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This Week at Physics

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Events on Tuesday, March 5th, 2013

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
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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
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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.
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