Rydberg pair interactions in divalent atoms in external fields

Neutral atom platforms for quantum computing rely on Rydberg excitation for the implementation of two- or multi-qubit gates. While the use of heavy divalent or open-shell elements, such as strontium or ytterbium, has benefits due to their optically active core and the multitude of possible qubit encodings, their Rydberg structure may be complicated, particularly for fermionic species with level densities much higher than e.g.~in alkali metals. Here, we report the in-depth analysis of different Rydberg series of ⁸⁷Sr employing multi-channel quantum defect theory to predict the Rydberg spectrum and building on the pairinteraction software [1], to analyse Rydberg pair-interaction potentials in various external field configurations. Specifically, we study an intrinsic Forster resonance in the 5sns F=9/2 series as predicted in [2], which provides long-range interactions and, thus, constitutes a promising candidate for the implementation of multi-qubit gates. Due to the combination of a high pair-state level density, large dipole interactions, and sensitivity to external fields, a perturbative treatment to obtain both single atom and pair properties is sometimes not sufficient to capture the relevant physics, e.g. pair-state degeneracies can lead to avoided crossings and external fields can cause the breakdown of blockade.

[1] S. Weber, C. Tresp, H. Menke, A. Urvoy, O. Firstenberg, H. P. Büchler, and S. Hofferberth, Tutorial: Calculation of Rydberg interaction potentials, J. Phys. B: At. Mol. Opt. Phys. 50, 133001 (2017) .

[2] F. Robicheaux, Calculations of long range interactions for 87Sr Rydberg states, J. Phys. B: At. Mol. Opt. Phys. 52, 244001 (2019).