Dr Mustafa Bakr

Abstract:

Decoherence in superconducting quantum circuits, caused by loss mechanisms like material imperfections and two-level system (TLS) defects, remains a major obstacle to improving the performance of quantum devices. In this work, we present atomic-level characterization of cross-sections of a Josephson junction and a spiral resonator to assess the quality of critical interfaces. Employing scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy and electron-energy loss spectroscopy, we identify structural imperfections associated with oxide layer formation and carbon-based contamination, and correlate these imperfections to the patterning and etching steps in the fabrication process and environmental exposure. These results suggest that TLS imperfections at critical interfaces significantly contribute to limiting device performance, emphasizing the need for an improved fabrication process.

Abstract:

A novel compact implementation is introduced, providing contiguous multiplexing of channels with extreme bandwidth differences. It relies on the combination of the direct branching approach with the ‘all-resonator’ multiplexing method. It accommodates the implementation of nearly arbitrary filter functions for different channels, incorporating transmission zeros (TZs) to meet the rejection demands between contiguous channels with a reasonable filter order. The proposed approach has been applied to a contiguous Ka-band multiplexer design with 15.5% overall bandwidth, serving four channels with three different channel bandwidths: 11.7%, 2.1%, and two channels of 0.6% each. Accordingly, different filter functions are applied - namely, 12th-, 8th-, and 6th-order filters - each providing two TZs. The wideband channel is separated at the common branching from the other three channels, which are served by an adapted all-resonator multiplexing method. The final multiplexer design proves high RF performance with a very compact implementation.