The spin resonance clock transition of the endohedral fullerene ${}^{15}{\mathrm{N}\mathrm{@}\mathrm{C}}_{60}$

The endohedral fullerene ${}^{15}{\mathrm{N}\mathrm{@}\mathrm{C}}_{60}$ has narrow electron paramagnetic resonance lines which have been proposed as the basis for a condensed-matter portable atomic clock. We measure the low-frequency spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. We infer a linewidth at the clock field of 100 kHz. Using experimental data, we are able to place a bound on the clock’s projected frequency stability. We discuss ways to improve the frequency stability to be competitive with existing miniature clocks.