Source code for qermit.taskgraph.measurement_reduction

# Copyright 2019-2023 Quantinuum
#
# 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
#
#     http://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,
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# See the License for the specific language governing permissions and
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from copy import copy
from typing import Dict, List, Tuple

from pytket import Bit
from pytket.backends import Backend
from pytket.partition import (
    GraphColourMethod,
    PauliPartitionStrat,
    measurement_reduction,
)
from pytket.pauli import QubitPauliString
from pytket.transform import CXConfigType

from .mitex import MitEx
from .mittask import MitTask, ObservableExperiment
from .utils import MeasurementCircuit




def measurement_reduction_task_gen(
    strategy: PauliPartitionStrat, method: GraphColourMethod, cx_config: CXConfigType
) -> MitTask:
    """
    Uses measurement reduction techniques available in the pytket.partition module to reduce
    the number of measurement circuits required to measure all expectations for an observable.

    :param strategy: Measurement reduction strategy used in measurement_reduction method
    :param method: Graph colouring method used in measurement_reduction method
    :param cx_config: Configuration of CX gates for diagonlisation methods.

    :return: MitTask object, taking and return same write type, but with added
        measurement circuits to ObservableTracker.
    """

    def task(
        obj,
        measurement_wires: List[ObservableExperiment],
    ) -> Tuple[List[ObservableExperiment]]:
        new_wires = []
        # for each experiment
        for measurement_wire in measurement_wires:
            # circ is ansatz circuit which measurement circuits are appended to
            circ = measurement_wire[0][0]
            # symbols dict object
            symbols = measurement_wire[0][2]
            # ObservableTracker object for given experiment
            tracker = measurement_wire[1]
            # use measurement reduction method to get MeasurementSetup object
            # with reduced number of measurement circuits
            new_setup = measurement_reduction(
                list(tracker._qubit_pauli_operator._dict.keys()),
                strat=strategy,
                method=method,
                cx_config=cx_config,
            )
            # attach measurement configurations from MeasurementSetup to
            # copies of AnsatzCircuit
            # new measurement circuits
            measurement_circuits = []
            for measurement_circuit in new_setup.measurement_circs:
                full_circ = circ.copy()
                full_circ.append(measurement_circuit)
                measurement_circuits.append(MeasurementCircuit(full_circ, symbols))
            # convert MeasurementBitMap objects to MeasurementInfo for ObservableTracker
            adder_info: Dict[int, List[Tuple[QubitPauliString, List[Bit], bool]]] = (
                dict()
            )
            for i in range(len(measurement_circuits)):
                adder_info[i] = list()

            for qps, value in new_setup.results.items():
                for v in value:
                    circ_index = v.circ_index
                    bits = [Bit(i) for i in v.bits]
                    invert = v.invert
                    adder_info[circ_index].append((qps, bits, invert))

            for key in adder_info:
                tracker.add_measurement_circuit(
                    measurement_circuits[key], adder_info[key]
                )

            new_wires.append(
                ObservableExperiment(
                    AnsatzCircuit=measurement_wire[0], ObservableTracker=tracker
                )
            )
        return (new_wires,)

    return MitTask(
        _label="MeasurementReduction", _n_in_wires=1, _n_out_wires=1, _method=task
    )





def gen_MeasurementReduction_MitEx(backend: Backend, **kwargs) -> MitEx:
    """
    Returns a MitEx object that produces measurement circuits via Measurement Reduction
    methods.

    :param backend: Backend experiment is built around.
    :key mitex: MitEx object measurement reduction task is prepended to.

    :return: MitEx object performing Measurement reduction
    """
    # mitex object to built measurement reduction
    _mitex = copy(
        kwargs.get("mitex", MitEx(backend, _label="MeasurementReductionMitEx"))
    )
    _mitex._label = "MeasurementReductionMitEx"
    _strategy = kwargs.get("strategy", PauliPartitionStrat.NonConflictingSets)
    _method = kwargs.get("method", GraphColourMethod.Lazy)
    _cx_config = kwargs.get("cx_config", CXConfigType.Snake)
    # add measurement reduction task to start of method
    _mitex.prepend(measurement_reduction_task_gen(_strategy, _method, _cx_config))
    return _mitex