Atomic nucleus excited by laser for the first time

A research group from Germany and Austria has succeeded for the first time in exciting an atomic nucleus with a laser. Until now, this has only been possible with atoms or molecules. The breakthrough, which has now been made public, should not only enable much more precise atomic nucleus clocks, but also much more accurate measurements of physical quantities, explains the Vienna University of Technology. The success was preceded by a decades-long search for the exact amount of energy required to excite the so-called thorium transition. The work now presented makes it possible to combine two areas of physics that previously had few points of contact, adds the team: classical quantum physics and nuclear physics.

Extremely lengthy search

As the researchers remind us, manipulating atoms or molecules with lasers has long been commonplace. With the right wavelength, they can be made to change from one state to another. In this way, their energies can be determined exactly, enabling many precision measurement techniques. However, applying this to the much smaller atomic nuclei seemed impossible for a long time. This would normally require much more energy, which photons would not have at their disposal. At the same time, atoms are actually "the perfect quantum objects for precision measurements". Because they are smaller and less susceptible to interference, they would enable measurements with unrivaled accuracy.

For more than 50 years, however, there has been speculation that there might be a very specific atomic nucleus that could perhaps be manipulated with a laser after all. Thorium-229 has two closely neighboring energy states between which a targeted change could be stimulated. However, the energy required for this would have to be known very precisely: "If you know the exact energy of this transition to within one electron volt, it's of little use if you have to hit it to within a millionth of an electron volt in order to detect it," explains Thorsten Schumm from TU Wien. It was the proverbial search for a needle in a haystack, which is why it took so long.

The team has now been successful because it has developed crystals that contain large numbers of thorium atoms, writes the university. Although this was technically complex, it had the advantage that not individual thorium nuclei were examined with a laser, but ¹⁰¹⁷ of them simultaneously - "millions of times more than there are stars in our galaxy". On November 21, 2023, the energy of the sought-after thorium transition was precisely hit for the first time. The laser switched the state in a targeted manner and successfully completed the search. The concrete preparatory work for this experiment had been underway since 2009. The milestone is now being presented in Physical Review Letters.

It will be exciting

A central long-term goal of the work was an atomic nuclear clock, the team explains. If the light that excites the thorium transition is used as a timer, it would be possible to build a new type of clock "that would be significantly more accurate than the best atomic clocks available today". The team predicts that the precision measurements that will soon be possible could also be used to measure the Earth's gravitational field so precisely that it could provide indications of earthquakes and even mineral resources. In addition, it would now also be possible to check whether natural constants are actually constant. "Our measurement method is just the beginning," promises Schumm and: "It will certainly be exciting."

(mho)