1.3.3. Push-button photon entanglement[7]

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.

The scheme is based on two-photon emission from biexciton–exciton cascade radiative decay in a single quantum dot. The key to the experiment is to pump the single quantum dot to the biexciton states. Müller and colleagues exploit a more elegant excitation method — coherent two-photon excitation . The photon energy of a pulsed laser (yellow arrows in Fig. 1) is set at the average energy of the biexciton and exciton photons, in resonance with the virtual biexciton two-photon excitation state. An extra bonus of the two-photon excitation scheme is that, because the laser energy differs from both the biexciton and exciton photons, the laser background can be spectrally filtered out, without resorting to a polarization suppression technique.

The approach of Müller et al. fulfils a number of criteria in the wish list. The results are clear evidence for the realization of a high-quality source of entangled photons.