channel_props.is_positive

Determines if a channel is positive.

Functions

is_positive(phi[, rtol, atol])

Determine whether the given channel is positive.

Module Contents

channel_props.is_positive.is_positive(phi, rtol=1e-05, atol=1e-08)

Determine whether the given channel is positive.

(Section: Linear Maps Of Square Operators from [1]).

A map \(\Phi \in \text{T} \left(\mathcal{X}, \mathcal{Y} \right)\) is positive if it holds that

\[\Phi(P) \in \text{Pos}(\mathcal{Y})\]

for every positive semidefinite operator \(P \in \text{Pos}(\mathcal{X})\).

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 \(\Phi\) defined as

\[\Phi(X) = X - U X U^*\]

where

\[\begin{split}U = \frac{1}{\sqrt{2}} \begin{pmatrix} 1 & 1 \\ -1 & -1 \end{pmatrix}.\end{split}\]

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 \(4\)-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

[1]

John Watrous. The Theory of Quantum Information. Cambridge University Press, 2018. URL: https://johnwatrous.com/wp-content/uploads/TQI.pdf, doi:10.1017/9781316848142.

Parameters:

  • phi (numpy.ndarray | list[list[numpy.ndarray]]) – The channel provided as either a Choi matrix or a list of Kraus operators.

  • rtol (float) – The relative tolerance parameter (default 1e-05).

  • atol (float) – The absolute tolerance parameter (default 1e-08).

Returns:

True if the channel is positive, and False otherwise.

Return type:

bool