The control of one light field by another, ultimately at the single photonlevel, is a challenging task which has numerous interesting applications withinnonlinear optics and quantum information science. Due to the extremely weakdirect interactions between optical photons in vacuum, this type of control canin practice only be achieved through highly nonlinear interactions within amedium. Electromagnetic induced transparency (EIT) constitutes one such meansto obtain the extremely strong nonlinear coupling needed to facilitateinteractions between two faint light fields. Here, we demonstrate for the firsttime EIT as well as all-optical EIT-based light switching using ion Coulombcrystals situated in an optical cavity. Unprecedented narrow cavity EIT featurewidths down to a few kHz and a change from essentially full transmission tofull absorption of the probe field within a window of only ~100 kHz areachieved. By applying a weak switching field, we furthermore demonstrate nearlyperfect switching of the transmission of the probe field. These resultsrepresent important milestones for future realizations of quantum informationprocessing devices, such as high-efficiency quantum memories, single-photontransistors and single-photon gates.
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