  A three-level atomic medium can be made transparent to a resonant probe field
in the presence of a strong control field acting on an adjacent atomic
transition to a long-lived state, which can be represented by a highly excited
Rydberg state. The long-range interactions between the Rydberg state atoms then
translate into strong, non-local, dispersive or absorptive interactions between
the probe photons, which can be used to achieve deterministic quantum logic
gates and single photon sources. Here we show that long-range dipole-dipole
exchange interaction with one or more spins -- two-level systems represented by
atoms in suitable Rydberg states -- can play the role of control field for the
optically-dense medium of atoms. This induces transparency of the medium for a
number of probe photons $n_p$ not exceeding the number of spins $n_s$, while
all the excess photons are resonantly absorbed upon propagation. In the most
practical case of a single spin atom prepared in the Rydberg state, the medium
is thus transparent only to a single input probe photon. For larger number of
spins $n_s$, all $n_p \leq n_s$ photon components of the probe field would
experience transparency but with an $n_p$-dependent group velocity.
