Stylus ion trap for enhanced access and sensing

Small, controllable, highly accessible quantum systems can serve as probes at the single-quantum level to study a number of physical effects, for example in quantum optics or for electricand magnetic-field sensing. The applicability of trapped atomic ions as probes is highly dependent on the measurement situation at hand and thus calls for specialized traps. Previous approaches for ion traps with enhanced optical access included traps consisting of a single ring electrode1,2 or two opposing endcap electrodes2,3. Other possibilities are planar trap geometries, which have been investigated for Penning traps4,5 and radiofrequency trap arrays6–8. By not having the electrodes lie in a common plane, the optical access can be substantially increased. Here, we report the fabrication and experimental characterization of a novel radiofrequency ion trap geometry. It has a relatively simple structure and provides largely unrestricted optical and physical access to the ion, of up to 96% of the total 4π solid angle in one of the three traps tested. The trap might find applications in quantum optics and field sensing. As a force sensor, we estimate sensitivity to forces smaller than 1 yN Hz−1/2. The basic electrode geometry is shown in Fig. 1 and is formed by two concentric cylinders over a ground plane. The design provides straightforward indexing and assembly of the trap electrodes, with large solid angle access to the ion. Four extra electrodes were placed on a circle between the grounded plane and the radiofrequency electrode to break the rotational symmetry of the radiofrequency pseudopotential about the vertical axis and to compensate for stray electric fields to minimize ion radiofrequency micromotion in the trap9. Three different traps were built adjacent to each other on the same test set-up (Fig. 2). These traps range from a conservative design with a larger trap depth, higher motional frequencies and a smaller accessible solid angle, to a weaker trap with greater optical access. This change in properties is achieved by varying the protrusion height1h of the central grounded electrode with respect to the radiofrequency electrode (Table 1). The degeneracy of motional frequencies in the radial direction was lifted by applying potentials of the order of 0.1–1V to the compensation electrodes A–D. This created a static quadrupole field defining the principal radial axes of the trap along the lines connecting compensation electrode A with D and B with C. Thus, the axes were oriented at angles of about 45 relative to the two cooling beams (Fig. 1b). In addition, the entire trap assembly was tilted by about 7.5 with respect to the direction defined by the laser beams, ensuring that the vertical axis of the traps was not orthogonal to the wave vectors of the cooling beams. In this way, all three normal modes of the ion were sufficiently Doppler cooled by a single laser beam. For further details, see the Methods section.