Source code for pennylane.transforms.transpile
"""
Contains the transpiler transform.
"""
from functools import partial
from typing import List, Union, Sequence, Callable
import networkx as nx
import pennylane as qml
from pennylane.transforms import transform
from pennylane import Hamiltonian
from pennylane.operation import Tensor
from pennylane.ops import __all__ as all_ops
from pennylane.ops.qubit import SWAP
from pennylane.queuing import QueuingManager
from pennylane.tape import QuantumTape
def state_transposition(results, mps, new_wire_order, original_wire_order):
"""Transpose the order of any state return.
Args:
results (ResultBatch): the result of executing a batch of length 1
Keyword Args:
mps (List[MeasurementProcess]): A list of measurements processes. At least one is a ``StateMP``
new_wire_order (Sequence[Any]): the wire order after transpile has been called
original_wire_order (.Wires): the devices wire order
Returns:
Result: The result object with state dimensions transposed.
"""
if len(mps) == 1:
temp_mp = qml.measurements.StateMP(wires=original_wire_order)
return temp_mp.process_state(results[0], wire_order=qml.wires.Wires(new_wire_order))
new_results = list(results[0])
for i, mp in enumerate(mps):
if isinstance(mp, qml.measurements.StateMP):
temp_mp = qml.measurements.StateMP(wires=original_wire_order)
new_res = temp_mp.process_state(
new_results[i], wire_order=qml.wires.Wires(new_wire_order)
)
new_results[i] = new_res
return tuple(new_results)
def _process_measurements(expanded_tape, device_wires, is_default_mixed):
measurements = expanded_tape.measurements.copy()
if device_wires:
for i, m in enumerate(measurements):
if isinstance(m, qml.measurements.StateMP):
if is_default_mixed:
measurements[i] = qml.density_matrix(wires=device_wires)
elif not m.wires:
measurements[i] = type(m)(wires=device_wires)
return measurements
[docs]@transform
def transpile(
tape: QuantumTape, coupling_map: Union[List, nx.Graph], device=None
) -> (Sequence[QuantumTape], Callable):
"""Transpile a circuit according to a desired coupling map
.. warning::
This transform does not yet support measurements of Hamiltonians or tensor products of observables. If a circuit
is passed which contains these types of measurements, a ``NotImplementedError`` will be raised.
Args:
tape (QNode or QuantumTape or Callable): A quantum tape.
coupling_map (list[tuple(int, int)] or nx.Graph): Either a list of tuples(int, int) or an instance of
`networkx.Graph` specifying the couplings between different qubits.
Returns:
qnode (QNode) or quantum function (Callable) or tuple[List[.QuantumTape], function]: The transformed circuit as described in :func:`qml.transform <pennylane.transform>`.
**Example**
Consider the following example circuit
.. code-block:: python
def circuit():
qml.CNOT(wires=[0, 1])
qml.CNOT(wires=[2, 3])
qml.CNOT(wires=[1, 3])
qml.CNOT(wires=[1, 2])
qml.CNOT(wires=[2, 3])
qml.CNOT(wires=[0, 3])
return qml.probs(wires=[0, 1, 2, 3])
which, before transpiling it looks like this:
.. code-block:: text
0: ──╭●──────────────╭●──╭┤ Probs
1: ──╰X──╭●──╭●──────│───├┤ Probs
2: ──╭●──│───╰X──╭●──│───├┤ Probs
3: ──╰X──╰X──────╰X──╰X──╰┤ Probs
Suppose we have a device which has connectivity constraints according to the graph:
.. code-block:: text
0 --- 1
| |
2 --- 3
We encode this in a coupling map as a list of the edges which are present in the graph, and then pass this, together
with the circuit, to the transpile function to get a circuit which can be executed for the specified coupling map:
>>> dev = qml.device('default.qubit', wires=[0, 1, 2, 3])
>>> transpiled_circuit = qml.transforms.transpile(circuit, coupling_map=[(0, 1), (1, 3), (3, 2), (2, 0)])
>>> transpiled_qnode = qml.QNode(transpiled_circuit, dev)
>>> print(qml.draw(transpiled_qnode)())
0: ─╭●────────────────╭●─┤ ╭Probs
1: ─╰X─╭●───────╭●────│──┤ ├Probs
2: ─╭●─│──╭SWAP─│──╭X─╰X─┤ ├Probs
3: ─╰X─╰X─╰SWAP─╰X─╰●────┤ ╰Probs
A swap gate has been applied to wires 2 and 3, and the remaining gates have been adapted accordingly
"""
if device:
device_wires = device.wires
is_default_mixed = getattr(device, "short_name", "") == "default.mixed"
else:
device_wires = None
is_default_mixed = False
# init connectivity graph
coupling_graph = (
nx.Graph(coupling_map) if not isinstance(coupling_map, nx.Graph) else coupling_map
)
# make sure every wire is present in coupling map
if any(wire not in coupling_graph.nodes for wire in tape.wires):
wires = tape.wires.tolist()
raise ValueError(
f"Not all wires present in coupling map! wires: {wires}, coupling map: {coupling_graph.nodes}"
)
if any(isinstance(m.obs, (Hamiltonian, Tensor)) for m in tape.measurements):
raise NotImplementedError(
"Measuring expectation values of tensor products or Hamiltonians is not yet supported"
)
if any(len(op.wires) > 2 for op in tape.operations):
raise NotImplementedError(
"The transpile transform only supports gates acting on 1 or 2 qubits."
