A single ion as a nanoscopic probe of an optical field

In near-field imaging, resolution beyond the diffraction limit of optical microscopy is obtained by scanning the sampling region with a probe of subwavelength size(1). In recent experiments, single molecules were used as nanoscopic probes to attain a resolution of a few tens of nanometres(2,3). Positional control of the molecular probe was typically achieved by embedding it in a crystal attached to a substrate on a translation stage. However, the presence of the host crystal inevitably led to a disturbance of the light field that was to be measured. Here we report a near-field probe with atomic-scale resolution-a single calcium ion in a radiofrequency trap-that causes minimal perturbation of the optical field. We measure the three-dimensional spatial structure of an optical field with a spatial resolution as high as 60 nm (determined by the residual thermal motion of the trapped ion), and scan the modes of a low-loss optical cavity over a range of up to 100 mum. The precise positioning we achieve implies a deterministic control of the coupling between ion and field. At the same time, the field and the internal states of the ion are not affected by the trapping potential. Our set-up is therefore an ideal system for performing cavity quantum electrodynamics(4,5) experiments with a single particle.