Atom Trap Trace Analysis Reaches a Part-per-quadrillion Sensitivity*

A quadrillion is 10$^{15}$. This is how many argon atoms one has to sift through in order to find just one atom of the radioactive isotope $^{39}$Ar. Atom Trap Trace Analysis (ATTA), a MOT-based atom counting method, is now able to unambiguously pick $^{39}$Ar out of a regular argon gas sample. The exceedingly rare $^{39}$Ar forms naturally in the environment by cosmic rays, decays with a half-life of 270 years, and is an ideal tracer to study ocean circulation or groundwater flow over the past few hundred years. In an ATTA apparatus, only $^{39}$Ar atoms are selectively captured by the MOT, appear as a bright dot, and can be counted one atom at a time using a sensitive camera. This work constitutes a major breakthrough in analytical capability, and promises to enable a wide range of applications in physics as well as earth sciences.