# This code is part of a Qiskit project.
#
# (C) Copyright IBM 2022, 2025.
#
# 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 import defaultdict
from collections.abc import Sequence
from qiskit.circuit import Parameter, QuantumCircuit
from ..base.base_sampler_gradient import BaseSamplerGradient
from ..base.sampler_gradient_result import SamplerGradientResult
from ..utils import _make_param_shift_parameter_values
from ...exceptions import AlgorithmError
[docs]
class ParamShiftSamplerGradient(BaseSamplerGradient):
"""
Compute the gradients of the sampling probability 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],
parameter_values: Sequence[Sequence[float]],
parameters: Sequence[Sequence[Parameter]],
*,
shots: int | None,
) -> SamplerGradientResult:
"""Compute the estimator gradients on the given circuits."""
g_circuits, g_parameter_values, g_parameters = self._preprocess(
circuits, parameter_values, parameters, self.SUPPORTED_GATES
)
if self._transpiler is not None:
g_circuits = self._transpiler.run(g_circuits, **self._transpiler_options)
results = self._run_unique(g_circuits, g_parameter_values, g_parameters, shots)
return self._postprocess(results, circuits, parameter_values, parameters)
def _run_unique(
self,
circuits: Sequence[QuantumCircuit],
parameter_values: Sequence[Sequence[float]],
parameters: Sequence[Sequence[Parameter]],
shots: int | None,
) -> SamplerGradientResult:
"""Compute the sampler gradients on the given circuits."""
metadata = []
all_n = []
has_transformed_shots = False
if isinstance(shots, int) or shots is None:
shots = [shots] * len(circuits)
has_transformed_shots = True
pubs = []
for circuit, parameter_values_, parameters_, shots_ in zip(
circuits, parameter_values, parameters, shots
):
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)
pubs.append((circuit, param_shift_parameter_values, shots_))
all_n.append(n)
# Run the single job with all circuits.
job = self._sampler.run(pubs)
try:
results = job.result()
except Exception as exc:
raise AlgorithmError("Estimator job failed.") from exc
# Compute the gradients.
gradients = []
for n, result_n in zip(all_n, results):
gradient = []
result = [
{label: value / res.num_shots for label, value in res.get_int_counts().items()}
for res in getattr(result_n.data, next(iter(result_n.data)))
]
for dist_plus, dist_minus in zip(result[: n // 2], result[n // 2 :]):
grad_dist: dict[int, float] = defaultdict(float)
for key, val in dist_plus.items():
grad_dist[key] += val / 2
for key, val in dist_minus.items():
grad_dist[key] -= val / 2
gradient.append(dict(grad_dist))
gradients.append(gradient)
if has_transformed_shots:
shots = shots[0]
if shots is None:
shots = results[0].metadata["shots"]
else:
for i, (shots_, result) in enumerate(zip(shots, results)):
if shots_ is None:
shots[i] = result.metadata["shots"]
return SamplerGradientResult(gradients=gradients, metadata=metadata, shots=shots)