Atomic clocks are the most precise instruments ever built, and underlie a wide range of infrastructure including the global positioning system (GPS), used by up to 7% of the European economy. However, the reliance on GPS for distributed timekeeping is now recognised to introduce systematic vulnerability to interference, deliberate jamming, or physical attack. There is thus demand to reduce atomic clock size, weight, and power consumption so that they can be incorporated into portable equipment. This will enable jam-resistant navigation, communication, and radar, potentially in individual mobile phones or munitions. However, it has proved difficult to miniaturise the vacuum and optical elements of vapour-based clocks.

We are developing a condensed-matter approach using nature’s own atom traps – endohedral fullerene molecules (see figure). In these remarkable structures, single nitrogen atoms float in the centre of a fullerene cage. Because the cage protects the nitrogen, the spin resonances are extremely sharp. Most excitingly, we have demonstrated the existence of a clock transition, where the transition frequency is (to first order) immune to magnetic field noise.

The next step is to build a desktop prototype and study its sensitivity to different kinds of perturbation (impurities, temperature, electromagnetic noise, ageing etc.). The device will then be miniaturised to make a chip-based clock working entirely at radio frequencies and compatible with battery power. This has a potential market in every GPS handset or telecoms receiver in the world.

More information:

• Keeping Perfect Time With Caged Atoms by K. Porfyrakis and E.A. Laird, IEEE Spectrum 54 34 (2017)
• Spin resonance clock transition of the endohedral fullerene ¹⁵N@C₆₀ by Reuben Harding et al., Physical Review Letters 119 140801 (2017)

The endohedral fullerene atomic clock (a) Overview of clock operation. The clock oscillator sends radio waves towards a reference medium, whose atoms absorb energy at one precise frequency. A detector monitors the absorption, and sends a feedback signal to the oscillator to stabilize its frequency. (b) An endohedral fullerene molecule. (c) Sample of this material. (d) Absorption spectrum, showing sharp resonances.