qiskit/primitives/estimator.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.
"""
Estimator class
"""

from __future__ import annotations

from collections.abc import Sequence
from typing import Any

import numpy as np

from qiskit.circuit import QuantumCircuit
from qiskit.exceptions import QiskitError
from qiskit.quantum_info import Statevector
from qiskit.quantum_info.operators.base_operator import BaseOperator

from .base import BaseEstimator, EstimatorResult
from .primitive_job import PrimitiveJob
from .utils import (
    _circuit_key,
    _observable_key,
    bound_circuit_to_instruction,
    init_observable,
)


class Estimator(BaseEstimator[PrimitiveJob[EstimatorResult]]):
    """
    Reference implementation of :class:`BaseEstimator`.

    :Run Options:

        - **shots** (None or int) --
          The number of shots. If None, it calculates the exact expectation
          values. Otherwise, it samples from normal distributions with standard errors as standard
          deviations using normal distribution approximation.

        - **seed** (np.random.Generator or int) --
          Set a fixed seed or generator for the normal distribution. If shots is None,
          this option is ignored.
    """

    def __init__(self, *, options: dict | None = None):
        """
        Args:
            options: Default options.

        Raises:
            QiskitError: if some classical bits are not used for measurements.
        """
        super().__init__(options=options)
        self._circuits = []
        self._parameters = []
        self._observables = []
        self._circuit_ids = {}
        self._observable_ids = {}

    def _call(
        self,
        circuits: Sequence[int],
        observables: Sequence[int],
        parameter_values: Sequence[Sequence[float]],
        **run_options,
    ) -> EstimatorResult:
        shots = run_options.pop("shots", None)
        seed = run_options.pop("seed", None)
        if seed is None:
            rng = np.random.default_rng()
        elif isinstance(seed, np.random.Generator):
            rng = seed
        else:
            rng = np.random.default_rng(seed)

        # Initialize metadata
        metadata: list[dict[str, Any]] = [{} for _ in range(len(circuits))]

        bound_circuits = []
        for i, value in zip(circuits, parameter_values):
            if len(value) != len(self._parameters[i]):
                raise QiskitError(
                    f"The number of values ({len(value)}) does not match "
                    f"the number of parameters ({len(self._parameters[i])})."
                )
            bound_circuits.append(
                self._circuits[i]
                if len(value) == 0
                else self._circuits[i].assign_parameters(dict(zip(self._parameters[i], value)))
            )
        sorted_observables = [self._observables[i] for i in observables]
        expectation_values = []
        for circ, obs, metadatum in zip(bound_circuits, sorted_observables, metadata):
            if circ.num_qubits != obs.num_qubits:
                raise QiskitError(
                    f"The number of qubits of a circuit ({circ.num_qubits}) does not match "
                    f"the number of qubits of a observable ({obs.num_qubits})."
                )
            final_state = Statevector(bound_circuit_to_instruction(circ))
            expectation_value = final_state.expectation_value(obs)
            if shots is None:
                expectation_values.append(expectation_value)
            else:
                expectation_value = np.real_if_close(expectation_value)
                sq_obs = (obs @ obs).simplify(atol=0)
                sq_exp_val = np.real_if_close(final_state.expectation_value(sq_obs))
                variance = sq_exp_val - expectation_value**2
                variance = max(variance, 0)
                standard_error = np.sqrt(variance / shots)
                expectation_value_with_error = rng.normal(expectation_value, standard_error)
                expectation_values.append(expectation_value_with_error)
                metadatum["variance"] = variance
                metadatum["shots"] = shots

        return EstimatorResult(np.real_if_close(expectation_values), metadata)

    def _run(
        self,
        circuits: tuple[QuantumCircuit, ...],
        observables: tuple[BaseOperator, ...],
        parameter_values: tuple[tuple[float, ...], ...],
        **run_options,
    ):
        circuit_indices = []
        for circuit in circuits:
            key = _circuit_key(circuit)
            index = self._circuit_ids.get(key)
            if index is not None:
                circuit_indices.append(index)
            else:
                circuit_indices.append(len(self._circuits))
                self._circuit_ids[key] = len(self._circuits)
                self._circuits.append(circuit)
                self._parameters.append(circuit.parameters)
        observable_indices = []
        for observable in observables:
            observable = init_observable(observable)
            index = self._observable_ids.get(_observable_key(observable))
            if index is not None:
                observable_indices.append(index)
            else:
                observable_indices.append(len(self._observables))
                self._observable_ids[_observable_key(observable)] = len(self._observables)
                self._observables.append(observable)
        job = PrimitiveJob(
            self._call, circuit_indices, observable_indices, parameter_values, **run_options
        )
        job._submit()
        return job