Sideband cooling of small ion Coulomb crystals in a Penning trap

Abstract

We have recently demonstrated the laser cooling of a single 40Ca+ ion to the motional ground state in a Penning trap using the resolved-sideband cooling technique on the electric quadrupole transition S1/2 ↔ D5/2. Here we report on the extension of this technique to small ion Coulomb crystals made of two or three 40Ca+ 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 n¯COM = 0.30(4) for the centre of mass (COM) mode and n¯B = 0.07(3) for the breathing mode of the two-ion string with corresponding heating rates of 11(2) s−1 and 1(1)s−1 at a trap frequency of 162 kHz. The occupation of the ground state of the axial modes (ntilt = nCOM = 0) is above 75% for the two-ion planar crystal and the associated heating rates 0.8(5)s−1 at a trap frequency of 355 kHz