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This Week at Physics

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Events During the Week of September 24th through October 1st, 2017

Monday, September 25th, 2017

R. G. Herb Condensed Matter Seminar
Spin-orbit interaction at the level of single electrons
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Dr. Andrea Hofmann, ETH Zurich
We measure the anisotropy of spin-orbit interaction (SOI) using real-time charge detection of single electrons tunneling between different states of GaAs/AlGaAs-based double quantum dots (DQDs). The strength of the SOI depends on the crystallographic direction of the electron tunneling, and on the relative alignment between the tunneling direction and the spin quantization axis. In the DQD, the tunneling direction is defined by the main axis of the device, and the spin quantization axis is chosen by the direction of an external in-plane magnetic field. This set-up allows us to control the strength of the spin-orbit interaction and leads to spin lifetimes of 10 s.

We fabricate two DQDs on a GaAs heterostructure, one with its main axis along the [110] crystal axis, and another one with the main axis rotated by 90 degrees, i.e. along [-110]. By applying suitable gate voltages to metallic top-gates, each DQD is brought into a configuration where two electrons reside in the device, and tunneling to the source and drain is suppressed. Using a charge detector, we distinguish between two resonant charge states: one state where both electrons reside in the right quantum dot, (0,2), and one state where each dot is occupied by a single electron, (1,1). We argue that in this configuration, the Pauli spin blockade can be used to measure the strength of the spin--orbit interaction experienced by tunneling electrons.

We use the two DQDs for measuring the different strengths of the SOI experienced by electrons moving along distinct crystallographic axes. We find that the SOI induces spin-flips for electrons moving along [110], and that the SOI vanishes for an electron moving along [-110]. For a given tunneling direction, we vary the strength of the experienced SOI by changing the alignment between the tunneling direction and spin-quantization axis by means of rotating the direction of the applied in-plane field. We find a sinusoidal dependence on the relative angle between the two directions.

A high magnetic field facilitates suppression of incoherent spin-relaxation processes within single dots. We measure the anisotropy of spin-flip tunneling rates between two energetically resonant quantum states in this setting and find that the spin--orbit interaction can be turned from on to almost completely off.
Host: Eriksson
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Plasma Physics (Physics/ECE/NE 922) Seminar
Reflection-driven Alfvén turbulence in the inner heliosphere in preparation for Parker Solar Probe observations
Time: 12:00 pm
Place: Chamberlin 2241
Speaker: Prof. Jean Carlos Perez, Florida Institute of Technology
Host: Stanislav Boldyrev
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Tuesday, September 26th, 2017

Chaos & Complex Systems Seminar
Intelligent extraterrestrial life: Does it exist? and, if so, what are the prospects for discovery and communication?
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Ed Churchwell, UW Department of Astronomy
Abstract: I will explain what I mean by intelligent life, review the high points about what is known about the evolution of "intelligent" life on Earth and apply some of what are thought to be global principles that are likely to govern the origin and evolution of life in the universe. In particular, I will spend some time on limitations to our prospects for discovery and communication. This is a very broad subject, and I certainly will not have time to cover all the issues associated with this subject, nor am I qualified to speak about many of them.
Host: Clint Sprott
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Theory Seminar (High Energy/Cosmology)
Weak gravity conjecture, Multiple point principle and standard model landscape
Time: 3:30 pm
Place: 5280 Chamberlin
Speaker: Yuta Hamada, University of Wisconsin-Madison & KEK Theory Center
Abstract: The requirement for an ultraviolet completable theory to be well-behaved upon compacti cation has been suggested as a guiding principle for distinguishing the landscape from the swampland. Motivated by the weak gravity conjecture and the multiple point principle, we investigate the vacuum structure of the standard model compacti ed on S^1 and T^2. The measured value of the Higgs mass implies, in addition to the electroweak vacuum, the existence of a new vacuum where the Higgs eld value is around the Planck scale. We explore two- and three-dimensional critical points of the moduli potential arising from compacti cations of the electroweak vacuum as well as this high scale vacuum, in the presence of Majorana/Dirac neutrinos and/or axions. We point out potential sources of instability for these lower dimensional critical points in the standard model landscape. We also point out that a high scale AdS_4 vacuum of the Standard Model, if exists, would be at odd with the conjecture that all non-supersymmetric AdS vacua are unstable. We argue that, if we require a degeneracy between three- and four-dimensional vacua as suggested by the multiple point principle, the neutrinos are predicted to be Dirac, with the mass of the lightest neutrino  O(1-10) meV, which may be tested by future CMB, large scale structure and 21cm line observations.
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Wednesday, September 27th, 2017

No events scheduled

Thursday, September 28th, 2017

R. G. Herb Condensed Matter Seminar
Dynamical control techniques with superconducting qubits
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Dr. Simon Gustavsson , MIT
Abstract: Dynamical error suppression techniques are commonly used to improve coherence in quantum systems. They reduce dephasing errors by applying control pulses designed to reverse erroneous coherent evolution driven by environmental noise. However, such methods cannot correct for irreversible processes such as energy relaxation (T1). In this work, we investigate a complementary, stochastic approach to reducing errors: instead of deterministically reversing the unwanted qubit evolution, we use control pulses to shape the noise environment dynamically. In the context of superconducting qubits, we implement a pumping sequence to reduce the number of unpaired electrons - quasiparticles - in close proximity to the device. We report a 70% reduction in the quasiparticle density, resulting in a threefold enhancement in qubit relaxation times, and a comparable reduction in coherence variability [1].
In a separate experiment, we investigate qubit dephasing (T2) due to photon shot noise in a flux qubit transversally coupled to a coplanar microwave resonator. Due to the AC Stark effect, photon fluctuations in the resonator cause frequency shifts of the qubit, which in turn lead to dephasing. While this is universally understood, we have made the first quantitative spectroscopy of this noise for both thermal (i.e., residual photons from higher temperature stages) and coherent photons (residual photons from the readout and control pulses). By mapping out the noise power spectral density seen by the qubit, we uniquely identify thermal shot noise as the dominant source of dephasing. When implementing the CPMG dynamical-decoupling protocol, we are able mitigate to the adverse influence of the photon shot noise, and improve T2 Echo ~ 40 us to reach T2 CPMG ~ 80 us ~ 2*T1. Furthermore, by improving the filtering for thermal noise in a subsequent cooldown, we are able to reduce the residual photon population to 0.0004, resulting in T2 echo times approaching 100 us [2].

[1] Science 354, 1573 (2016)
[2] Nature Communications 7, 12964 (2016)
Host: McDermott
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Friday, September 29th, 2017

Graduate Introductory Seminar
Plasma Physics
Time: 12:00 pm
Place: Chamberlin Hall
Speaker: Boldyrev, Egedal, Forest, Sarff, Terry, Zweibel
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