α-Tantalum (α-Ta) is an emerging material for superconducting qubit fabrication due to the low microwave loss of its stable native oxide. However, hydrogen absorption during fabrication,particularly when removing the native oxide, can degrade performance by increasing microwave loss. In this work, we demonstrate that hydrogen can enter α-Ta thin films when exposed to 10 vol% hydrofluoric acid for 3 minutes or longer, leading to an increase in power-independent ohmic loss in high-Q resonators at millikelvin temperatures. Reduced resonator performance is likely caused by the formation of non-superconducting tantalum hydride (TaHx) precipitates. We further show that annealing at 500°C in ultra-high vacuum (10−8 Torr) for one hour fully removes hydrogen and restores the resonators‘ intrinsic quality factors to ~4 million at the single-photon level. These findings identify a previously unreported loss mechanism in α-Ta and offer a pathway to reverse hydrogen-induced degradation in quantum devices based on Ta and, by extension also Nb, enabling more robust fabrication processes for superconducting qubits.
Tailoring the decay rate of structured quantum emitters into their environment opens new avenues for nonlinear quantum optics, collective phenomena, and quantum communications. Herewe demonstrate a novel coupling scheme between an artificial molecule comprising two identical, strongly coupled transmon qubits, and two microwave waveguides. In our scheme, the coupling is engineered so that transitions between states of the same (opposite) symmetry, with respect to the permutation operator, are predominantly coupled to one (the other) waveguide. The symmetry-based coupling selectivity, as quantified by the ratio of the coupling strengths, exceeds a factor of 30 for both the waveguides in our device. In addition, we implement a two-photon Raman process activated by simultaneously driving both waveguides, and show that it can be used to coherently couple states of different symmetry in the single-excitation manifold of the molecule. Using that process, we implement frequency conversion across the waveguides, mediated by the molecule, with efficiency of about 95%. Finally, we show that this coupling arrangement makes it possible to straightforwardly generate spatially-separated Bell states propagating across the waveguides. We envisage further applications to quantum thermodynamics, microwave photodetection, and photon-photon gates.
We fabricate and characterize superconducting through-silicon vias and electrodes suitable for superconducting quantum processors. We measure internal quality factors of a million fortest resonators excited at single-photon levels, when vias are used to stitch ground planes on the front and back sides of the wafer. This resonator performance is on par with the state of the art for silicon-based planar solutions, despite the presence of vias. Via stitching of ground planes is an important enabling technology for increasing the physical size of quantum processor chips, and is a first step toward more complex quantum devices with three-dimensional integration.