sampler_result_decoder.py

# This code is part of Qiskit.
#
# (C) Copyright IBM 2022.
#
# 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.

"""Sampler result decoder."""

from typing import Dict
from math import sqrt

from qiskit.result import QuasiDistribution
from qiskit.primitives import SamplerResult

from ..program.result_decoder import ResultDecoder


class SamplerResultDecoder(ResultDecoder):
    """Class used to decode sampler results."""

    @classmethod
    def decode(cls, raw_result: str) -> SamplerResult:
        """Convert the result to SamplerResult."""
        decoded: Dict = super().decode(raw_result)
        quasi_dists = []
        for quasi, meta in zip(decoded["quasi_dists"], decoded["metadata"]):
            shots = meta.get("shots", float("inf"))
            overhead = meta.get("readout_mitigation_overhead", 1.0)

            # M3 mitigation overhead is gamma^2
            # https://github.com/Qiskit-Partners/mthree/blob/423d7e83a12491c59c9f58af46b75891bc622949/mthree/mitigation.py#L457
            #
            # QuasiDistribution stddev_upper_bound is gamma / sqrt(shots)
            # https://github.com/Qiskit/qiskit-terra/blob/ff267b5de8b83aef86e2c9ac6c7f918f58500505/qiskit/result/mitigation/local_readout_mitigator.py#L288
            stddev = sqrt(overhead / shots)
            quasi_dists.append(
                QuasiDistribution(quasi, shots=shots, stddev_upper_bound=stddev)
            )
        return SamplerResult(
            quasi_dists=quasi_dists,
            metadata=decoded["metadata"],
        )