Strontium, as an earth-alkaline element with two valence electrons, in contrast to alkali atoms as commonly used in ultracold atomic labs, exhibits few specific features such as a narrow intercombination line and an advantageous electronic structure, which enable efficient cooling schemes. Few of them have already been demonstrated, namely the production of very big Bose-Einstein condensates (BECs)1, the production of very fast BEC creation cycles2 and to the first BEC without the need for evaporative cooling3.
These atoms we excite to high-lying electronic states – so-called Rydberg states – which, due to their high polarizability, generally have much larger interactions among each other than ground-state atoms. These interactions scale, dependent on their character, as n4 or n11 with n being the principal quantum number of the electronic state, and are around 1011 higher than for ground-state atoms4. These strongly interacting systems can be useful as quantum simulators, but also exhibit interesting properties such as the Rydberg blockade whose non-linearity can be used for the realization of quantum gates. The study of two-electron Rydberg gases is a relatively new field of research5.
The electronic energy levels of strontium being most important for our envisioned setup are shown on the picture: we first cool down the atoms in a blue (461 nm) broadband magneto-optical trap (MOT). Due to a small leak in the pumping cycle an excited atom de-excites to the long-lived 5P2 state with a probability of 1:150,000. Those atoms, not interacting with the trapping light, are kept in a magnetic trap before they are de-excited via the 481 nm repump beam. Then a red (689 nm) narrowband MOT stage with a Doppler temperature below 1 µK follows. Finally, we excite the atoms to triplet Rydberg states.
- Stellmer, S., Grimm, R., & Schreck, F. (2013). Production of quantum-degenerate strontium gases. Physical Review A, 87(1), 013611.
- Stellmer, S. Degenerate quantum gases of strontium, PhD thesis, University of Innsbruck, 2013
- Stellmer, S., Pasquiou, B., Grimm, R., & Schreck, F. (2013). Laser cooling to quantum degeneracy. Physical review letters, 110(26), 263003.
- Saffman, M., Walker, T. G., & Mølmer, K. (2010). Quantum information with Rydberg atoms. Reviews of Modern Physics, 82(3), 2313.
- Millen, J., Lochead, G., & Jones, M. P. A. (2010). Two-electron excitation of an interacting cold Rydberg gas. Physical review letters, 105(21), 213004.
- McQuillen, P., Zhang, X., Strickler, T., Dunning, F. B., & Killian, T. C. (2013). Imaging the evolution of an ultracold strontium Rydberg gas. Physical Review A, 87(1), 013407.