# This code is part of a Qiskit project.
#
# (C) Copyright IBM 2022, 2023.
#
# 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.
"""
Gradient of probabilities with parameter shift
"""
from __future__ import annotations
from collections.abc import Sequence
from qiskit.circuit import Parameter, QuantumCircuit
from qiskit.quantum_info.operators.base_operator import BaseOperator
from ..base.base_estimator_gradient import BaseEstimatorGradient
from ..base.estimator_gradient_result import EstimatorGradientResult
from ..utils import _make_param_shift_parameter_values
from ...exceptions import AlgorithmError
[docs]class ParamShiftEstimatorGradient(BaseEstimatorGradient):
"""
Compute the gradients of the expectation values by the parameter shift rule [1].
**Reference:**
[1] Schuld, M., Bergholm, V., Gogolin, C., Izaac, J., and Killoran, N. Evaluating analytic
gradients on quantum hardware, `DOI <https://doi.org/10.1103/PhysRevA.99.032331>`_
"""
SUPPORTED_GATES = [
"x",
"y",
"z",
"h",
"rx",
"ry",
"rz",
"p",
"cx",
"cy",
"cz",
"ryy",
"rxx",
"rzz",
"rzx",
]
def _run(
self,
circuits: Sequence[QuantumCircuit],
observables: Sequence[BaseOperator],
parameter_values: Sequence[Sequence[float]],
parameters: Sequence[Sequence[Parameter]],
**options,
) -> EstimatorGradientResult:
"""Compute the gradients of the expectation values by the parameter shift rule."""
g_circuits, g_parameter_values, g_parameters = self._preprocess(
circuits, parameter_values, parameters, self.SUPPORTED_GATES
)
results = self._run_unique(
g_circuits, observables, g_parameter_values, g_parameters, **options
)
return self._postprocess(results, circuits, parameter_values, parameters)
def _run_unique(
self,
circuits: Sequence[QuantumCircuit],
observables: Sequence[BaseOperator],
parameter_values: Sequence[Sequence[float]],
parameters: Sequence[Sequence[Parameter]],
**options,
) -> EstimatorGradientResult:
"""Compute the estimator gradients on the given circuits."""
job_circuits, job_observables, job_param_values, metadata = [], [], [], []
all_n = []
for circuit, observable, parameter_values_, parameters_ in zip(
circuits, observables, parameter_values, parameters
):
metadata.append({"parameters": parameters_})
# Make parameter values for the parameter shift rule.
param_shift_parameter_values = _make_param_shift_parameter_values(
circuit, parameter_values_, parameters_
)
# Combine inputs into a single job to reduce overhead.
n = len(param_shift_parameter_values)
job_circuits.extend([circuit] * n)
job_observables.extend([observable] * n)
job_param_values.extend(param_shift_parameter_values)
all_n.append(n)
# Run the single job with all circuits.
job = self._estimator.run(
job_circuits,
job_observables,
job_param_values,
**options,
)
try:
results = job.result()
except Exception as exc:
raise AlgorithmError("Estimator job failed.") from exc
# Compute the gradients.
gradients = []
partial_sum_n = 0
for n in all_n:
result = results.values[partial_sum_n : partial_sum_n + n]
gradient_ = (result[: n // 2] - result[n // 2 :]) / 2
gradients.append(gradient_)
partial_sum_n += n
opt = self._get_local_options(options)
return EstimatorGradientResult(gradients=gradients, metadata=metadata, options=opt)