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In this research project the interaction of light and matter is studied. The aim is to reach high coupling efficiencies of a single photon with a single ion in free space. The concept of the experimental approach is based upon the time reversal symmetry of the spontaneous emission process [1].

In the experiment a single Ytterbium ion will be located in the focal point of a parabolic mirror [2]. The Ytterbium ion is produced via photo ionization and trapped with a needle-like Paul trap [3], that is combined with the mirror. An excitation light field reflected by the deep parabolic mirror is focused onto the ion from nearly the whole solid angle [4]. To maximize the absorption probability care is taken in optimizing the overlap between the incident light wave and the radiation pattern that corresponds to the atomic transition [2,4] which is in our case a linear dipole transition. With a radially polarized incident light field collimated along the mirror axis a spatial overlap of 98% can be reached [5]. In order to achieve high coupling efficiencies one also has to tailor the spectral/temporal shape of the photon. This will be done by sending highly attenuated light pulses with an exponential increasing amplitude onto the ion. Addressing the 1S0 - 3P1 transition of doubly ionized Ytterbium, the planned setup can enable coupling efficiencies of about 90%.

The following building blocs have been developed: an ion trap with high optical access has been built [3] and tested successfully with single Yb+ ions. The incident exponentially increasing laser pulse has been generated using an acousto-optic modulator. The radially polarized doughnut mode is generated by shining a linearly polarized Gaussian beam through a segmented half wave plate [6]. Furthermore the ion trap has been rebuilt with a similar geometry and inserted into a parabolic mirror. As soon as the single ions are trapped and cooled efficiently the coupling of the ion to the free space light field will be studied.

[1] S. Quabis et al, Opt. Commun. 179, 1 (2000)

[2] M. Sondermann et al, Appl. Phys. B 89, 489 (2007)

[3] R. Maiwald et al., Nature Phys. 5, 551 (2009)

[4] N. Lindlein et al, Laser Physics 17, 927 (2007)

[5] M. Sondermann et al. arXiv:0811.2098

[6] R. Dorn et al, Phys. Rev. Lett. 91, 233901 (2003)

Figure: One single trapped Ytterbium ion.