Practical photon source for quantum communication
The use of individual light quanta as information carriers in quantum communication and quantum computer technology is being researched worldwide. A single light quantum (photon) is to be used as an information carrier. One of the most important challenges: The technology is rather fragile, as it usually works almost exclusively in vacuum and at temperatures close to absolute zero. This requires complex laboratory equipment and thus severely restricts the use of this technology. Professor Dr. Stephan Reitzenstein’s research group at the Institute for Solid State Physics at the Technical University of Berlin has now succeeded in building a robust, “plug&play-ready” source of light quanta.
The system is based on an artificial atom – a quantum dot on a semiconductor chip. The scientists applied a microlens to this quantum dot using a specially developed, unique technique that enables extreme accuracy. This lens collects the photons emitted by the quantum dot efficiently, so that a high data transfer rate in quantum communication can be realized, “explains Reitzenstein fellow scientist Dr. Tobias Heindel.
For the nanostructuring of the semiconductor chip, a proprietary technique was used, with which a microlens is placed exactly above a selected quantum point. However, in order to be able to use a quantum or photon source in practice in quantum communication outside the laboratories, the photons must also be efficiently coupled into an optical fiber optic cable. Precisely such fibers form the basis for worldwide data transmission on the Internet today and should also enable quantum-based Internet in the future.
The highlight of the Berlin team’s effort: The scientists succeeded positioning an optical fiber exactly above the quantum dot, which allows the radiated photons to be captured directly and transmitted over long distances. “The decisive factor here is that we optically ‘scan’ the surface of the semiconductor on which our quantum dot sits, and then mount the fiber optic cable in a relatively robust process at room temperature exactly above the microlens with epoxy resin,” Heindel said.
The system of semiconductor chip with quantum dot, microlens and precisely aligned and fixed glass fiber is built into a Stirling cooler. This is a commercially available device that serves to cool the semiconductor chip down to the required low temperatures, only a few ten Kelvin above absolute temperature zero.
The advantage of this arrangement is that the entire so-called Q-source (quantum source) including Stirling cooler can be stored in an average desk drawer and only requires a mains connection. Typical quantum light sources for research purposes, in contrast, usually require an entire laboratory with complex and expensive helium cooling technology.
The fiber optic cable enables the transport of the quantum even over long distances. “This opens up completely new possibilities for use in quantum communication,” believes Heindel. Thus, such a Q-Source could in future become an integral part of tap-proof communication channels in the quantum internet.
The work on this compact quantum light source was carried out as part of the BMBF VIP project “QSOURCE”. They were published in the current issue of the scientific journal Scientific Reports (https://www.nature.com/articles/s41598-017-19049-4 ).
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