Design and fabrication of a merged element transmon with a phononic bandgap shield – Part 1

Lossy two-level systems (TLSs) originating in amorphous surface layers and material interfaces represent one of the dominant decay and decoherence mechanisms in superconducting qubits. By incorporating a phononic bandap shield around the junction of a transmon qubit, one can mitigate the dominant decay channels for TLS interacting with the local junction field. Further, by concentrating the energy of a transmon to a small volume around the junction while ensuring the corresponding TLS spectral density can remain in a discrete limit, then one can realize distinct frequency windows in which the TLS-mediated qubit energy decay can be suppressed.

Past work by Chen et al. [1] has shown that the shielding technique is highly effective in suppressing the energy decay of TLS localized to the Josephson junction, resulting in TLS T₁ times over a few milliseconds. In this first part of a two-part talk, we build on this work to design a merged element transmon (mergemon) and read-out resonator circuit. In this architecture, parasitic capacitance to structures outside the shielded region is three-orders of magnitude less than the shielded junction capacitance, indicating that such designs should enable mergemon qubit T₁ times > 1ms, assuming typical numbers for TLS density and coupling strength.

[1] M. Chen et al., Sci. Adv. 10, eado6240, (2024), doi:10.1126/sciadv.ado6240.