Source code for toqito.state_props.is_mixed

"""Checks if a quantum state is mixed."""

import numpy as np

from toqito.state_props import is_pure




def is_mixed(state: np.ndarray) -> bool:
    r"""Determine if a given quantum state is mixed [@WikiMixedSt].

    A mixed state by definition is a state that is not pure.

    Examples:
        Consider the following density matrix:

        \[
            \rho =  \begin{pmatrix}
                        \frac{3}{4} & 0 \\
                        0 & \frac{1}{4}
                    \end{pmatrix} \in \text{D}(\mathcal{X}).
        \]

        Calculating the rank of \(\rho\) yields that the \(\rho\) is a mixed state. This can be
        confirmed in `|toqito⟩` as follows:

        ```python exec="1" source="above"
        from toqito.states import basis
        from toqito.state_props import is_mixed
        e_0, e_1 = basis(2, 0), basis(2, 1)
        rho = 3 / 4 * e_0 @ e_0.conj().T + 1 / 4 * e_1 @ e_1.conj().T
        print(is_mixed(rho))
        ```

    Args:
        state: The density matrix representing the quantum state.

    Returns:
        `True` if state is mixed and `False` otherwise.

    """
    return not is_pure(state)