Laser cooling of ions in rf traps of different symmetry : from an ion string to an ion ring
Dr Caroline Champenois (CNRS-Universite de Provence)

Ions trapped in radiofrequency electric fields are among the best candidates for the realisation of optical frequency clocks or quantum information protocols. So far, the trap of choice has been the quadrupole trap where one or several ions can be confined in a volume nearly free of rf electric field. With the appropriate set of trapping parameters, laser cooled ions in a linear quadrupole trap organise as a chain which allows one to entangle their internal state by means of their common vibrational modes. Alternative structures could be of use for metrology or quantum simulations and can be formed in traps of different geometry. Indeed, molecular dynamics simulations show that a small set of laser cooled ions form a ring or a tube when they are trapped in a tight enough linear multipole trap, an octupole trap for example.

During the talk, I will first introduce the main differences between quadrupole and multipole traps on the point of view of the ions self-organisation [1]. I will then focus on the ring configuration whose main advantage with respect to a chain in a quadrupole is to be periodic. Thanks to the decoupling of the motion along the axial and radial directions [2], laser cooling allows the ions to reach the Doppler limit in the axial direction even if the ions do not sit in a rf field free position. All these features will be analysed in the context of a recent proposal [3] where the ion ring is the basis for an optical frequency clock. The possibility offered by sympathetic cooling of an ion ring will be also discussed.

[1] C. Champenois, Tutorial: About the dynamics and thermodynamics of trapped ions, J. Phys. B 42 (2009) 154002.
[2] M. Marciante, et al., Ion dynamics in a linear radio-frequency trap with a single cooling laser, Phys. Rev. A 82 (2010) 033406 [3] C. Champenois, et al., Ion ring in a linear multipole trap for optical frequency metrology, Phys. Rev. A. 81 (2010) 043410.