TL;DW
Cesium atomic clocks are good, and work by probing the behaviour of the unpaired electron. Unfortunately to do that you need isolated cesium atoms and that means a very complicated vacuum chamber, cesium atom source, magnets etc.
By jamming a lonely nitrogen atom into the middle of a buckyball (carbon 60 molecule), you can achieve a similar effect but hopefully without vacuum chambers etc. This might open a door to a very small low power implementation. Edit: some modest magnets likely still required.