We propose and demonstrate gates between two superconducting qubits based on quantum interference of consecutive Landau-Zener (LZ) transitions. This gate mechanism bridges between basebandand parametric two-qubit control, enabling in situ tuning of the control frequency across a continuous interval up to hundreds of MHz. Another advantage compared to dispersive couplers is that the speed of the LZ gate is on the order of the full coupling strength. We experimentally demonstrate the gate on two platforms, a modular chiplet architecture of coupled generalized flux qubits, and on a monolithic transmon architecture. The combination of tunability and gate speed establishes the LZ gate as a unique tool for multiplexing control pulses and interconnecting superconducting chiplet architectures.
For large-scale superconducting quantum technology, e.g. quantum computing, the homogeneity of wafer-scale superconducting thin films is vital for consistent performance of the fabricateddevices. Terahertz (THz) spectroscopy as a contactless and non-destructive measurement technique is a powerful tool to characterize the superconducting films. In this work, a set of niobium titanium nitride (NbTiN) thin films on 4-inch and 6-inch silicon wafers, grown via plasma-enhanced magnetron sputtering, are investigated via THz spectroscopy: full wafers are mapped at room temperatures and exemplary segments are characterized at cryogenic temperatures. The deviations in observed sheet resistance depend on the used deposition device and the film thickness. While the deviations in superconducting sheet kinetic inductance match those of the normal-state sheet resistance, the critical temperature and energy gap exhibit little variation. This THz mapping technique demonstrates the feasibility of evaluating wafer-scale superconducting thin films before lithography, facilitating preparation of the thin films for reproducible device fabrication.
High quality crystalline growth of a thin film on sapphire requires sufficient substrate preparation, often achieved via the use of aggressive chemical cleaning. Direct thermal reconstructionof the sapphire substrate via a CO2 laser beam may allow for an alternative way to prepare the substrate for epitaxy without the use of any chemical processing. Within this work, we demonstrate that thermal annealing of sapphire into its (31‾‾‾√×31‾‾‾√)R±9° reconstruction is a valid alternative preparation technique for sapphire substrates. TiN films grown via plasma-assisted molecular beam epitaxy upon these substrates exhibit greater crystallinity than those grown on chemically cleaned sapphire substrates. Superconducting resonators fabricated from these films exhibit similar performance, with many possessing internal quality factors at single photon levels greater than 106 for both substrate preparation methods.