Sympathetic cooling with Be ions for precision measurements – BESCOOL

In the last two decades, spectacular advances have been made in the quantum state control of ions confined in radiofrequency traps. Methods have been developed for laser cooling of the external motion down to the vibrational ground-state. Sympathetic cooling extended these techniques to many atomic and molecular species that cannot be directly laser cooled. Progress in ion transport allows performing each task in a separate, specifically optimized trap. As a result, trapped ions now represent one of the most advanced systems in the fields of quantum information and high precision measurements.

This project aims at overcoming two important open questions of these techniques, which are still preventing investigation of highly interesting light ion species.
Firstly, the above results were obtained with ions directly created inside the trap by electron impact or photo-ionization. However, many species such as antimatter ions, state-selected molecular ions, or highly charged ions are produced in external sources. We plan to develop a universal setup for transport, capture, and cooling of externally produced ions. The second challenge comes from the sympathetic cooling process, which becomes less efficient as the difference between the charge-to-mass (q/m) ratios of the laser-cooled and sympathetically cooled species increases, both for Doppler and ground-state cooling. We will experimentally investigate sympathetic cooling dynamics in the regime of high q/m differences, and develop cooling methods and trap geometries specifically adapted to this case.

Taking advantage of both partners’ skills, a complete set of methods and instrumentation to load multi-species ion crystals will be developed. The proposal relies on a sequence of two linear, segmented traps: a “capture trap” where Doppler sympathetic cooling will allow reducing the temperature to the mK range, before transport of spectroscopic ions to a “precision trap” optimized for ground-state cooling of a two-ion crystal. We aim at optimizing the scheme so that even a pair of ions with highly different q/m ratios can be cooled.

These developments will allow scientific breakthroughs in the fields of fundamental gravitation and physical constants determination.
The first application is the GBAR project accepted by CERN in 2012; it concerns the first test of the equivalence principle with antimatter, through a free-fall experiment on neutral antihydrogen atoms. Both partners of the present project are among the 14 members of this international collaboration, and are involved in a key step: ground-state cooling of an antihydrogen positive ion in a Hbar+/Be+ ion pair. Our objective is to deliver the core of the GBAR experiment, complete setup for Hbar+ capture and cooling, tested with H+ ions before final assembly in 2017 at CERN.
The second application is spectroscopy of state-selected cold H2+ ions, that will result in a new determination of the proton-to-electron mass ratio with about 10-10 accuracy (limited by theory), i.e. an improvement by a factor of 4.

This binational project sets up a new collaboration joining rich competences in complementary fields of ion trapping and spectroscopy to overcome the demanding technical and physical challenges. The French partner’s experience in light ion production and manipulation, high-resolution spectroscopy, Be+ ion cooling laser sources, and the German partner’s experience in trap design, quantum control and transport of trapped ions are essential assets for the positive outcome of the project
The BESCOOL project will provide novel and universal instrumentation for future spectroscopic applications and fundamental tests using light atomic and molecular ions. Among the exciting prospects is laser spectroscopy of highly charged ions for tests of quantum electrodynamics, quantum logic clock applications, and studies on time variations of fundamental constants.