Computable lower bounds on the entanglement cost of quantum channels

TL;DR

This paper introduces computable lower bounds for the entanglement cost of quantum channels, extending previous state-based bounds to channels and providing a semidefinite program for practical computation.

Contribution

It generalizes entanglement monotones to quantum channels, establishing a lower bound on their entanglement cost that can outperform existing bounds and is computationally feasible.

Findings

Derived a semidefinite programming bound for channel entanglement cost

Established a link between state and channel entanglement robustness

Proved lower semicontinuity of channel entanglement robustness

Abstract

A class of lower bounds for the entanglement cost of any quantum state was recently introduced in [arXiv:2111.02438] in the form of entanglement monotones known as the tempered robustness and tempered negativity. Here we extend their definitions to point-to-point quantum channels, establishing a lower bound for the asymptotic entanglement cost of any channel, whether finite or infinite dimensional. This leads, in particular, to a bound that is computable as a semidefinite program and that can outperform previously known lower bounds, including ones based on quantum relative entropy. In the course of our proof we establish a useful link between the robustness of entanglement of quantum states and quantum channels, which requires several technical developments such as showing the lower semicontinuity of the robustness of entanglement of a channel in the weak*-operator topology on bounded linear maps between spaces of trace class operators.