AVS 70 Session QS-ThP: Quantum Science and Technology Mini-Symposium Poster Session

Transmon qubits are one of the leading technologies for quantum information science (QIS) with fabrication and characterization techniques accessible by numerous laboratories around the world.In recent years, efforts at coherence time optimization have shifted focus to the performance of the underlying superconducting material and accompanying surface oxides.While earlier devices were fabricated from Al and Nb, the current state-of-the-art transmons are fabricated from Ta with coherence times up to 0.5 milliseconds.Our research focuses on Ta coplanar waveguide (CPW) resonators fabricated by a damascene technique which yields pristine metal structures entrenched in silicon substrates.While the participation ratio of the dielectric substrate is increased, the device edges remain oxide free from the entrenchment and a nitride capping layer.Our recent studies revealed a current best Q value on the order of 105 at low power.This work, performed in collaboration with NY CREATES/SUNY Poly and BNL, will be discussed along with the prospect of further improvements.

This poster covers a bit of metrology history of how we got to where we are today and gives a forward-looking vision for the future of measurement science and its important role in our daily lives. The role of NIST as a National Metrology institute (NMI) is briefly described considering the world-wide redefinition of units that occurred on May 20^th, 2019. The re-definition of units is now aligned with physical constants of nature and fundamental physics which opens new realization routes with quantum-based sensors and standards. The NIST on a Chip program (NOAC) is briefly introduced in this context.

The technical core is a deeper dive into research on measurement methods for pressure, the Fixed Length Optical Cavity (FLOC) and for vacuum the Cold Atom Vacuum Standard (CAVS). What is exciting about many of these new measurement approaches is that they are both primary (relying on fundamental physics), are quantum-based and use photons for the measurement readout which is key for taking advantage of the fast-growing field of photonics. The FLOC will enable the elimination of mercury barometers pressure standards worldwide and the CAVS will be first primary standard for making vacuum measurements below 1.3x10^-5 Pa. A Sensitive Photonic Thermometer (SPOT) will be introduced, along with other sensing technologies that the NOAC program is currently investigating such as flux, magnetic field, and mass and torque.

Superconducting quantum materials research is increasingly focused towards the fabrication of robust quantum processors to enable practical quantum computation. As these processors scale in size and complexity, they become more susceptible to various loss mechanisms, with material-related losses being a major source of decoherence. Traditional fabrication methods, such as evaporation and sputtering, have shown to introduce defects in thin metallic films, as well as lossy and noisy dielectrics. Advances in fabrication techniques and the exploration of new materials are necessary to develop qubits that are more robust to decoherence. Achieving this requires exploring various material growth techniques in a controlled manner, guided by real-time feedback from in situ characterization. Additionally, employing novel fabrication techniques that allow simultaneous application of multiple growth techniques without breaking vacuum are crucial.

In this context, we present the use of a cluster deposition and characterization system at Berkeley Lab’s Molecular Foundry user facility to advance the exploration and optimization of quantum materials. The integrated system combines multiple deposition tools — including e-beam evaporation (deposition, oxidation and ion milling), reactive sputtering and nitride and oxide ALD chambers—with a suite of characterization systems including XPS, ellipsometry, optical emission spectroscopy and Auger spectroscopy. The system also includes a glove box for inert sample loading and unloading, and automated wafer transfer integrated in a unified vacuum system. By facilitating multi-deposition processes, this setup opens up new possibilities for creating novel materials that were previously unattainable. The in situ characterization capability during growth provides a deeper understanding of growth mechanisms and defect control, facilitating rapid feedback and maintaining pristine interfaces between device layers.

We demonstrate the capabilities of this system through the optimization and integration of Josephson junctions incorporating different material stacks and superconducting nitrides (NbN, TiN, and NbTiN) backed up by electrical and material characterization techniques. We will discuss the ongoing and future directions for device development, highlighting how this cluster tool system, along with the characterization suite at the Molecular Foundry, provides an in-depth platform for materials growth and exploration.