Current research / employment

I am working on an experiment which facilitates the strong interaction between photons and a single ion trapped in free space inside an ion trap. Such a strengthening of the coupling between the optical and the matter domain could enhance measurements in the field of quantum computing as well as in experiments which rely on the mediated coupling of atomic system by means of light beams.

One of the main experimental goals of the group I am working in is the full saturation of a single two-level atom in free space by a single photon, hence the atom fully absorbs a single incident photon. To make such an effect feasible one should take guidance from the opposite effect, namely the spontaneous emission of a single photon by a single initially excited two-level atom.

Taking a single two-level atom in free space, of which the two levels are coupled by a linear-dipole transition, such an atom couples exclusively to the free space optical linear dipole modes which by themselves are only distinguished by their optical frequency (actually, it is this broadband coupling of the atom to these optical modes which makes up the spontaneous emission, hence the exponential decay of the atomic excitation). To make the preparation of the single photon in these optical dipole modes feasible, the demand for an electric linear dipole transition is raised in the experiment. Additionally, the single ion used in the experiment should have an isolated two-level system which can then be exclusively excited by the photon. The ground state based 1S0->3P1 transition in 174Yb2+ fulfils all these requirements.

The 174Yb isotope features a vanishing nuclear spin component, hence there is no hyperfine splitting of the ground state. The second ionization step ensures, that this ion features a J=0 to J=1 transition which is free of Zeeman shifts.

The wavelength of this 1S0->3P1 transition is at 251.8nm and has a line-width of 692kHz. Laser radiation (cw) at this wavelength is produced by a frequency quadrupled diode laser system.

My current work concentrates on the realization of the second ionization step from Yb+ towards the creation of Yb2+. This ionization will be realized by a two-photon photo-ionization with one resonant intermediate level starting from a single Yb+ ion trapped inside an ion trap. The wavelength of this resonant intermediate step was found using a cloud of buffer gas (He) cooled Yb+ ions inside a ring-formed ion-trap.

Figure: This photograph shows a side view into the vacuum chamber housing the ion-trap system. The tiny ring in its centre is the actual ion trap electrode.

Publications S. Heugel, A. S. Villar, M. Sondermann, U. Peschel, G. Leuchs, On the analogy between a single atom and an optical resonator, Laser Phys., 20(1), 100-106, (2010)

F. Mueller, S. Heugel, L. J. Wang, Optomechanical stochastic resonance in a macroscopic torsion oscillator, Phys. Rev. A, 79, 031804, (2009)

F. Mueller, S. Heugel, L. J. Wang, Femto-Newton light force measurement at the thermal noise limit, Opt. Lett., 33, 6, (2008)

F. Mueller, S. Heugel, L. J. Wang, Observation of optomechanical multi-stability in a high-Q torsion balance oscillator, Phys. Rev. A, 77, 031802, (2008)

F. Mueller, S. Heugel, L. J. Wang, Sub-Kelvin cooling of a gram-sized oscillator, Appl. Phys. Lett., 92, 044101, (2008)