title: Spatial Confinement of Atomic Excitation in a Doped Solid
Creators: name: Stabel, Markus
Publisher: name: UNSPECIFIED
Year: 2024

description: This research project dealt with the experimental implementation of theory proposals to spatially confine atomic excitations by different techniques of coherent and adiabatic interactions—potentially also below the diffraction limit imposed by the driving light fields. This tight confinement requires a strong nonlinear dependence of the coherent excitation probability on the laser intensities, which we achieved using adiabatic passage processes or narrowband composite pulse (NCP) sequences.

In the first chapter, we investigated the adiabatic passage processes stimulated Raman adiabatic passage (STIRAP) and electromagnetically induced transparency (EIT). They show pronounced robustness against variations of experimental parameters, which leads to a threshold-like behavior of the transfer efficiency vs. laser intensity and, hence, enables spatially tightly confined population dynamics. We applied a STED-like beam geometry with a Gaussian Stokes and a "donut"-shaped pump beam to localize population in the node of the latter. We presented a convincing experimental demonstration and a thorough investigation of both techniques. Our data confirmed that adiabatic passage confines population to spatial extensions far below the beam diameter. With a pump beam waist of 100µm, we confined the population to 20µm for EIT and 3µm for STIRAP. This is the first implementation of EIT-driven localization in a solid, and the first implementation of the STIRAP-based approach at all. Furthermore, we confirmed that the localization improves with increasing pump intensity and that STIRAP converges to smaller population regions much faster than EIT, as predicted by theory. The data agree very well with numerical simulations and the analytic treatment. We published these results in the special issue "Coherent Control: Photons, Atoms and Molecules" of the Journal of Physics B.

Moreover, in the second chapter, we implemented NCP sequences for high-resolution addressing. They show a strong dependence of the excitation probability on the laser intensity. Our data confirmed that this confines excitation below the diameter of the driving Gaussian laser profile. This is the first implementation of NCP-driven localization in a solid. With 31 pulses, we reached a localization to 25% of the beam diameter, which is far below the previously reported value of 72%. Furthermore, we found that most previously proposed NCP sequences cannot be applied on an inhomogeneously broadened transition. Hence, we collaborated with our theory partners in the team of Nikolay V. Vitanov (University of Sofia) to develop specific sequences matched to our medium. We compared them to several previously published classes of sequences and confirmed that the confinement improves with the number of pulses but also strongly depends on the class of sequence, as predicted by theory. The results, in particular regarding the inhomogeneously broadened line, agree very well with the numerical simulation. We are currently preparing a manuscript to publish these results.

These proof-of-principle experiments on localization by STIRAP or NCP sequences still operated well above the diffraction limit. Nevertheless, they permit extrapolation toward obtaining spatially confined population in the subdiffraction regime. This will be relevant to quantum information technology and well beyond.

Finally, in the last chapter, we investigated composite pulse sequences in the context of error compensation. We provided a simple showcase experiment where we increased the coherence time of a quantum memory using dynamical decoupling (DD), but intentionally introduced pulse errors in the form of inhomogeneous driving fields. We fully characterized these inhomogeneities and performed systematic measurements to compare the ability of various robust DD sequences to compensate for the errors. Our data showed that even in the case of homogeneous driving fields, robust DD sequences improve the coherence time. This difference becomes even more pronounced when the inhomogeneity increases. In particular, we found that the universal robust sequences outperform other sequences of the same or similar order.

As the next step, we will perform additional measurements without dark state beating and then publish the results of this showcase experiment.