Göteborg, Sweden and Madrid, Spain--Physicists and coauthors led by Per Delsing and Chris Wilson from the Chalmers University of Technology and the Spanish National Research Council have developed the first single-photon router, which could one day serve as a quantum node in a quantum information network, providing basic processing and routing of communications data.
Their research, described in a recent issue of Physical Review Letters (see http://prl.aps.org/abstract/PRL/v107/i7/e073601), demonstrates that an artificial atom embedded in a transmission line can route a single photon from an input port to one of two output ports. As the scientists explain, controlling and directing photons is more difficult than controlling and directing electrons, which are used in most of today’s routers. The difficulty stems from the fact that, unlike electrons, photons do not strongly interact with each other. However, one of the important requirements of a quantum channel is the ability to distribute information over large distances. Since photons can coherently travel much further distances than other quantum systems such as atoms, it makes sense to use photons as the information carriers in quantum information networks.
To build their single-photon router, the scientists used a superconducting qubit as the “artificial atom” (although the qubit is actually made of several atoms, it has discrete energy states like a real atom). Then the scientists coupled the qubit to a one-dimensional transmission line through which microwave photons travel. Next they applied a weak, continuous photon probe and sometimes added a stronger control pulse. Without the strong control pulse, the artificial atom reflects the incoming photons, which travel to output 1. When the strong control pulse is on, it leads to the phenomenon of electromagnetically induced transparency (EIT). EIT causes the atom to become transparent to the weak probe beam, resulting in the photons traveling past the atom to output 2. In this way, the scientists could direct incoming photons to one of two output ports.
The researchers achieved an order of magnitude increase in the extinction efficiency (reaching 99.6%), which shows that the photons are efficiently coupled to the artificial atom, enabling the photons to be better controlled. The device’s switching time (the time it takes to switch incoming photons from one output port to the other) is just a few nanoseconds. The scientists also noted that the router can easily be extended to have multiple output ports, which will be necessary for use as a quantum node.
For applications in quantum channels, the scientists explain that it will be better to use optical photons instead of microwave photons, which were used here. The device could also have important applications in research.
SOURCE: PhysOrg.com; www.physorg.com/news/2011-08-physicists-single-photon-router.html