Sub-kHz "clock" transition linewidths in a cold trapped 88Sr+ ion in low magnetic fields using 1092-nm polarisation switching

A small RF (Paul) trap has been developed, which allows Lamb-Dicke confinement of a single strontium ion. This paper describes the trap and the diagnostics used to verify Lamb-Dicke confinement. In particular, it is necessary to operate in a ?ear zero magnetic field, so that the Zeeman splitting is minimised. In such low magnetic fields, the ion fluorescence at the S-2(1/2)-P-2(1/2) cooling wavelength at 422 nm normally falls to zero. This arises from optical pumping into some of the Zeeman sub-levels of the D-2(3/2) State, Which occurs because the laser which drives the 1092 nm transition (P-2(1/2)-D-2(3/2)) has a fixed polarisation state. However, if the polarisation state of this laser is rapidly switched, the fluorescence is recovered. A scan over the strontium ion S-2(1/2)-D-2(5/2) quadrupole transition shows only the carrier and sidebands corresponding to the radial and axial secular frequencies. Short range high resolution scans over a part of the carrier reveal Zeeman structure with less than or equal to 1 kHz-linewidth features. (C) 1998 The National Physical Laboratory. Published by Elsevier Science B.V. All rights reserved.