A refreshing look on ruby. Ruby is an archetypal material for the spectroscopy of transition metal ions. The so-called R-lines of chromium, because of their unique electronic properties, are extremely narrow and reveal a weak coupling to the environment. The authors achieve optical spin polarization of very lightly doped ruby. This persistent hyperpolarization, surviving out of thermal equilibrium for a fraction of a second, has two beneficial effects: absorption enhancement and line narrowing. Both properties magnify the abrupt variation of the refractive index and are further exploited to demonstrate a group velocity reduction by three orders of magnitude.

The article is also a remarkable review of magneto-optical (high resolution spectroscopy) and coherence (dephasing and relaxation mechanisms) properties of a material that has been studied for decades.

There cannot be a better way to give tribute to the work of Alex Szabo than showing that ruby is still shining in the context of slow-light.

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