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Events on Wednesday, January 14th, 2015

R. G. Herb Condensed Matter Seminar
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
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