Abstract
We have recently demonstrated the laser cooling of a single ion to the motional ground state in a Penning trap using the resolved-sideband cooling technique on the electric quadrupole transition S
D
. Here we report on the extension of this technique to small ion Coulomb crystals made of two or three
ions. Efficient cooling of the axial motion is achieved outside the Lamb-Dicke regime on a two-ion string along the magnetic field axis as well as on two- and three-ion planar crystals. Complex sideband cooling sequences are required in order to cool both axial degrees of freedom simultaneously. We measure a mean excitation after cooling of
for the centre of mass (COM) mode and
for the breathing mode of the two-ion string with corresponding heating rates of 11(2)
and
at a trap frequency of 162 kHz. The occupation of the ground state of the axial modes (
) is above 75% for the two-ion planar crystal and the associated heating rates 0.8(5)
at a trap frequency of 355 kHz.
Notes
No potential conflict of interest was reported by the authors.