Clock made from a single atom could lead to a precise mass measurement

A special technique allows researchers to use a particle's mass as the driver for an atomic clock. Turning the process around may lead to precise measurments of microscopic mass.

Pei-Chen Kuan

Most of the units we rely on are based on precision microscopic measurements—regular fluctuations in certain atoms define one second of time, for example. A major exception to this is the basic unit of mass, the kilogram, which is defined by a platinum-iridium cylinder in a vault in Paris, along with a number of supposedly identical replicas distributed around the world. The problem: these chunks of metal no longer are precisely the same mass, thanks to accumulation of surface gunk and tiny variations on the atomic scale.

A concept from fundamental physics may come to the kilogram's rescue. According to quantum physics, all matter behaves as a wave, vibrating at a set frequency proportional to its mass—if we measure the vibrations, we get the mass. Reliably measuring this frequency is a major challenge, however, since it is huge even for low-mass particles like electrons.

Shau-Yu Lan and colleagues exploited advanced techniques to construct an atomic clock based on a single cesium atom, a device capable of dividing the huge natural frequencies of the atom into more manageable quantities. This provided a strong demonstration of the ability to construct clocks based on a single microscopic mass. And, because we already have excellent clocks to compare them with, this can potentially work in the opposite direction, leading to accurate mass measurements in the future.