Spectroscopy with an Extra Twist

Work on excitation of a single ion in the dark core of a vortex laser beam has been published in Nature Communications

Laser beams can carry optical angular momentum, an extra twist due to the spatial structure of the optical field. These beams, generated by using holographical plates, feature a revolution of the optical phase around their propagation direction, and consequentially a dark spot – a penumbra – in their center. Such beams have already been used to explore a wide range of scientific and technological applications, such as stirring a gas of ultracold atoms or multiplexed optical communication. However, their interaction with well-defined atomic systems has so far mostly remained unexplored.

In the group of Prof. Ferdinand Schmidt-Kaler at the Institute for Physics of the University of Mainz, Christian Schmiegelow and colleagues have employed a laser beam with optical angular momentum for spectroscopic measurements on an isolated Calcium ion, confined in a Paul trap. The laser has been used to drive an electric quadrupole transition. On this transition, two quanta of angular momentum can be transferred to the atomic system at the same time upon excitation from the ground state to the excited state. The transition rates between different sublevels are governed by selection rules, which can be found in many standard textbooks on atomic physics. However, the measurements with the OAM beam have shown that the selection rules can substantially deviate from the standard ones if the transverse structure of the laser beam deviates strongly from commonly employed beams.

As the most prominent result, transition rates comparable to the rates attained with standard Gaussian beams were observed even if the ion placed in the penumbra of the beam, where the intensity is zero. This possibility of this excitation in the dark is a particular feature of the quadrupole transition, which is not driven by the laser’s electric field but rather by the field gradient. Furthermore, transitions with a change of atomic angular momentum of two quanta were driven for a scenario where angular momentum conservation would not permit this for a standard Gaussian beam: Here, one quantum of angular momentum is provided by the photon’s intrinsic spin, and the other one by the beam’s orbital angular momentum.

These results confirm that optical angular momentum actually enters the angular momentum balance for atomic excitation, and open up interesting possible applications for quantum technology. In particular, the possibility of excitation in the dark might enable the suppression of undesired effects for quantum computing and metrology applications.