R. G. Herb Condensed Matter Seminars

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Events During the Week of November 1st through November 7th, 2015

Monday, November 2nd, 2015

No events scheduled

Tuesday, November 3rd, 2015

No events scheduled

Wednesday, November 4th, 2015

No events scheduled

Thursday, November 5th, 2015

Detecting frequency fluctuations in mesoscopic oscillators
Time: 10:00 am - 11:00 am
Place: Chamberlin 5310
Speaker: Yaxing Zhang, Michigan State University
Abstract: Mesoscopic oscillators, such as nanomechanical resonators or optomechanical systems, usually experience comparatively large fluctuations due to their small size. An important type of fluctuations is frequency fluctuations, that is, the oscillator eigenfrequency is subject to a random perturbation in time. For nanomechanical resonators, frequency fluctuations can come from tension and mass fluctuations, fluctuating charges in the substrate, or dispersive inter-mode coupling; for electromagnetic cavity modes, they can come from fluctuations of dielectric constant. Frequency fluctuations lead to the broadening of oscillator linewidth. However, this broadening is partly masked by the broadening due to oscillator decay, making the identification of frequency noise difficult. In this talk, I will show that the interplay of a resonant driving and frequency noise can lead to specific features in the oscillator power spectrum that allow sensitive detection and quantitive characterization of frequency noise. I will start with a generic analysis of a harmonic oscillator with frequency noise, corroborated by measurements on a carbon nanotube resonantor [1]. Then I will discuss a particular type of frequency noise that comes from dispersive inter-mode coupling based on microscopic descriptions [2].

[1] Zhang, Y., Moser, J., Güttinger, J., Bachtold, A. & Dykman, M. I. Interplay of driving and frequency noise in the spectra of vibrational systems. Phys. Rev. Lett. 113, 255502 (2014).
[2] Zhang, Y. & Dykman, M. I. Spectral effects of dispersive mode coupling in driven mesoscopic systems. Phys. Rev. B 92, 165419 (2015).

Host: Vavilov
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Friday, November 6th, 2015

Meissner qubit - characterization and vortex-probing applications
Time: 11:00 am
Place: 5310 Chamberlin Hall
Speaker: Jaseung Ku , University of Illinois- Urbana Champaign
Abstract: I will present a new type of transmon split-junction qubit which can be tuned by Meissner screening currents in the adjacent superconducting film electrodes. The best detected relaxation time (T1) was of the order of 50 us and the dephasing time (T2) about 40 us. The achieved period of oscillation with magnetic field was much smaller than in usual SQUID-based transmon qubits, thus a strong effective field amplification has been realized. This Meissner qubit allows an efficient coupling to superconducting vortices. I will present a quantitative analysis of the radiation-free energy relaxation in qubits coupled to Abrikosov vortices. The observation of coherent quantum oscillations provides evidence that vortices can exist in coherent quantum superpositions of different position states. According to our suggested model, the wave function collapse is defined by Caldeira-Leggett dissipation associated with viscous motion of the vortex cores.
Host: McDermott
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