  The control of one light field by another, ultimately at the single photon
level, is a challenging task which has numerous interesting applications within
nonlinear optics and quantum information science. Due to the extremely weak
direct interactions between optical photons in vacuum, this type of control can
in practice only be achieved through highly nonlinear interactions within a
medium. Electromagnetic induced transparency (EIT) constitutes one such means
to obtain the extremely strong nonlinear coupling needed to facilitate
interactions between two faint light fields. Here, we demonstrate for the first
time EIT as well as all-optical EIT-based light switching using ion Coulomb
crystals situated in an optical cavity. Unprecedented narrow cavity EIT feature
widths down to a few kHz and a change from essentially full transmission to
full absorption of the probe field within a window of only ~100 kHz are
achieved. By applying a weak switching field, we furthermore demonstrate nearly
perfect switching of the transmission of the probe field. These results
represent important milestones for future realizations of quantum information
processing devices, such as high-efficiency quantum memories, single-photon
transistors and single-photon gates.
