Phys. Rev. Lett. 113, 053602 (2014)
The idea of gating the flow of single photons is not new. However, a single-photon transistor that is able to not only gate flow, but also exhibit gain would be even more useful as it would allow the construction of a network of cascaded devices. A number of experiments have shown gain, but for gating with just a single-photon unity gain remained elusive. Now, Daniel Tiarks and a team from Germany have demonstrated an all-optical transistor controlled by a single photon (on average) with a gain of 20. The team optically trapped 150,000 87Rb atoms at a temperature of 0.33 μK and used a signal laser beam with a wavelength of 795 nm and two control beams at 474 nm. According to the authors, the improved response compared with previous work is largely due to targeting relatively small principal quantum numbers (n = 69). This led to the ability to increase the length of the target pulse by about two orders of magnitude, enabling the higher gain. And, the authors note that the energy mismatch between atom-pair states near n = 70 can be very close to zero, enabling the scheme to benefit from a Förster resonance.
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