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

## TL;DR

This paper investigates the spin resonance properties of $^{15} ext{N@C}_{60}$ endohedral fullerenes, identifying a magnetic field-insensitive transition suitable for portable atomic clocks and analyzing its potential stability.

## Contribution

It reports the measurement and characterization of a low-frequency clock transition in $^{15} ext{N@C}_{60}$, providing insights into its potential for miniaturized atomic clock applications.

## Key findings

- Identified a magnetic field-insensitive clock transition at low frequency.
- Measured a linewidth of approximately 100 kHz at the clock field.
- Estimated bounds on the projected frequency stability of the system.

## Abstract

The endohedral fullerene $^{15}\mathrm{N@C}_{60}$ 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.