Endohedral fullerenes as frequency standards in portable atomic clocks*

Stable frequency references underpin a range of navigation, communication, and sensing infrastructure. Over the past decade, advances in miniaturization have enabled the use of highly-stable atomic clocks in a range of portable applications. However, further miniaturization and improved stability are required to enable broader adoption of atomic frequency standards. The endohedral fullerene ¹⁵N@C₆₀ has been proposed as the basis for an alternative condensed-matter portable atomic clock that could address these challenges. Here, we measure the low-frequency electron paramagnetic resonance spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. From this experimental data, we infer a linewidth at the clock field of 100 kHz and estimate a bound on the clock’s projected frequency stability. We then discuss ways to improve the frequency stability of the proposed fullerene clock such that it is competitive with existing miniaturized portable atomic clocks.