R. G. Herb Condensed Matter Seminars |
Events During the Week of January 11th through January 18th, 2015
Monday, January 12th, 2015
- Studies of Correlations in Low-Disorder 2D Electron Gases in MBE-Grown GaAs
- Time: 10:00 am
- Place: 5310 Chamberlin Hall
- Speaker: John Watson, Purdue
- Abstract: GaAs/AlGaAs heterostructures have served as one of the most ideal material systems for studies of electron interactions in mesoscopic physics for the past three decades. The high quality of the 2D electron gases that can be generated in these structures has enabled a wide range of research in topics such as the search for non-Abelian states of matter in the fractional quantum Hall regime and scaleable spin-based quantum computation. In this talk, I will present my work on advancing the state-of-the-art in GaAs materials and devices. In particular, I will discuss advances our group has made to reach low temperature electron mobilities as high as 32 x 106 cm2/Vs and the impact this has had on work in the quantum Hall regime and with spin qubits. In addition, I will discuss progress with density-tuneable devices intended to search for phase transitions in the fractional quantum Hall state at ν = 5/2. Finally, I will present initial results from our work to improve the charge stability of devices useful for spin-based qubits.
- Host: Eriksson
Tuesday, January 13th, 2015
- No events scheduled
Wednesday, January 14th, 2015
- The pumpistor: understanding the parametrically flux-pumped SQUID by its electrical impedance
- Time: 10:00 am
- Place: 5310 Chamberlin Hall
- Speaker: Kyle Sundqvist , Texas A&M
- Abstract: Parametric amplifiers based on superconducting circuits have experienced recent popularity. It is possible to produce superconducting circuits which may sustain and amplify coherent states of microwaves close to the quantum limit. To this end, we describe a circuit understanding of the flux-pumped Superconducting QUantum Interference Device (SQUID). An unpumped SQUID acts as an inductance, the Josephson inductance, whereas a flux-pumped SQUID develops an additional, parallel element which we have coined the "pumpistor.'' Parametric gain can be understood as a result of a negative resistance of the pumpistor. In the degenerate case, the gain is sensitive to the relative phase between the pump and signal. In this case, a phase-sensitive impedance provides a classical analogy to quantum squeezing found in our experiments.
Conversely, in the nondegenerate case, gain is independent of phase. Here the pump frequency is not a multiple of the signal frequency, and it becomes necessary to consider idler tones. For the nondegenerate three-wave case, we present an intuitive picture for a parametric amplifier containing a flux-pumped SQUID where current at the signal frequency depends upon the load impedance at an idler frequency. This understanding has recently lead to the experimental realization of a SQUID parametric amplifier with strong environmental coupling, allowing for substantially increased bandwidth and dynamic range.
The use of equivalent impedances offers insights not always apparent from the Hamiltonian equations of motion. In particular, our "pumpistor" impedance models immediately provide readily testable predictions for many other circuits containing flux-pumped SQUIDs. - Host: Sue Coppersmith
Thursday, January 15th, 2015
- No events scheduled
Friday, January 16th, 2015
- No events scheduled