Source code for jet.state

"""Module containing the ``State`` class in addition to all ``State`` subclasses."""
from abc import ABC, abstractmethod
from typing import List, Optional, Sequence

import numpy as np

from .factory import Tensor, TensorType

__all__ = [
    "State",
    "Qudit",
    "QuditRegister",
    "Qubit",
    "QubitRegister",
]


class State(ABC):
    """State represents a quantum state.

    Args:
        name (str): Name of the state.
        num_wires (str): Number of wires the state is connected to.
    """

    def __init__(self, name: str, num_wires: int):
        self.name = name

        self._indices = None
        self._num_wires = num_wires

    @property
    def indices(self) -> Optional[List[str]]:
        """Returns the indices of this state. An index is a label associated with
        an axis of the tensor representation of a state; the indices of a tensor
        determine its connectivity in the context of a tensor network.
        """
        return self._indices

    @indices.setter
    def indices(self, indices: Optional[Sequence[str]]) -> None:
        """Sets the indices of this state. The ``indices`` property of a state
        is used to construct its tensor representation (unless ``indices`` is
        None). See @indices.getter for more information about tensor indices.

        Raises:
            ValueError: If the given indices are not a sequence of unique
                strings or the number of provided indices is invalid.

        Args:
            indices (Sequence[str] or None): New indices of the state.
        """
        # Skip the sequence property checks if `indices` is None.
        if indices is None:
            pass

        # Check that `indices` is a sequence of unique strings.
        elif (
            not isinstance(indices, Sequence)
            or not all(isinstance(idx, str) for idx in indices)
            or len(set(indices)) != len(indices)
        ):
            raise ValueError("Indices must be a sequence of unique strings.")

        # Check that `indices` has the correct length (or is None).
        elif len(indices) != self.num_wires:
            raise ValueError(
                f"States must have one index per wire. "
                f"Received {len(indices)} indices for {self.num_wires} wires."
            )

        self._indices = indices

    @property
    def num_wires(self) -> int:
        """Returns the number of wires connected to this state."""
        return self._num_wires

    def __eq__(self, other) -> bool:
        """Reports whether this state is equivalent to the given state."""
        return np.all(self._data() == other._data())

    def __ne__(self, other) -> bool:
        """Reports whether this state is not equivalent to the given state."""
        return not self == other

    @abstractmethod
    def _data(self) -> np.ndarray:
        """Returns the vector representation of this state."""

    def tensor(self, dtype: np.dtype = np.complex128) -> TensorType:
        """Returns the tensor representation of this state.

        Args:
            dtype (np.dtype): Data type of the tensor.
        """
        data = self._data()

        indices = self.indices
        if indices is None:
            indices = list(map(str, range(self.num_wires)))

        dimension = int(round(len(data) ** (1 / len(indices))))
        shape = [dimension] * len(indices)

        return Tensor(indices=indices, shape=shape, data=data, dtype=dtype)


class Qudit(State):
    """Qudit represents a qudit state.

    Args:
        dim (int): Dimension of the qudit.
        data (np.ndarray or None): Optional state vector.
    """

    def __init__(self, dim: int, data: Optional[np.ndarray] = None):
        name = "Qubit" if dim == 2 else f"Qudit(d={dim})"
        super().__init__(name=name, num_wires=1)

        if data is None:
            self._state_vector = (np.arange(dim) == 0).astype(np.complex128)
        else:
            self._state_vector = data.flatten()

    def _data(self) -> np.ndarray:
        return self._state_vector


class QuditRegister(State):
    """QuditRegister represents a qudit register state.

    Args:
        dim (int): Dimension of the qudits.
        size (int): Number of qudits.
        data (np.ndarray or None): Optional state vector.
    """

    def __init__(self, dim: int, size: int, data: Optional[np.ndarray] = None):
        name = f"Qubit[{size}]" if dim == 2 else f"Qudit(d={dim})[{size}]"
        super().__init__(name=name, num_wires=size)

        if data is None:
            self._state_vector = (np.arange(dim**size) == 0).astype(np.complex128)
        else:
            self._state_vector = data.flatten()

    def _data(self) -> np.ndarray:
        return self._state_vector


def Qubit(data: Optional[np.ndarray] = None) -> Qudit:
    """Constructs a qubit state using an optional state vector.

    Args:
        data (np.ndarray or None): Optional state vector.

    Returns:
        Qudit: Qudit with the specified state vector.
    """
    return Qudit(dim=2, data=data)


def QubitRegister(size: int, data: Optional[np.ndarray] = None) -> QuditRegister:
    """Constructs a qubit register state with the given size and optional state vector.

    Args:
        size (int): Number of qubits.
        data (np.ndarray or None): Optional state vector.

    Returns:
        QuditRegister: QuditRegister with the specified state vector.
    """
    return QuditRegister(dim=2, size=size, data=data)

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