Sideband cooling of small ion Coulomb crystals in a Penning trap

Abstract

We have recently demonstrated the laser cooling of a single ${}^{40}{\text{Ca}}^{+}$ ion to the motional ground state in a Penning trap using the resolved-sideband cooling technique on the electric quadrupole transition S${}_{12}\leftrightarrow$ D${}_{52}$. Here we report on the extension of this technique to small ion Coulomb crystals made of two or three ${}^{40}{\text{Ca}}^{+}$ 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 ${\overline{n}}_{}\text{COM}=0.30(4)$ for the centre of mass (COM) mode and ${\overline{n}}_{}\text{B}=0.07(3)$ for the breathing mode of the two-ion string with corresponding heating rates of 11(2) ${\text{s}}^{-1}$ and $1(1){\text{s}}^{-1}$ at a trap frequency of 162 kHz. The occupation of the ground state of the axial modes ($n_{}\text{tilt}=n_{}\text{COM}=0$) is above 75% for the two-ion planar crystal and the associated heating rates 0.8(5) ${\mathrm{s}}^{-1}$ at a trap frequency of 355 kHz.

Keywords:

• Penning trap
• trapped ions
• sideband cooling
• ion Coulomb crystals

View correction statement:

Publisher’s Note

Notes

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the UK Engineering and Physical Sciences Research Council [grant number EP/D068509/1]; the European Commission STREP PICC [FP7 2007-2013], [grant number 249958]. We gratefully acknowledge financial support towards networking activities from COST Action MP 1001 – Ion Traps for Tomorrow’s Applications. The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme [FP7/2007-2013] under REA [grant agreement number 31723]. JFG acknowledges support from an EPSRC Doctoral Prize award. The optical system used for the images shown in Figure 1 and the spectra in Figures 4 and 5 was designed by Mr Jieyi Liu.