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This Week at Physics

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Events During the Week of November 27th through December 4th, 2016

Monday, November 28th, 2016

R. G. Herb Condensed Matter Seminar
High-fidelity entangling gate for double-quantum-dot spin qubits
Time: 11:00 am
Place: 5310 Chamberlin Hall
Speaker: John Nichol, University of Rochester
Abstract: Electron spins in semiconductors are promising qubits, because their long coherence times enable nearly a billion coherent quantum gate operations. However, developing a scalable high-fidelity two-qubit gate remains challenging. We discuss a new entangling gate between two double-quantum-dot spin qubits in GaAs, which uses a magnetic field gradient between the two dots in each qubit to suppress decoherence due to charge noise. When the magnetic gradient dominates the voltage-controlled exchange interaction between electrons, qubit coherence times increase by an order of magnitude. Using randomized benchmarking, we measure single-qubit gate fidelities of approximately 99%, and through self-consistent quantum measurement, state, and process tomography, we measure an entangling gate fidelity of 90%. In the future, operating double quantum dot spin qubits with large gradients in nuclear-spin-free materials, such as Si, should enable a two-qubit gate fidelity surpassing the threshold for fault tolerant quantum information processing.
Host: Eriksson
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Plasma Physics (Physics/ECE/NE 922) Seminar
TBD
Time: 12:00 pm
Place: 2241 Chamberlin Hall
Speaker: Alex Ivanov
Host: Cary Forest
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Tuesday, November 29th, 2016

Chaos & Complex Systems Seminar
The chemistry of primes
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Melanie Matchett Wood, UW Department of Mathematics
Abstract: We are familiar with the prime numbers as those integers which cannot be factored into smaller integers, but if we consider systems of numbers larger than the integers, the primes may indeed factor in those larger systems. We discuss various questions mathematicians ask about how primes may factor in larger systems, talk about both classical results and current research on the topic, and give a sense of the kind of tools needed to tackle these questions.
Host: Clint Sprott
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Council Meeting
Council Meeting
Time: 1:00 pm
Place: 2314 Chamberlin hall
Speaker: Albrecht Karle
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Theory Seminar (High Energy/Cosmology)
Title to be announced
Time: 3:30 pm
Place: 5280 Chamberlin
Speaker: Angelo Monteux, Rutgers University
Abstract: TBA
Host: Yang Bai
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Wednesday, November 30th, 2016

Department Meeting
Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin hall
Speaker: Albrecht Karle
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Thursday, December 1st, 2016

R. G. Herb Condensed Matter Seminar
Controlling Spin Qubits in Diamond with a Mechanical Resonator
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Evan MacQuarrie, Cornell University
Abstract:
The spin state of the nitrogen-vacancy (NV) center in diamond offers a promising platform for the development of quantum technologies and investigations into spin dynamics at the nanoscale. With lengthy coherence times even at room temperature, NV centers present one path towards quantum information in the solid state and enable precision metrology with atomic scale spatial resolution. The NV center spin state can be coherently manipulated with resonant magnetic fields, electric fields, or, at cryogenic temperatures, optical fields. Here, we demonstrate direct mechanical control of an NV center spin by coherently driving magnetically-forbidden spin transitions with the resonant lattice strain generated by a bulk-mode mechanical resonator [1,2]. We then employ this mechanical driving to perform continuous dynamical decoupling and extend the inhomogeneous dephasing time of a single NV center spin [3]. Finally, we experimentally demonstrate that a spin-strain coupling exists within the NV center room temperature excited state and theoretically analyze a dissipative protocol that uses this newly discovered coupling to cool a mechanical resonator [4]. The methods of mechanical spin control developed here unlock a new degree of freedom within the NV center Hamiltonian that may enable new sensing modes and could provide a route to diamond-mechanical resonator hybrid quantum systems.


[1] E. R. MacQuarrie, et al, Phys. Rev. Lett. 111, 227602 (2013).

[2] E. R. MacQuarrie, et al, Optica 2, 233 (2015).

[3] E. R. MacQuarrie, et al, Phys. Rev. B 92, 224419 (2015).

