Ultra-slow light propagation by self-induced transparency in ruby in the superhyperfine limit.

Self-induced transparency is reported for circularly polarized light in the R₁(-3/2) line of a 30 ppm ruby (α-Al₂O₃:Cr³⁺) at 1.7 K in a magnetic field of B‖c=4.5  T. In such a field and temperature, a 30 ppm ruby is in the so-called superhyperfine limit resulting in a long phase memory time, T_M=50  μs, and a thousand-fold slower pulse propagation velocity of ∼300  m/s was observed, compared to the ∼300  km/s measured in the first observation of self-induced transparency (SIT) ∼50 years ago, that employed a ruby with a 500 ppm chromium concentration in zero field and at 4.2 K. To date, this is the slowest pulse propagation ever observed in a SIT experiment.