qiskit_machine_learning/utils/adjust_num_qubits.py

# 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.
"""Helper functions to adjust number of qubits."""
from __future__ import annotations

from typing import Tuple

from qiskit import QuantumCircuit
from qiskit.circuit.library import RealAmplitudes, ZFeatureMap, ZZFeatureMap

from qiskit_machine_learning import QiskitMachineLearningError


# pylint: disable=invalid-name
def derive_num_qubits_feature_map_ansatz(
    num_qubits: int | None = None,
    feature_map: QuantumCircuit | None = None,
    ansatz: QuantumCircuit | None = None,
) -> Tuple[int, QuantumCircuit, QuantumCircuit]:
    """
    Derives a correct number of qubits, feature map, and ansatz from the parameters.

    If the number of qubits is not ``None``, then the feature map and ansatz are adjusted to this
    number of qubits if required. If such an adjustment fails, an error is raised. Also, if the
    feature map or ansatz or both are ``None``, then :class:`~qiskit.circuit.library.ZZFeatureMap`
    and :class:`~qiskit.circuit.library.RealAmplitudes` are created respectively. If there's just
    one qubit, :class:`~qiskit.circuit.library.ZFeatureMap` is created instead.

    If the number of qubits is ``None``, then the number of qubits is derived from the feature map
    or ansatz. Both the feature map and ansatz in this case must have the same number of qubits.
    If the number of qubits of the feature map is not the same as the number of qubits of
    the ansatz, an error is raised. If only one of the feature map and ansatz are ``None``, then
    :class:`~qiskit.circuit.library.ZZFeatureMap` or :class:`~qiskit.circuit.library.RealAmplitudes`
    are created respectively.

    If all the parameters are none an error is raised.

    Args:
        num_qubits: Number of qubits.
        feature_map: A feature map.
        ansatz: An ansatz.

    Returns:
        A tuple of number of qubits, feature map, and ansatz. All are not none.

    Raises:
        QiskitMachineLearningError: If correct values can not be derived from the parameters.
    """

    # check num_qubits, feature_map, and ansatz
    if num_qubits in (0, None) and feature_map is None and ansatz is None:
        raise QiskitMachineLearningError(
            "Need at least one of number of qubits, feature map, or ansatz!"
        )

    if num_qubits not in (0, None):
        if feature_map is not None:
            if feature_map.num_qubits != num_qubits:
                _adjust_num_qubits(feature_map, "feature map", num_qubits)
        else:
            feature_map = ZFeatureMap(num_qubits) if num_qubits == 1 else ZZFeatureMap(num_qubits)
        if ansatz is not None:
            if ansatz.num_qubits != num_qubits:
                _adjust_num_qubits(ansatz, "ansatz", num_qubits)
        else:
            ansatz = RealAmplitudes(num_qubits)
    else:
        if feature_map is not None and ansatz is not None:
            if feature_map.num_qubits != ansatz.num_qubits:
                raise QiskitMachineLearningError(
                    f"Mismatching number of qubits in the feature map ({feature_map.num_qubits}) "
                    f"and the ansatz ({ansatz.num_qubits})!"
                )
            num_qubits = feature_map.num_qubits
        elif feature_map is not None:
            num_qubits = feature_map.num_qubits
            ansatz = RealAmplitudes(num_qubits)
        else:
            num_qubits = ansatz.num_qubits
            feature_map = ZFeatureMap(num_qubits) if num_qubits == 1 else ZZFeatureMap(num_qubits)

    return num_qubits, feature_map, ansatz


def _adjust_num_qubits(circuit: QuantumCircuit, circuit_name: str, num_qubits: int) -> None:
    """
    Tries to adjust the number of qubits of the circuit by trying to set ``num_qubits`` properties.

    Args:
        circuit: A circuit to adjust.
        circuit_name: A circuit name, used in the error description.
        num_qubits: A number of qubits to set.

    Raises:
        QiskitMachineLearningError: if number of qubits can't be adjusted.

    """
    try:
        circuit.num_qubits = num_qubits
    except AttributeError as ex:
        raise QiskitMachineLearningError(
            f"The number of qubits {circuit.num_qubits} of the {circuit_name} does not match "
            f"the number of qubits {num_qubits}, and the {circuit_name} does not allow setting "
            "the number of qubits using `num_qubits`."
        ) from ex