The inductively shunted transmon: A simple building block for multi-species qubit lattices*

The transmon has become the most widely used superconducting qubit owing to its simplicity, while its limitations have spurred the recent development of other more complex qubit designs, such as the fluxonium and zero-pi qubit. The inductively shunted transmon (IST) has also been proposed [1] as a simple alternative to complement the conventional transmon and enable statically coupled qubit pairs free of unwanted ZZ interaction. Here, we realize an IST in our 3D-integrated coaxial circuit architecture [2]. This qubit is simple to fabricate, has a stable single-well plasmonic spectrum and shows good coherence at the half flux bias point. By coupling it directly to a transmon, we show built-in ZZ suppression thanks to the positive sign of the IST anharmonicity. Numerical simulations indicate that this effect is realizable for a wide variety of circuit parameters. We investigate blue and red sideband transitions, allowed by the asymmetry of the IST potential when biased away from the flux symmetry points, which can be used to implement entangling gates. This work paves the way towards simple multi-species superconducting qubit lattices with direct static coupling and low crosstalk, without the need for tunable coupling elements or multi-path couplers.

[1] K. Zuo et al., Abstract X31.00010, APS March Meeting (2021)

[2] J. Rahamim et al., Appl. Phys. Lett. 110, 222602 (2017)