[4] E. R. MacQuarrie, et al, arXiv:1605.07131 (2016).
Host: Eriksson
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Cosmology Journal Club
An Informal discussion about a broad variety of arXiv papers related to Cosmology
Time: 12:15 pm
Place: 5242 Chamberlin Hall
Abstract: Please visit the following link for more details:
http://cmb.physics.wisc.edu/journal/index.html
Please feel free to bring your lunch!
If you have questions or comments about this journal club, would like to propose a topic or volunteer to introduce a paper, please email Amol Upadhye (aupadhye@wisc.edu).
Host: Amol Upadhye
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
STUDIES OF GALACTIC COSMIC RAYS WITH DIRECT AND INDIRECT MEASUREMENTS
Time: 2:30 pm
Place: 5280 Chamberlin hall
Speaker: Nahee Park, University of Chicago
Abstract: Cosmic rays, high energy particles originating from outside of the solar system, are believed to be dominated by particles from our Galaxy at least up to the energy of 10^15 eV. Recent results from direct measurements of cosmic rays, including the rise of the positron flux, the hardening of the light nuclei, and the different spectral indexes of the proton and helium spectra, challenge the classical models of the Galactic cosmic rays. Meanwhile, the development of gamma-ray experiments has opened a new window to study the acceleration and propagation of high-energy particles in the vicinity of the source sites, such as supernova remnants.

I will present the Galactic gamma-ray measurements from the VERITAS experiment, an imaging atmospheric Cherenkov telescope measuring gamma rays with energies higher than 85 GeV and up to ~ 30 TeV. Focusing on the supernova remnants, I will discuss what we have learned about the acceleration of high-energy particles with gamma-ray observations. I will also introduce the near-future balloon-borne experiment, HELIX (High Energy Light Isotope eXperiment), which is designed to measure the clock isotope 10^Be up to 10 GeV/n to study the propagation of Galactic cosmic rays. Finally, I will highlight how measurements from different disciplines, such as cosmic-ray and gamma-ray astrophysics, will broaden our perspectives on high-energy particles and advance us towards a new paradigm of Galactic cosmic rays.
Host: Stefan Westerhoff
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Astronomy Colloquium
Do High Equivalent Width LAEs Exist in the Local Universe? Insights from a Flux-Limited GALEX LAE sample at z~0.3
Time: 2:30 pm
Place: 4421 Sterling Hall, NOTE EARLIER TIME 2:30 PM
Speaker: Isak Wold, University of Texas - Austin
Abstract: Observational surveys of Lya emitters (LAEs) have proven to be an efficient method to identify and study large numbers of galaxies over a wide redshift range. To understand what types of galaxies are selected in LAE surveys - and how this evolves with redshift - it is important to establish a low-redshift reference sample that can be directly compared to high-redshift samples. The lowest redshift where a direct Lya survey is possible is at a redshift of z~0.3 via the Galaxy Evolution Explorer (GALEX ) FUV grism data. Using the z~0.3 GALEX sample as an anchor point, it has been suggested that at low redshifts high equivalent width (EW) LAEs become less prevalent and that the amount of escaping Lya emission declines rapidly. A number of explanations for these trends have been suggested including increasing dust content, increasing neutral column density, and/or increasing metallicity of star-forming galaxies at lower redshifts. However, the published z~0.3 GALEX sample is pre-selected from bright NUV objects. Thus, objects with strong Lya emission but faint continuum (high-EW LAEs) could be missed. In this talk, I will present my efforts to re-reduce the deepest archival GALEX FUV grism data and obtain a sample that is not biased against high-EW LAEs. I will discuss the implications of this new sample on the evolutionary trends listed above.
Host: Amy Barger
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Friday, December 2nd, 2016

Faculty Search Committee Meeting (PVL 88215)
Time: 10:00 am
Place: 2314 Chamberlin Hall
Abstract: The Neutrino/Astro/Cosmo search committee will meet to review applications for the tenure-track position of Assistant Professor
Host: Sridhara Dasu
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Physics Department Colloquium
Neutrinos from the Sky and Through the Earth
Time: 3:30 pm
Place: 2241 Chamberlin hall
Speaker: Kate Scholberg, Duke
Abstract: The progress in neutrino physics over the past fifteen years has been<br>
tremendous: we have learned that neutrinos have mass and change<br>
flavor. This discovery won the 2015 Nobel Prize. I will pick out one<br>
of the threads of the story-- the measurement of flavor oscillation in<br>
neutrinos produced by cosmic ray showers in the atmosphere, and<br>
further measurements by long-baseline beam experiments. In this talk,<br>
I will present the latest results from the Super-Kamiokande and T2K<br>
(Tokai to Kamioka) long-baseline experiments, and will discuss how the<br>
next generation of high-intensity beam experiments will address some<br>
of the remaining puzzles.
Host: Baha Balantekin
Presentation: ks_photo.pdf
Video: https://vod.physics.wisc.edu/media/2016_12_02.m4v
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