Events at Physics
Events During the Week of March 3rd through March 10th, 2013
- Cosmology Journal Club
- An Informal discussion about a broad variety of arXiv papers related to Cosmology
- Time: 12:30 pm
- Place: 5242 Chamberlin Hall
- Abstract: Please visit the following link for more details:
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 Le Zhang (firstname.lastname@example.org)
- Host: Peter Timbie
- R. G. Herb Condensed Matter Seminar
- Analysis of high-fidelity gate design and error thresholds for fault-tolerant superconducting quantum computing architectures
- Time: 10:00 am
- Place: 5280 Chamberlin Hall
- Speaker: Joydip Ghosh, University of Georgia
- Abstract: Quantum computing with superconducting elements promises scalability and is widely regarded as a viable approach to develop a fault-tolerant architecture of a candidate quantum computer. In this talk, I first discuss our recent proposal to design high-fidelity controlled-σz (CZ) operations using only DC bias control and then explore the performance of various existing superconducting surface code based architectures under a realistic multi-parameter error model. Assuming phase or transmon qubits and using only low frequency qubit-bias control, our CZ operation exhibits threshold fidelity (intrinsic) with a realistic two-parameter pulse profile. In addition we have an analytic model that estimates the fidelities of CZ gates as a function of various pulse parameters as well as quantifies the error due to any perturbation over an optimal pulse shape. Next we consider a realistic, multi-parameter error model and investigate the performance of the surface code for three possible fault-tolerant superconducting architectures. We map amplitude and phase damping to an asymmetric depolarization channel via the Pauli twirl approximation, and obtain the logical error rate as a function of the qubit coherence time, intrinsic state preparation and gate and readout errors. A numerical Monte Carlo simulation is performed to obtain the logical error rates and a leading order analytic model is constructed to estimate their scaling behavior below threshold. Our results suggest that large-scale fault-tolerant quantum computation should be possible with existing superconducting devices.
- Host: Friesen & Coppersmith
- Chaos & Complex Systems Seminar
- How are complex adaptations built? Using cultural and genetic convergence to understand evolving systems
- Time: 12:05 pm
- Place: 4274 Chamberlin (Refreshments will be served)
- Speaker: John Hawks, UW Department of Anthropology
- Abstract: Adaptation by natural selection is a genetically heterogeneous process. Some adaptive phenotypes are the result of simple genetic changes under positive natural selection. But some adaptive phenotypes are more complex, requiring changes to a network of interacting genes, possibly in addition to gene-environment interactions. Is there any general process by which such complex adaptations can be understood, or are they a simple stochastic accumulation of simple changes? The record of recent human evolution provides a wealth of cases of genetic and cultural changes that have unfolded convergently in different populations. Genetic adaptation to new pathogens, new diets and new physical environments allows us to probe the networks of genetic interactions and the timing of changes on multiple human genes. Cultural adaptation to new diets and modes of social organization also allow us to examine how evolutionary dynamics may constrain the path taken by complex adaptations. I lay out a research agenda that distinguishes functional networks from evolutionary networks, giving us a way to discuss the origins of complexity through evolutionary time.
- Host: Sprott
- Theory Seminar (High Energy/Cosmology)
- A Metastable Vacuum and the Higgs to Diphoton Rate
- Time: 4:00 pm
- Place: 5280 Chamberlin Hall
- Speaker: Stefania Goris, University of Chicago
- Abstract: We analyze the implications for New Physics theories of an enhanced Higgs to di-photon rate. We show that models predicting a sizable enhancement have generically an electroweak vacuum that is not absolutely stable. In particular we discuss the only viable scenario, in the framework of the MSSM, that can predict sizable New Physics effects in the di-photon rate with all the other Higgs rates SM like: a scenario with light and highly mixed staus. We conclude with the phenomenology of the model and with the prospects of probing it at the LHC, through a multilepton signature.
