,
)
logger.debug("layer_permutation: final distance for this trial = %s", dist)
if dist == len(gates) and depth_step < best_depth:
logger.debug("layer_permutation: got circuit with improved depth %s", depth_step)
best_edges = optim_edges
best_layout = trial_layout
best_depth = min(best_depth, depth_step)
# Break out of trial loop if we found a depth 1 circuit
# since we can't improve it further
if best_depth == 1:
break
# If we have no best circuit for this layer, all of the
# trials have failed
if best_layout is None:
logger.debug("layer_permutation: failed!")
return False, None, None, None
edges = best_edges.edges()
for idx in range(best_edges.size // 2):
swap_src = self.trivial_layout[edges[2 * idx]]
swap_tgt = self.trivial_layout[edges[2 * idx + 1]]
trial_circuit.apply_operation_back(SwapGate(), [swap_src, swap_tgt], [])
best_circuit = trial_circuit
# Otherwise, we return our result for this layer
logger.debug("layer_permutation: success!")
best_lay = best_layout.to_layout(qregs)
return True, best_circuit, best_depth, best_lay
def _layer_update(self, dag, layer, best_layout, best_depth, best_circuit):
"""Add swaps followed by the now mapped layer from the original circuit.
Args:
dag (DAGCircuit): The DAGCircuit object that the _mapper method is building
layer (DAGCircuit): A DAGCircuit layer from the original circuit
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
"""
layout = best_layout
logger.debug("layer_update: layout = %s", layout)
logger.debug("layer_update: self.trivial_layout = %s", self.trivial_layout)
# Output any swaps
if best_depth > 0:
logger.debug("layer_update: there are swaps in this layer, " "depth %d", best_depth)
dag.compose(best_circuit)
else:
logger.debug("layer_update: there are no swaps in this layer")
# Output this layer
layer_circuit = layer["graph"]
order = layout.reorder_bits(dag.qubits)
dag.compose(layer_circuit, qubits=order)
def _mapper(self, circuit_graph, coupling_graph, trials=20):
"""Map a DAGCircuit onto a CouplingMap using swap gates.
Use self.trivial_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
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"])
qubit_subset = self.trivial_layout.get_virtual_bits().keys()
# Find swap circuit to precede each layer of input circuit
layout = self.trivial_layout.copy()
# Construct an empty DAGCircuit with the same set of
# qregs and cregs as the input circuit
dagcircuit_output = None
if not self.fake_run:
dagcircuit_output = circuit_graph._copy_circuit_metadata()
logger.debug("trivial_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 = 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", success_flag, str(best_depth))
# 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 = 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,", success_flag, str(best_depth)
)
# Give up if we fail again
if not success_flag:
raise TranspilerError(
"swap mapper failed: " + "layer %d, sublayer %d" % (i, j)
)
# Update the record of qubit positions
# for each inner iteration
layout = best_layout
# Update the DAG
if not self.fake_run:
self._layer_update(
dagcircuit_output,
serial_layerlist[j],
best_layout,
best_depth,
best_circuit,
)
else:
# Update the record of qubit positions for each iteration
layout = best_layout
# Update the DAG
if not self.fake_run:
self._layer_update(
dagcircuit_output, layerlist[i], best_layout, best_depth, best_circuit
)
# This is the final edgemap. We might use it to correctly replace
# any measurements that needed to be removed earlier.
logger.debug("mapper: self.trivial_layout = %s", self.trivial_layout)
logger.debug("mapper: layout = %s", layout)
if self.fake_run:
self.property_set["final_layout"] = layout
return circuit_graph
return dagcircuit_output