Source code for psipose.feature_maps.angle

"""Angle encoding: each feature becomes a rotation angle."""

import pennylane as qml

from .base import FeatureMap




class AngleFeatureMap(FeatureMap):
    """Angle encoding of classical data into quantum rotations.

    Each feature :math:`x_i` is encoded as a rotation angle on a separate
    qubit via :math:`R_{\\text{axis}}(x_i)`. By default, uses Y rotations.
    Supports X, Y, or Z rotations.

    This follows the angle encoding approach from:

    - Farhi & Neven (2018), "Classification with Quantum Neural Networks
      on Near Term Processors" (arXiv:1802.06002). Uses parametrized
      rotation gates to encode classical data into quantum states.
    - Schuld, Bocharov, Svore & Wiebe (2018), "Circuit-centric
      quantum classifiers" (arXiv:1804.00633). Encodes features as
      rotation angles in variational quantum circuits.

    Mathematically, for a feature vector :math:`\\mathbf{x} = (x_1, \\dots, x_n)`,
    the encoded state is:

    .. math::
        |\\phi(\\mathbf{x})\\rangle = \\bigotimes_{i=1}^{n}
        R_{\\text{axis}}(x_i) |0\\rangle

    where :math:`R_{\\text{axis}}
    \\in \\{R_X, R_Y, R_Z\\}`.

    Parameters
    ----------
    rotation : {"X", "Y", "Z"}, default="Y"
        Which rotation gate to use for encoding.
    n_qubits : int, optional
        Number of qubits to use. If None, uses one qubit per feature
        (or the minimum of n_features and 10).

    Examples
    --------
    >>> feature_map = AngleFeatureMap(rotation="Y")
    >>> feature_map.fit(X_train)  # determines n_qubits_
    >>> circuit = feature_map.encode(x_sample, wires=range(feature_map.n_qubits_))
    """

    def __init__(self, rotation="Y", n_qubits=None):
        super().__init__(n_qubits)
        if rotation not in {"X", "Y", "Z"}:
            raise ValueError(f'rotation must be one of "X", "Y", "Z", got {rotation}')
        self.rotation = rotation



    def encode(self, x, wires):
        """Apply angle encoding to the given wires.

        Only encodes as many features as there are wires. If x has more
        features than wires, the excess features are truncated.
        """
        for i, wire in enumerate(wires):
            if i >= len(x):
                break
            feature = x[i]
            if self.rotation == "X":
                qml.RX(feature, wires=wire)
            elif self.rotation == "Y":
                qml.RY(feature, wires=wire)
            else:  # Z
                qml.RZ(feature, wires=wire)