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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 size1. In recent experiments, single molecules were used as nanoscopic probes to attain a resolution of a few tens of nanometres2,3. 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 radio-frequency 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 µm. 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 electrodynamics4,5 experiments with a single particle.

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Figure 1: Experimental arrangement of trap electrodes and cavity.
Figure 2: Transverse profiles of the Hermite–Gauss modes of the cavity, obtained by monitoring the ion's fluorescence while scanning over a range of 120 µm.
Figure 3: Two-dimensional images of the cavity field taken over an area of 100 × 60 µm2.
Figure 4: Single-ion mapping of the longitudinal structure of the cavity field.


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This work was supported by the European IST/FET programme QUBITS and by the Deutsche Forschungsgemeinschaft.

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Correspondence to W. Lange.

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Guthöhrlein, G., Keller, M., Hayasaka, K. et al. A single ion as a nanoscopic probe of an optical field. Nature 414, 49–51 (2001).

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