"""Is channel positive."""
from __future__ import annotations
import numpy as np
from toqito.channel_ops import kraus_to_choi
from toqito.matrix_props import is_positive_semidefinite
[docs]
def is_positive(
phi: np.ndarray | list[list[np.ndarray]],
rtol: float = 1e-05,
atol: float = 1e-08,
) -> bool:
r"""
Determine whether the given channel is positive [WatPM18]_.
A map :math:`\Phi \in \text{T} \left(\mathcal{X}, \mathcal{Y} \right)` is *positive* if it
holds that
.. math::
\Phi(P) \in \text{Pos}(\mathcal{Y})
for every positive semidefinite operator :math:`P \in \text{Pos}(\mathcal{X})`.
Alternatively, a channel is positive if the corresponding Choi matrix of the channel is both
Hermitian-preserving and positive semidefinite.
Examples
==========
We can specify the input as a list of Kraus operators. Consider the map :math:`\Phi` defined as
.. math::
\Phi(X) = X - U X U^*
where
.. math::
U = \frac{1}{\sqrt{2}}
\begin{pmatrix}
1 & 1 \\
-1 & -1
\end{pmatrix}.
This map is not completely positive, as we can verify as follows.
>>> from toqito.channel_props import is_positive
>>> import numpy as np
>>> unitary_mat = np.array([[1, 1], [-1, -1]]) / np.sqrt(2)
>>> kraus_ops = [[np.identity(2), np.identity(2)], [unitary_mat, -unitary_mat]]
>>> is_positive(kraus_ops)
False
We can also specify the input as a Choi matrix. For instance, consider the Choi matrix
corresponding to the :math:`4`-dimensional completely depolarizing channel and may verify
that this channel is positive.
>>> from toqito.channels import depolarizing
>>> from toqito.channel_props import is_positive
>>> is_positive(depolarizing(4))
True
References
==========
.. [WatPM18] Watrous, John.
"The theory of quantum information."
Section: "Linear maps of square operators".
Cambridge University Press, 2018.
:param phi: The channel provided as either a Choi matrix or a list of Kraus operators.
:param rtol: The relative tolerance parameter (default 1e-05).
:param atol: The absolute tolerance parameter (default 1e-08).
:return: True if the channel is positive, and False otherwise.
"""
# If the variable `phi` is provided as a list, we assume this is a list
# of Kraus operators.
if isinstance(phi, list):
phi = kraus_to_choi(phi)
return is_positive_semidefinite(phi, rtol, atol)