- No events scheduled
- Special Seminar
- The two sides of a semiclassical spin
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Feifei Li, Northwestern University
- Abstract: A spinning electron and a spinning gyroscope represent the two ultra limits that spin can behave. One is purely quantum, while the other is purely classical. In this talk, I would like to discuss what happens for a semiclassical spin with intermediate magnitude of angular momentum. My talk consists two parts. In the first half, I would like to tell a story about the single-molecular-magnet Fe8. Fe8 is a molecule made of about a hundred atoms, yet it behaves like a single giant spin of J = 10 at low temperatures. Quantum interference causes the tunneling gap of this molecule to oscillate with applied magnetic field and to vanish at certain magnitude and direction of the magnetic fields, known as diabolical points. My story is about how these diabolical points were discovered, missed and rediscovered. The second half of my talk will focus on the quantum-classical correspondence for spin. The quantum-classical correspondence for a particle has been formulated by Moyal, who in a seminal paper, showed that quantum mechanics can be expressed as a quasi-statistical theory in the phase space of coordinate and momentum. Moyal's formalism unified Weyl ordering and Wigner quasi-distribution function, providing an invertible map between dynamical variables on the classical phase space and operators on the quantum mechanical Hilbert space. Moyal has also shown that the commutator of two operators is the Poisson bracket to leading order of $hbar$. All this was done for position and momentum. Here I present a Moyal treatment for spin, and show that, in the classical limit, the Weyl symbol for a spin commutator is i times the Poisson bracket of the corresponding Weyl symbols.
 Feifei Li and Anupam Garg, Numerical search for diabolical points in the energy spectrum of the single-molecule magnet Fe8, Phys. Rev. B 83, 132401 (2011).
 José E. Moyal, Quantum Mechanics as a Statistical Theory, Proc. Cambridge Philos. Soc. 45, 99 (1949).
 Feifei Li, Carol Braun, and Anupam Garg, The Weyl-Wigner-Moyal Formalism for Spin, arXiv:1210.4075v2 (2012).
- Host: Friesen & Coppersmith
- Astronomy Colloquium
- Stellar Mergers and Interactions: Yes, Virginia, Stars Do Collide.
- Time: 3:30 pm
- Place: 4421 Sterling Hall
- Speaker: Alison Sills, McMaster University
I will discuss strong interactions between stars in a variety of
environments. Despite the vast (average) interstellar distances, stars are social creatures and tend to live in pairs, multiples, or groups. Under these circumstances, stars can, and do, modify each other's mass, radius, composition, and overall evolution through gravitational encounters ranging from wind mass transfer in a binary system to complete stellar collisions and mergers. I will show how such events can change our understanding of particular stellar systems, how they can explain the properties of many unusual objects, and how interactions could change the environment these stars live in. The emphasis for this
talk will be on the modelling of these interactions, and I will
demonstrate how a combination of stellar evolution, stellar dynamics, and hydrodynamics can bring some understanding to these complicated systems.
- Host: Robert Mathieu
- Physics Department Colloquium
- Meeting the Energy needs of a Growing Population
- Time: 3:30 pm
- Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
- Speaker: Timothy Donohue, UW-Madison Dept. of Bacteriology & Director, Great Lakes Bioenergy
- Abstract: In 2007, UW-Madison was named as home of the Great Lakes Bioenergy Research Center (GLBRC), a 25M/year DOE-funded cellulosic biofuels research center. Great Lakes Bioenergy conducts basic, genomics-based research to design the microbial and plant systems needed to produce cellulosic biofuels. Combining innovative science, a critical mass of natural assets and the corporate horsepower to build and advance a new bioenergy economy, Great Lakes Bioenergy has become a worldwide center of excellence for research and development of cellulosic biofuels and co-products. GLBRC activities are led by experts in plant biology, microbiology, molecular or cell biology, biochemistry, protein design, engineering, computer sciences, systems analysis, and ecology. Great Lakes Bioenergy is part of the Wisconsin Energy Institute, a State, UW-Madison and private sector partnership to make advances needed to provide a suite of renewable energy systems.
- Host: Forest