Single CA+ Ions in a Paul-Straubel Trap

Singly-ionized calcium confined in a rf trap has become an ion of choice for widespread applications in spectroscopy, metrology and quantum computing [1,2]. In fact, the wavelengths of the transitions between the lowest energy levels lie in the visible and nearinfrared range, easily accessible by all-solid-state laser systems. Laser-cooling may be carried out on a strong electric-dipole transition at 397nm, while the natural linewidth of the 729nm transition which interconnects the metastable 3D5/2-state and the 4S1/2 ground-state is inferior to one Hertz. To obtain the small spectral linewidths necessary for a frequency standard performance, it is essential to access the Lamb-Dicke regime where the ion's amplitude of motion is inferior to the emitted wavelength. We have set up and thoroughly characterized the trapping performance of a miniature ion trap of the Paul-Straubel-type [3]. Two different techniques have been applied to a small, laser-cooled ion cloud: measurement of the frequencies of motion of the confined particles by application of a resonant "tickle" frequency, and scanning of the UDC-potential. The stored ions are observed by a photomultiplier in the photon-counting mode as well as by an intensified CCD camera, permitting the visualization of their spatial behavior. Our aim is to localize a single ion as precisely as possible in the trapping device. In a first step, we have used the observation of the spatial distribution of the ion's fluorescence as a monitor to reduce its micromotion in the trap. This allows for a rough compensation of static electric stray fields in the plane of observation. Correlation measurement of the emitted photons with the rf trapping frequency has been chosen for fine-tuning of the potentials applied to the compensation electrodes. In a final stage of our experiment, the laser-cooling and probing of the ions in the trap will be carried out exclusively by diode lasers. Laser cooling of the Ca ions is made on the 4S1/2-4P1/2 resonance line at 397 nm. One way to obtain this wavelength is frequency doubling of a 794 nm laser with a non-linear SHG crystal. We have set up an injection system using a broad-area laser diode (500mW) as slave laser. About 120 mW of spectrally and spatially single-mode output power at 794 nm could be reached [4]. To produce radiation at 729 nm we use a high-power broad-area laser diode (BAL) put into an external cavity. The initial spectral linewidth of the free-running laser is about 2 nm, in a Littrow-configuration cavity the linewidth is reduced to a couple of MHz. Making use of the Pound-Drever-stabilization method on an low finesse cavity (F=300), the laser linewidth is reduced to less than 400 kHz. The next step will be the stabilization using the same technique on a high finesse ULE-cavity (F=15000) which is thermally and mechanically controlled.