Infinitesimal Change in Mass of Individual Atoms Observed for the First Time

Superconducting 7 T magnet situated in the temperature controlled PENTATRAP lab.

A team at Max Planck Institute for Nuclear Physics in Heidelberg has successfully measured an infinitesimal change in the mass of an individual atom. PENTATRAP – is a novel cryogenic multi-Penning trap experiment for high-precision mass measurements on highly charged ions.

PENTATRAP aims for a relative precision of the mass measurement of δm/m ≤10-11. It can measure the minuscule change in the mass of a single atom when an electron absorbs or releases energy.

When an atom absorbs or releases energy via the quantum leap of an electron, it becomes heavier or lighter. This can be explained by Einstein’s theory of relativity (E = mc²). But, the effect is too small for a single atom and has never been observed before.

The team of Klaus Blaum and Sergey Eliseev were able to discover a previously unobserved quantum state in rhenium. This state appears to have special properties. Pentatrap can detect the jump of an electron into this quantum state via the mass change of a rhenium atom.

Dr. Rima Schüssler, a postdoctoral fellow at MPI Nuclear Physics who helped build Pentatrap since 2014, puts this discovery in perspective. She says “By weighing a six-tonne elephant, we were able to determine whether a ten-milligram ant was crawling on it.

The team discovered an extremely long-lived metastable quantum state in rhenium.

The Experimental Setup

Pentatrap consists of five Penning traps. In order for such a trap to be able to weigh an atom, it must be electrically charged. Because rhenium was stripped of 29 of its 75 electrons, it is highly charged. This dramatically increases the accuracy of the measurement. The trap captures this highly charged rhenium ion in a combination of a magnetic field and a specially shaped electric field. Inside, it travels in a circular path, which is intricately twisted into itself. In principle, it can be thought of as a ball on a rope, which is allowed to rotate in the air. If this is done with constant force, a heavier ball rotates slower than a lighter one.

You can read more about the experiment setup here.

Application in future atomic clocks

Such excited electronic states in highly charged ions are interesting for basic research. This even gives rise to a possible application in future atomic clocks.

The metastable state in rhenium is attractive for several reasons.

  • First, because of its longevity, it corresponds to a sharp orbital frequency of the electron around the atomic nucleus.
  • Second, the electron can be excited with soft X-ray light to jump into this quantum state.

In principle, such a clock could tick faster and therefore more accurately than current generation of optical atomic clocks.

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