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Source code for qiskit_machine_learning.algorithms.classifiers.vqc

# This code is part of Qiskit.
#
# (C) Copyright IBM 2021, 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.
"""An implementation of variational quantum classifier."""

from typing import Union, Optional, Callable, cast
import numpy as np

from qiskit import QuantumCircuit
from qiskit.utils import QuantumInstance
from qiskit.circuit.library import ZZFeatureMap, RealAmplitudes
from qiskit.algorithms.optimizers import Optimizer

from ...exceptions import QiskitMachineLearningError
from ...neural_networks import CircuitQNN
from ...utils.loss_functions import Loss

from .neural_network_classifier import NeuralNetworkClassifier


[docs]class VQC(NeuralNetworkClassifier): """Variational quantum classifier. The variational quantum classifier is a variational algorithm where the measured expectation value is interpreted as the output of a classifier. Only supports one-hot encoded labels; e.g., data like ``[1, 0, 0]``, ``[0, 1, 0]``, ``[0, 0, 1]``. Multi-label classification is not supported; e.g., data like ``[1, 1, 0]``, ``[0, 1, 1]``, ``[1, 0, 1]``, ``[1, 1, 1]``. """ def __init__( self, num_qubits: int = None, feature_map: QuantumCircuit = None, ansatz: QuantumCircuit = None, loss: Union[str, Loss] = "cross_entropy", optimizer: Optional[Optimizer] = None, warm_start: bool = False, quantum_instance: QuantumInstance = None, initial_point: np.ndarray = None, callback: Optional[Callable[[np.ndarray, float], None]] = None, ) -> None: """ Args: num_qubits: The number of qubits for the underlying CircuitQNN. If None, derive from feature_map or ansatz. If neither of those is given, raise exception. feature_map: The feature map for underlying CircuitQNN. If None, use ZZFeatureMap. ansatz: The ansatz for the underlying CircuitQNN. If None, use RealAmplitudes. loss: A target loss function to be used in training. Default is cross entropy. optimizer: An instance of an optimizer to be used in training. When `None` defaults to SLSQP. warm_start: Use weights from previous fit to start next fit. quantum_instance: The quantum instance to execute circuits on. 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. Raises: QiskitMachineLearningError: Needs at least one out of num_qubits, feature_map or ansatz to be given. """ # check num_qubits, feature_map, and ansatz if num_qubits is None and feature_map is None and ansatz is None: raise QiskitMachineLearningError( "Need at least one of num_qubits, feature_map, or ansatz!" ) num_qubits_: int = None feature_map_: QuantumCircuit = None ansatz_: QuantumCircuit = None if num_qubits: num_qubits_ = num_qubits if feature_map: if feature_map.num_qubits != num_qubits: raise QiskitMachineLearningError("Incompatible num_qubits and feature_map!") feature_map_ = feature_map else: feature_map_ = ZZFeatureMap(num_qubits) if ansatz: if ansatz.num_qubits != num_qubits: raise QiskitMachineLearningError("Incompatible num_qubits and ansatz!") ansatz_ = ansatz else: ansatz_ = RealAmplitudes(num_qubits) else: if feature_map and ansatz: if feature_map.num_qubits != ansatz.num_qubits: raise QiskitMachineLearningError("Incompatible feature_map and ansatz!") feature_map_ = feature_map ansatz_ = ansatz num_qubits_ = feature_map.num_qubits elif feature_map: num_qubits_ = feature_map.num_qubits feature_map_ = feature_map ansatz_ = RealAmplitudes(num_qubits_) elif ansatz: num_qubits_ = ansatz.num_qubits ansatz_ = ansatz feature_map_ = ZZFeatureMap(num_qubits_) # 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) # construct circuit QNN neural_network = CircuitQNN( self._circuit, self.feature_map.parameters, self.ansatz.parameters, interpret=self._get_interpret(2), output_shape=2, quantum_instance=quantum_instance, 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
[docs] def fit(self, X: np.ndarray, y: np.ndarray): # pylint: disable=invalid-name """ Fit the model to data matrix X and targets y. Args: X: The input data. y: The target values. Required to be one-hot encoded. Returns: self: returns a trained classifier. """ num_classes = y.shape[-1] cast(CircuitQNN, self._neural_network).set_interpret( self._get_interpret(num_classes), num_classes ) return super().fit(X, y)
def _get_interpret(self, num_classes: int): def parity(x: int, num_classes: int = num_classes) -> int: return x % num_classes return parity

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