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Probing Ground-State Degeneracies of a Strongly Interacting Fermi-Hubbard Model with Superconducting Correlations

Sebastiaan L. D. ten Haaf*, Sebastian Miles*, Qingzhen Wang, A. Mert Bozkurt, Ivan Kulesh, Yining Zhang, Christian G. Prosko, Michael Wimmer, and Srijit Goswami†

  • *These authors contributed equally to this work.
  • †Contact author: s.goswami@tudelft.nl
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Phys. Rev. Lett. 137, 126503 – Published 15 September, 2026

DOI: https://doi.org/10.1103/nz6v-xxrv

Abstract

The Fermi-Hubbard model describes a large variety of condensed matter systems with spinful fermions and strong interactions. On the other hand, the Kitaev chain model deals with noninteracting spinless fermions and produces robust ground-state degeneracies that give rise to Majorana bound states. In this Letter, we connect these two domains by experimentally studying small arrays of quantum dots (QDs) coupled via superconductors. Without a magnetic field, this system constitutes a Fermi-Hubbard model with intersite superconducting correlations. Using two coupled QDs, we find sweet spots in parameter space where we observe zero-bias conductance peaks that are robust against on-site perturbations of the QDs, similar to the behavior expected of a two-site Kitaev chain. This observation is consistent with the presence of Majorana Kramers pairs or Z3 parafermions in this minimal system. Extending to three sites, we find that the spinful system scales very differently compared to the spinless Kitaev chain. When the same sweet-spot conditions are satisfied, the ground state degeneracy of the full three-site system is lifted. The degeneracy can be restored by tuning the superconducting phase difference between the hybrid segments, but the robustness against on-site detunings is lost. Our findings are a first step in studying ground-state degeneracies in strongly interacting Fermi-Hubbard systems with superconducting correlations.

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Phys. Rev. Lett. 137, 126503– Published 15 September, 2026
  • Received 15 December 2025
  • Accepted 16 July 2026
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