# This code is part of a Qiskit project.
#
# (C) Copyright IBM 2021, 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.
"""An implementation of variational quantum classifier."""
from __future__ import annotations
from typing import Callable
import numpy as np
from qiskit import QuantumCircuit
from qiskit.primitives import BaseSampler
from qiskit_algorithms.optimizers import Optimizer, OptimizerResult, Minimizer
from ...neural_networks import SamplerQNN
from ...utils import derive_num_qubits_feature_map_ansatz
from ...utils.loss_functions import Loss
from .neural_network_classifier import NeuralNetworkClassifier
[docs]class VQC(NeuralNetworkClassifier):
r"""A convenient Variational Quantum Classifier implementation.
The variational quantum classifier (VQC) is a variational algorithm where the measured
bitstrings are interpreted as the output of a classifier.
Constructs a quantum circuit and corresponding neural network, then uses it to instantiate a
neural network classifier.
Labels can be passed in various formats, they can be plain labels, a one dimensional numpy
array that contains integer labels like `[0, 1, 2, ...]`, or a numpy array with categorical
string labels. One hot encoded labels are also supported. Internally, labels are transformed
to one hot encoding and the classifier is always trained on one hot labels.
Multi-label classification is not supported. E.g., :math:`[[1, 1, 0], [0, 1, 1], [1, 0, 1]]`.
"""
def __init__(
self,
num_qubits: int | None = None,
feature_map: QuantumCircuit | None = None,
ansatz: QuantumCircuit | None = None,
loss: str | Loss = "cross_entropy",
optimizer: Optimizer | Minimizer | None = None,
warm_start: bool = False,
initial_point: np.ndarray | None = None,
callback: Callable[[np.ndarray, float], None] | None = None,
*,
sampler: BaseSampler | None = None,
) -> None:
"""
Args:
num_qubits: The number of qubits for the underlying QNN.
If ``None`` is given, the number of qubits is derived from the
feature map or ansatz. If neither of those is given, raises an exception.
The number of qubits in the feature map and ansatz are adjusted to this
number if required.
feature_map: The (parametrized) circuit to be used as a feature map for the underlying
QNN. If ``None`` is given, the :class:`~qiskit.circuit.library.ZZFeatureMap`
is used if the number of qubits is larger than 1. For a single qubit
classification problem the :class:`~qiskit.circuit.library.ZFeatureMap`
is used by default.
ansatz: The (parametrized) circuit to be used as an ansatz for the underlying QNN.
If ``None`` is given then the :class:`~qiskit.circuit.library.RealAmplitudes`
circuit is used.
loss: A target loss function to be used in training. Default value is ``cross_entropy``.
optimizer: An instance of an optimizer or a callable to be used in training.
Refer to :class:`~qiskit_algorithms.optimizers.Minimizer` for more information on
the callable protocol. When `None` defaults to
:class:`~qiskit_algorithms.optimizers.SLSQP`.
warm_start: Use weights from previous fit to start next fit.
initial_point: Initial point for the optimizer to start from.
callback: a reference to a user's callback function that has two parameters and
returns ``None``. The callback can access intermediate data during training.
On each iteration an optimizer invokes the callback and passes current weights
as an array and a computed value as a float of the objective function being
optimized. This allows to track how well optimization / training process is going on.
sampler: an optional Sampler primitive instance to be used by the underlying
:class:`~qiskit_machine_learning.neural_networks.SamplerQNN` neural network. If
``None`` is passed then an instance of the reference Sampler will be used.
Raises:
QiskitMachineLearningError: Needs at least one out of ``num_qubits``, ``feature_map`` or
``ansatz`` to be given. Or the number of qubits in the feature map and/or ansatz
can't be adjusted to ``num_qubits``.
"""
num_qubits, feature_map, ansatz = derive_num_qubits_feature_map_ansatz(
num_qubits, feature_map, ansatz
)
# construct circuit
self._feature_map = feature_map
self._ansatz = ansatz
self._num_qubits = num_qubits
self._circuit = QuantumCircuit(self._num_qubits)
self._circuit.compose(self.feature_map, inplace=True)
self._circuit.compose(self.ansatz, inplace=True)
neural_network = SamplerQNN(
sampler=sampler,
circuit=self._circuit,
input_params=self.feature_map.parameters,
weight_params=self.ansatz.parameters,
interpret=self._get_interpret(2),
output_shape=2,
input_gradients=False,
)
super().__init__(
neural_network=neural_network,
loss=loss,
one_hot=True,
optimizer=optimizer,
warm_start=warm_start,
initial_point=initial_point,
callback=callback,
)
@property
def feature_map(self) -> QuantumCircuit:
"""Returns the used feature map."""
return self._feature_map
@property
def ansatz(self) -> QuantumCircuit:
"""Returns the used ansatz."""
return self._ansatz
@property
def circuit(self) -> QuantumCircuit:
"""Returns the underlying quantum circuit."""
return self._circuit
@property
def num_qubits(self) -> int:
"""Returns the number of qubits used by ansatz and feature map."""
return self.circuit.num_qubits
def _fit_internal(self, X: np.ndarray, y: np.ndarray) -> OptimizerResult:
"""
Fit the model to data matrix X and targets y.
Args:
X: The input feature values.
y: The input target values. Required to be one-hot encoded.
Returns:
Trained classifier.
"""
X, y = self._validate_input(X, y)
num_classes = self._num_classes
# instance check required by mypy (alternative to cast)
if isinstance(self._neural_network, SamplerQNN):
self._neural_network.set_interpret(self._get_interpret(num_classes), num_classes)
function = self._create_objective(X, y)
return self._minimize(function)
def _get_interpret(self, num_classes: int):
def parity(x: int, num_classes: int = num_classes) -> int:
return x % num_classes
return parity