Molecular Nitrogen Ion Clock

The experiment team at the University of Sussex is building a clock system based on trapped molecular nitrogen ions. Nitrogen ions are very good candidates for the search of physics beyond the standard models of particle physics and cosmology. They have transitions that are sensitive to changes in the electron-to-proton mass ratio and, at the same time, have great immunity against changes in their environment. Their vibrational transitions, offer the potential to achieve a frequency uncertainly of better that 10-20, better that what has been achieved with the best atomic clocks to date.
To perform spectroscopy on the vibrational transition, a single nitrogen ion is trapped alongside a co-trapped calcium ion. The calcium ions serves to sympathetically cool the nitrogen ions and to serve as a ‘read-out’ of its internal state via quantum logic spectroscopy. This is necessary due to the lack of cycling transitions within the nitrogen ion.

Schematic of the electrode structure of an rf-ion trap with trapped nitrogen and calcium ions (left). Image of a real trapped coulomb crystal of a single nitrogen ion and a singe calcium ion (right). Visible is only the calcium ions as it can continuously emit light.

The figure below shows the setup of the clock system with the vacuum chamber of the ion trap and the molecular beam line.

Schematic and photo of the spectroscopy setup.

The ion trap is under ultra-high vacuum to prevent collisions and hence disturbance of the trapped nitrogen’s transition. To maintain this UHV environment during the loading of the ions into the ion trap, the nitrogen is injected into the ion trap through a supersonic molecular beam where it is ionised with an intense laser pulse. Through the coulomb repulsion between the nitrogen ion and the trapped calcium ion, the nitrogen in then cooled to ~500uK the through Doppler cooling of the calcium ion.
More information can be found here.