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This Week at Physics

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Event Number 3500

  Thursday, November 13th, 2014

R. G. Herb Condensed Matter Seminar
Generalized Tunneling Model (GTM) for Two Level System (TLS) in amorphous materials and its predictions for their dephasing and the noise in superconducting microresonators
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Lara Faoro, Universites Paris 6 et 7 and Rutgers University
Abstract: Thin film high quality superconductor microresonators are important for a number of diverse applications that range from quantum computation to submillimeter and far-infrared astronomy. The performance of these devices has improved dramatically over the past decades and resonator quality factors above 10^6 are now routinely achieved using single-layer structures deposited on high-quality low-loss crystalline substrate. Achieving high-quality factors requires minimizing all potential sources of dissipation and noise. One prominent source of dissipation in microresonators has been found to be Two Level Systems (TLSs) in thin amorphous dielectric surface layers of the microresonators. These TLS are also responsible for an excess frequency noise (jitter) in the resonators. TLS in amorphous materials are usually described by the Standard Tunneling Model (STM). In this theory, TLS are represented as independent quantum two level systems with constant distribution of low energy states. Unfortunately, STM fails to explain the data of noise in the superconducting microresonators so a consistent model for the noise due to TLS in superconducting resonators remains an open issue.
In this talk we shall first review data of recent experiments performed on high quality superconducting microresonator where low frequency noise spectra have been studied in varying temperature and different applied power and argue that the data are inconsistent with the STM. We shall then propose a new model, the Generalized Tunneling Model (GTM) which includes strong interactions between TLS and a slow power law dependence of their density of states. We show that the predictions of this model are in a perfect agreement with the recent studies of the noise in high quality superconducting resonators. The predictions also agree with the temperature dependence of the TLS dephasing rates observed in phase qubits. Finally, we discuss the origin of the universal dimensionless parameter that controls the interaction between TLS in glasses and show that it is consistent with the assumptions of the GTM.
Host: McDermott
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