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.

Direct measurement of tiny masses is never as simple as placing them on a scale. Progress has been made by using the vibration of molecules, such as carbon nanotubes. However, as with atomic clocks, these systems are all based on collections of particles and their interactions. That places inherent limitations on precision. While those limits are pretty small (to put it mildly), we could always do better in order to get truly precise definitions of a second of time or a kilogram of mass.

Single particles also have fundamental frequencies of vibration, based on their wave-like character. Every particle type has a unique frequency proportional to its mass, known as the Compton frequency.

Compton frequencies are huge: for an electron, the frequency is 1.23×1020 Hz, or 123 billion GHz, far larger than typical laboratory experiments can track. Heavier particles like protons have even higher Compton frequencies: it's a linear relationship, so if a particle has double the mass, its Compton frequency also doubles. But these frequencies have a big advantage: they are also as basic as can be. Compton frequencies are independent of any interactions of the particle, and can be defined for any particle, atom, molecule, or (if one wants to be ridiculous) a macroscopic object.

The researchers accessed the Compton frequency of a cesium atom by trapping it in a Ramsey-Bordé interferometer. This device sent two laser pulses into the atom, which absorbed the photons from one pulse stream and reemitted them into the second. The interferometer controlled the atom's response by varying the pulse duration and number of photons it contained. Tuning the difference between the timing of the two laser paths led to a new frequency, just as adding water waves together produces a new wave with its own frequency.

In the case of the Ramsey-Bordé interferometer, this new frequency was a precise fraction of the Compton frequency—a small enough fraction that it was within a range accessible to measurement. The researchers used this as the basis for an atomic clock, involving a single atom. While their accuracy was much less than modern atomic clocks built on other principles, it marked an important proof of concept. With refinement, this type of experiment could be used to define the second of time in a more precise way than is possible using other methods, many of which rely on collections of atoms.

Additionally, the experiment could be turned around conceptually: measuring the mass of a particle or atom using the interferometer. This could make it possible to define the kilogram in a replicable way. With this in hand, the researchers suggested new experiments to measure some of the physical constants of nature (such as Planck's constant, important for all of quantum physics). However, they could also test some of the fundamental principles—the equivalence between inertial mass (resistance to motion) and gravitational mass, for example—in a sensitive way.

Science, 2013. DOI: 10.1126/science.1230767 (About DOIs).