We have been developing superconducting devices using novel approaches to making tunnel barriers. Traditionally, tunnel junctions use amorphous, thermal oxide barriers because their thickness is controllable and they tend to be pin-hole free. However, they have energy-absorbing defects at random frequencies due to their amorphous nature. We developed a process to grow epitaxial tunnel junctions. This is challenging because the tunnel current depends exponentially on the barrier thickness, and epitaxial barriers require high temperatures to crystallize. We worked around these challenges using precision, ultra-high vacuum molecular-beam epitaxy (MBE) growth and lattice-matched, superconducting rhenium films on sapphire substrates. We are able to fabricate, characterize, and successfully integrate them into devices, the performance of which devices have improved significantly. We recently developed a new type of tunnel barrier: the overlap tunnel junction. It is superior to junctions made by shadow evaporation because it is more scalable to smaller dimensions. We are making readout resonators using Nb-Ti-N and Ti-N on Si.
Qubits
Summary
The Qubits Project develops new geometries for superconducting quantum binary devices as fundamental building blocks for quantum computing.
Description
Major Accomplishments
1) Superconducting qubit development and spectroscopy – Superconducting qubits and their applications is one of our main focuses. In this area, we have developed high quality factor materials and radiation suppression techniques that allow for reliable T1 times on the order of 30 – 40 us. We have developed measurement infrastructure of large arrays of qubit and are currently testing adiabatic state transfer, dark state transfer, and novel gates. These devices are designed using combinations of readout-resonating circuits, capacitors, inductors, and superconducting Josephson junctions. The interactions between these elements is important because it allows us to extract the quantum information from the qubits. Below is an image of the susceptibility of a qubit that is strongly coupled to a resonant cavity. From these spectra we were able to completely describe the system in terms of it's total quantum state, successfully predicting the transition frequencies. For example the energy difference associated with the transitions 4-7, 1-3, 2-4, and 0-1 are indicated in the figure.
Radiation-suppressed superconducting quantum bit in a planar geometry, M. Sandberg, M. R. Vissers, T. A. Ohki, Jiansong Gao, J. Aumentado, M. Weides, D. P. Pappas, Appl. Phys. Lett. 102, 072601 (2013).
Detailed modelling of the susceptibility of a thermally populated, strongly driven circuit-QED system, Anton Frisk Kockum, Martin Sandberg, Michael R Vissers, Jiansong Gao, Goran Johansson and David P Pappas, J. Phys. B: At. Mol. Opt. Phys. 46, 224014 (2013), article featured on the cover.