Similar to quantum logic spectroscopy, photon-recoil spectroscopy uses an auxiliary ion trapped together with the ion under investigation to provide cooling and efficient signal detection as required for precision spectroscopy. The trap keeps the two ions together whereas they would normally repel each other due to their respective electric charge. Similar to Siamese twins, these two ions are forced to do everything together. In this way, it is possible to obtain information on the ion to be investigated (in this experiment: calcium) by observing the behaviour of the easily controllable so-called “auxiliary” or “logic ion” (magnesium). Close to an atomic resonance, photons from the spectroscopy laser pulses are absorbed and cause the spectroscopy ion to oscillate due to their recoil kick. Due to the strong coupling, the auxiliary ion also oscillates. Its oscillation can be detected with high efficiency using quantum logic techniques, which amplify the signal of a few absorbed photons into thousands of photons. Recoil kicks from less than 10 photons suffice to generate a measurable oscillation.

In this way, the absolute frequency of a certain transition in calcium was measured to with an accuracy of 88 kHz; this corresponds to a relative inaccuracy of 10−10 at a transition width of approx. 30 MHz. Previous measurements were less precise by more than an order of magnitude. One of the main features of the experiment is its versatility. The reason why this experiment is so special is that it is flexible. The only thing it takes to iInvestigatinge other spectroscopy ions requires just a change in the spectroscopy laseris to re-adjust the laser used; the auxiliary ion and the elaborate laser set-ups required for cooling and detection remain unchanged.

The objective of this new method is to carry out absolute frequency measurements on many different ions with the greatest possible precision. This new technique extends quantum logic spectroscopy to the investigation of ions which remain in their excited state for a few micro- or even nano-seconds only. Together with the greater sensitivity, this opens up new possibilities in the precision spectroscopy of molecular and metal ions which are found in space and are often used as a reference by astronomers – for example to compare atomic resonance lines from some billion years ago with the respective resonances today in order to detect possible changes in the fine-structure constant.