Project Team

Archan Banerjee, Ahmed Hajr, Cassidy Berk, Frank Ogletree, Maria Altoe


Mohammed Alghadeer

Start Date

June 2019


In Process

Relevant Publications

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We started by fabricating and characterizing state-of-the-art niobium based superconducting CPW resonators with single photon internal quality factors (Q_i) of 〖~1×10〗^6 (figure on the right). The power and temperature dependence of the quality factor, resonance frequency shift, and noise spectrum were explained in the framework of TLSs and residual quasiparticles. We also carried out various surface treatments (post processing chemical treatments and in-situ surface passivation) to mitigate the interface loss with improvement in Q_i up to a factor of 4. Several material characterization tools were used to analyze the samples including TEM, XPS, X-ray EDS mapping and EELS and the results provided insight into the chemistry and structure of the surface oxides which alongside measurements of the device dynamics elucidated the oxides’ role in limiting the performance. Finally, we implemented some finite-element simulations using HFSS and COMSOL to optimize our chip to achieve critical coupling and implemented changes in the resonators geometry to study the contribution of different interfaces to the TLS loss

So far there are two phenomenological physical mechanisms which are believed to be the dominant sources of loss in superconducting resonators. The first is due to a bath of two-level systems (TLSs) which couple to the electromagnetic field and are thought to reside mostly in the oxide layers. The second stems from a modification of the surface resistance due to quasiparticles within the superconductor. Unfortunately, neither of those mechanisms is well-understood in terms of its theoretical description and physical origin. So, one of the experimentally challenging tasks would be to separate the influence of each mechanism on the device performance and obtain a large set of data to help guide the theoretical models.