)
# we wrap all manipulations inside stop_recording() so that we don't queue anything due to unrolling of templates
# or newly applied swap gates
with QueuingManager.stop_recording():
# this unrolls everything in the current tape (in particular templates)
def stop_at(obj):
return (obj.name in all_ops) and (not getattr(obj, "only_visual", False))
expanded_tape = tape.expand(stop_at=stop_at)
# make copy of ops
list_op_copy = expanded_tape.operations.copy()
wire_order = device_wires or tape.wires
measurements = _process_measurements(expanded_tape, device_wires, is_default_mixed)
gates = []
while len(list_op_copy) > 0:
op = list_op_copy[0]
# gates which act only on one wire
if len(op.wires) == 1:
gates.append(op)
list_op_copy.pop(0)
continue
# two-qubit gates which can be handled by the coupling map
if (
op.wires in coupling_graph.edges
or tuple(reversed(op.wires)) in coupling_graph.edges
):
gates.append(op)
list_op_copy.pop(0)
continue
# since in each iteration, we adjust indices of each op, we reset logical -> phyiscal mapping
wire_map = {w: w for w in wire_order}
# to make sure two qubit gates which act on non-neighbouring qubits q1, q2 can be applied, we first look
# for the shortest path between the two qubits in the connectivity graph. We then move the q2 into the
# neighbourhood of q1 via swap operations.
source_wire, dest_wire = op.wires
# pylint:disable=too-many-function-args
shortest_path = nx.algorithms.shortest_path(coupling_graph, source_wire, dest_wire)
path_length = len(shortest_path) - 1
wires_to_swap = [shortest_path[(i - 1) : (i + 1)] for i in range(path_length, 1, -1)]
for w0, w1 in wires_to_swap:
# swap wires
gates.append(SWAP(wires=[w0, w1]))
# update logical -> phyiscal mapping
wire_map = {
k: (w0 if v == w1 else (w1 if v == w0 else v)) for k, v in wire_map.items()
}
# append op to gates with adjusted indices and remove from list
gates.append(op.map_wires(wire_map))
list_op_copy.pop(0)
list_op_copy = [op.map_wires(wire_map) for op in list_op_copy]
wire_order = [wire_map[w] for w in wire_order]
measurements = [m.map_wires(wire_map) for m in measurements]
new_tape = type(tape)(gates, measurements, shots=tape.shots)
# note: no need for transposition with density matrix, so type must be `StateMP` but not `DensityMatrixMP`
# pylint: disable=unidiomatic-typecheck
any_state_mp = any(type(m) is qml.measurements.StateMP for m in measurements)
if not any_state_mp or device_wires is None:
def null_postprocessing(results):
"""A postprocesing function returned by a transform that only converts the batch of results
into a result for a single ``QuantumTape``.
"""
return results[0]
return (new_tape,), null_postprocessing
return (new_tape,), partial(
state_transposition,
mps=measurements,
new_wire_order=wire_order,
original_wire_order=device_wires,
)
@transpile.custom_qnode_transform
def _transpile_qnode(self, qnode, targs, tkwargs):
"""Custom qnode transform for ``transpile``."""
if tkwargs.get("device", None):
raise ValueError(
"Cannot provide a 'device' value directly to the defer_measurements decorator "
"when transforming a QNode."
)
tkwargs.setdefault("device", qnode.device)
return self.default_qnode_transform(qnode, targs, tkwargs)
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