Gated photon transistor

Science http://dx.doi.org/10.1126/science.1238169 (2013)

Photons are excellent carriers of quantum information, but it is difficult to induce the strong interactions between individual photons required for all-optical quantum information processing. Therefore, the realization of an optical transistor exhibiting gain with gate signals at the few-photon level remains a challenge. Now, Wenlan Chen and co-workers from USA, Austria and Japan have realized an optical transistor that is gated by a single stored photon. The system consists of an ensemble of laser-cooled ¹³³Cs atoms optically trapped in a high-finesse optical cavity. Each atom has a four-state N-type level structure with two stable ground states (|g〉 and |s〉) and two electronic excited states (|d〉 and |e〉). The group found that the control laser addressing the |d〉 → |s〉 transition produced electromagnetically induced transparency for gate photons addressing the |g〉 → |d〉 transition. By reducing the control laser power to zero, a weak gate photon pulse could be stored in the atomic ensemble. This pulse could later be retrieved by adiabatically reapplying the control beam. When the average number of stored gate photons reached 2.9, the transmission of the source light addressing the |s〉 → |e〉 transition was reduced by a factor of five. The available gain outside the cavity was 1.4, demonstrating a gain exceeding unity for transistor operation.