Isolating metal-air and conductor loss in superconducting circuits

There is a continued effort to improve the coherence times of superconducting circuits as a platform for quantum computing. Recent works have targeted the metal-air interface as a significant source of loss, giving rise to approaches utilizing improved surface treatments and surface encapsulation. However, in a planar architecture, dielectric loss in the metal-air interface is not separable from other surface losses. By employing a flip-chip architecture, we magnify sensitivity to metal-air loss by orders of magnitude, enabling an unambiguous extraction of a metal-air loss factor. The additional geometric degree of freedom provided by a flip-chip architecture also allows separation of the contribution from conductor loss. To demonstrate our technique, we characterize loss contributions in leading materials platforms for superconducting circuits, tantalum and rhenium on sapphire. We present results from our loss characterization, in addition to coherence times of rhenium-based transmons that rival those of tantalum.