Nb-based Superconducting Qubits with Capping Layers: Optimizing the Proximity Effect

Recent studies within the SQMS center of transmon qubits, utilizing thin capping layers on the Nb capacitor to eliminate the RF lossy Nb oxide, have shown significant increases in the T₁ relaxation time. Such superconducting/normal metal (S/N) bilayers necessarily involve the proximity effect and we discuss how this can be optimized. First, a review is presented of the extensive tunneling research that shows that thin capping layers of Ta, Al and Mg on Nb can be fabricated in the specular limit where the Arnold theory of the proximity effect is realized. In this case, the continuum of quasiparticle states above the induced gap Δ_N << Δ_S are replaced with a discrete set of Andreev bound states near the top of the potenial well Δ_S - Δ_N. This allows the capping layer to transmit the superconducting properties of the underlying Nb including an effective gap parameter close to the bulk value of 1.55 meV for Nb. The inclusion of a quasiparticle scattering term in the Arnold model shows how low-lying quasiparticle states near Δ_N re-emerge which might be detrimental to qubit performance. This gives guidance to the materials processing, suggesting that the capping layer should be clean and the N/S interface should not introduce significant scattering. The model, as well as experiment with Nb/Mg, also shows that the N layer need not be superconducing at all in bulk. This opens the door to using capping layers of normal metals such as Au or other metals that have a minimal native oxide layer that might cause RF losses.