Enhancing quantum simulators with neural networks


Wednesday, December 18th 2019, 04:00 PM, MAPTC/0G/017

Speaker: G. Torlai (Flatiron Institute)

The recent advances in qubit manufacturing and coherent control of synthetic quantum matter are leading to a new generation of intermediate-scale quantum hardware, with promising progress towards simulation of quantum matter and materials. In order to enhance the capabilities of this class of quantum devices, some of the more arduous experimental tasks can be off-loaded to data-driven classical algorithms running on conventional computers. I will review recent efforts in deploying machine learning on data generated by cold-atom quantum simulators and superconducting quantum circuits.

Combining experiments and modeling in defect studies with positron annihilation


Wednesday, December 11th 2019, 04:00 PM, Bell Lecture Theatre

Speaker: I. Makkonen (University of Helsinki)

Positron annihilation spectroscopy is a method sensitive to charge neutral and negative open volume defects in solids. The decreased electron density in a vacancy manifests itself as an increase of positron lifetime and the narrowing of the-511 keV photo peak in the annihilation gamma spectrum, compared to a defect free crystal. Positron lifetime spectroscopy provides information on the atomic structure, the charge state and often the concentration of the vacancies while the Doppler broadening of the photopeak enables the identification of atoms surrounding the vacancy defects. The combination of positron annihilation spectroscopy and supporting first principles modeling of the measured annihilation parameters has been efficiently used to identify and quantify technologically important vacancy related defects in, for example, group IV semiconductors, III nitrides and ZnO [1]. In best cases, thorough experimental work combined with simulations can provide a very detailed picture of the structural and chemical identities of vacancy defects detected [2].

In this talk, I will discuss the experimental technique and how it can be complemented with density-functional-based modeling of defect structures, positron states and associated annihilation parameters. I will also discuss our recent progress in the development of accurate quantum Monte Carlo techniques to describe electron-positron interactions, positron annihilation rate and the momentum density of annihilating pairs in realistic periodic solids [3].

[1] F. Tuomisto and I. Makkonen, Reviews of Modern Physics 85 (2013) 1583.

[2] C. Rauch, I. Makkonen and F. Tuomisto, Physical Review B 84 (2011) 125201; K. M. Johansen, A. Zubiaga, I. Makkonen, F. Tuomisto, P. T. Neuvonen. K. E. Knutsen, E. V. Monakhov, A. Y. Kuznetsov, and B. G. Svensson, Physical Review B 83 (2011) 245208; F. Tuomisto, V. Prozheeva, I. Makkonen, T. H. Myers, M. Bockowski, and H. Teisseyre, Physical Review Letters 119 (2017) 196404.

[3] K. Simula, I. Makkonen, N. D. Drummond et al. (unpublished).

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