Controllable optical phase shift over one radian from a single isolated atom.

Fundamental optics such as lenses and prisms work by applying phase shifts of several radians to incoming light, and rapid control of such phase shifts is crucial to telecommunications. However, large, controllable optical phase shifts have remained elusive for isolated quantum systems. We have used a single trapped atomic ion to induce and measure a large optical phase shift of 1.3±0.1 radians in light scattered by the atom. Spatial interferometry between the scattered light and unscattered illumination light enables us to isolate the phase shift in the scattered component. The phase shift achieves the maximum value allowed by atomic theory over the accessible range of laser frequencies, pointing out new opportunities in microscopy and nanophotonics. Single-atom phase shifts of this magnitude open up new quantum information protocols, in particular long-range quantum phase-shift-keying cryptography.

Wavelength-scale imaging of trapped ions using a phase Fresnel lens.

A microfabricated phase Fresnel lens was used to image ytterbium ions trapped in a radio frequency Paul trap. The ions were laser cooled close to the Doppler limit on the 369.5 nm transition, reducing the ion motion so that each ion formed a near point source. By detecting the ion fluorescence on the same transition, near-diffraction-limited imaging with spot sizes of below 440 nm (FWHM) was achieved. To our knowledge, this is the first demonstration of wavelength-scale imaging of trapped ions and the highest imaging resolution ever achieved with atoms in free space.

An automated submicron beam profiler for characterization of high numerical aperture optics.

An automated, interferometrically referenced scanning knife-edge beam profiler with submicron resolution is demonstrated by directly measuring the focusing properties of three aspheric lenses with numerical aperture (NA) between 0.53 and 0.68, with spatial resolution of 0.02 microm. The results obtained for two of the three lenses tested were in agreement with paraxial Gaussian beam theory. It was also found that the highest NA aspheric lens, which was designed for 830 nm, was not diffraction limited at 633 nm. This process was automated using motorized translation stages and provides a direct method for testing the design specifications of high numerical aperture optics.

High-Q measurements of fused-silica microspheres in the near infrared.

Measurements of the quality factor Q approximately 8x10(9) are reported for the whispering-gallery modes (WGM's) of quartz microspheres for the wavelengths 670, 780, and 850 nm; these results correspond to finesse f approximately 2.2x10(6) . The observed independence of Q from wavelength indicates that losses for the WGM's are dominated by a mechanism other than bulk absorption in fused silica in the near infrared. Data obtained by atomic force microscopy combined with a simple model for surface scattering suggest that Q can be limited by residual surface inhomogeneities. Absorption by absorbed water can also explain why the material limit is not reached at longer wavelengths in the near infrared.