The ideal source will have the following four characteristics. (i) Deterministic generation — on a pulsed excitation, the source should emit one, and only one, pair of entangled photons with a vanishingly small chance of multi-pair emission. (ii) High fidelity — the created two-photon state should closely resemble the ideal desired entangled state. (iii) Indistinguishability — individual photons emitted in different trials should be quantum mechanically identical to each other. (iv) High collection efficiency — radiated photons should be extracted with a high efficiency so that they are not lost.
It is anticipated that s-shell resonant excitation, a more controlled method widely used in standard atomic physics experiments, would overcome these shortcomings, uncontrolled carrier recombination, emission time jitter and reduced photon indistinguishability, and yield single photons of better quality.
Going beyond single-photon generation, Müller et al. have now taken another step forward and produced entangled photons using coherent excitation.