Jul 10, 2012
Nanoquakes pump quantum posts
Low-jitter quantum light sources are key devices for future quantum communication networks. However, spontaneous emission of a single photon is a statistical process and, therefore, the actual emission time is not well defined. A radio frequency surface acoustic wave (SAW) provides a precise clock to trigger this emission beyond existing technologies. An international team of researchers at the University of Augsburg in Germany and UC Santa Barbara in the US has made substantial progress towards such an acoustically pumped single photon source by combining the nanoquake-like process with a novel type of nanostructure, a so-called quantum post.
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.
About the author
This study was conducted in the Junior Research Group led by Hubert Krenner at Augsburg University, Germany, which is supported by the Emmy Noether Program of the Deutsche Forschungsgemeinschaft. His group is associated with the chair of Achim Wixforth, one of the pioneers of the application of surface acoustic waves in nanotechnology. The samples for the experiments were fabricated in the group of Pierre Petroff, at the University of California in Santa Barbara, US. This work was supported by the Emmy Noether Program of the Deutsche Forschungsgemeinschaft (DFG), the Cluster of Excellence “Nanosystems Initiative Munich” (NIM) and the Alexander von Humboldt Foundation.