Comment on "Entanglement on Demand through Time Reordering"

TL;DR

This paper critically examines a proposed scheme for generating entangled photon pairs via time reordering in quantum dots, deriving an exact concurrence expression and revealing limitations in previous theoretical predictions.

Contribution

It provides an exact formula for photon entanglement concurrence and clarifies the inaccuracies in prior approximate analyses.

Findings

Exact concurrence expression derived

Previous predictions found to be unachievable

Highlights limitations of earlier approximations

Abstract

In a recent Letter [Phys. Rev. Lett. {\bf 100}, 120501 (2008)], Avron {\em et al.} discuss a time reordering scheme to achieve efficient "across-generation" of entangled photon pairs in a semiconductor quantum dot with a suppressed biexciton binding energy. They demonstrate that the scheme can be implemented using time delay between the generated photons. In this Comment, we derive an exact expression for the concurrence of the time delayed photons, and show that the predicted values by Avron {\em et al.} are not achievable, which stems from an approximation used in their approximate theoretical analysis.