Entangled photons from quantum dot devices: efficiency of post-selection

We theoretically investigate the production of polarization‐entangled photons through the biexciton cascade decay in a single semiconductor quantum dot. A biexciton radiatively decays through two intermediate exciton states, where polarization‐entangled photons are emitted if the two decay paths differ in polarization but are indistinguishable otherwise. This ideal performance is usually spoiled by the electron–hole exchange interaction splitting the intermediate exciton states by a small amount and consequently attaching a which‐path information to the photon frequencies. We discuss post‐selection schemes to mask this which‐path information to an outside observer. We show how spectral filtering and time shifts at a single photon level affect the photon state. Here the solid state environment plays a crucial role in the effective measurement of intermediate exciton states. Evaluating our analytical results with realistic quantum dot parameters we quantify the applicability of suggested protocols for solid‐state based quantum cryptography. Our results indicate, that a high degree of entanglement is only reached by spectral alignment of the exciton states. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)