In a conventional semiconductor single photon source, a pump laser excites the material. The generated charges, electrons and holes, are randomly captured by a quantum dot, where they recombine and emit single photons. In recent experiments, a SAW is used to break up electrons and holes along its propagation direction. The carriers “surf” this sound wave in swiftly moving potential pockets away from the pump laser position towards a remotely located quantum post. Since electrons and holes are spatially separated by half the acoustic wavelength, these two species are injected one after the other with a well defined delay of exactly half the acoustic period. Therefore, the time of single photon emission becomes defined with high precision.

High-mobility quantum well

In the study, the Augsburg team shows that self-assembled quantum posts, which were fabricated in Santa Barbara, are well suited to the project because these nanostructures are embedded in a high-mobility quantum well. This unique property yields highly efficient acoustic carrier transport over long distances and an increase in emission intensity compared with the quantum post being directly excited by the pump laser.

Without the SAW, the number of electrons and holes being captured by the emitter is not well defined. However, when the SAW injects the carrier species sequentially, the quantum posts predominantly contain an equal number of electrons and holes. In particular the so-called biexciton state (two electrons and two holes) can be prepared with unprecedented fidelity. This state can emit a pair of quantum mechanically entangled photons for advanced quantum communication protocols.

Full details can be found in the journal Nanotechnology.