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Quantum memory and random number generation in a trapped ion system
Kihwan Kim (Kihwan Kim Center for Quantum Information, Institute for Interdisciplinary Information Sciences, Tsinghua University, Beijing)

ABC Physics (Atomic/Bio/Condensed Matter) Special Seminar

Date: Friday, August 11, 2017 11:30 AM
Location: PAT C-520

Abstract
In this seminar, Iʼd like to discuss two experimental results that recently have been performed in our trapped ion system. Firstly, I present the observation of coherence time over 10 minutes for a single qubit system in a single ion. For the ensemble of qubits, there have been reported about ten minutes and hours of coherence time in trapped ion systems [1] and in solid state systems [2], respectively. For a single qubit, however, the longest coherence time reported is less than one minute [3], which was realized in trapped ion systems, to our knowledge. The main limitation of the coherence time in the trapped ion system comes from the heating of the ion without cooling. We overcome the limit by realizing a hybrid trapped ion system, which composes of qubit and cooling ion. Secondly, we demonstrate the protocol of exponential expansion of randomness certified by quantum contextuality in a trapped ion system. The intrinsic unpredictability of measurements in quantum mechani! cs can be used to produce genuine randomness. Recently, randomness expansion protocols based on inequality of Bell-text and Kochen-Specker (KS) theorem [4], have been demonstrated. These schemes have been theoretically innovated to exponentially expand the randomness and amplify the randomness from weak initial random seed. Here, we report the experimental evidence that the randomness can be exponentially expanded with the certification by KS theorem, in particular, the Klyachko-Can-Binicioglu-Shumovsky (KCBS) inequality. In the experiment, we use three states of a 138Ba+ ion between a ground state and two quadrupole states. 

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[2] Kamyar Saeedi, et al., Science 342, 830 (2013); Manjin Zhong, et al., Nature 517, 177 (2015). 
[3] C. Langer, et al., Phys. Rev. Lett. 95, 060502 (2005); T. P. Harty, et al., Phys. Rev. Lett. 113, 220501 (2014). 
[4] S. Pironio, et al., Nature 464, 1021 (2010); Mark Um, et al., Sci. Rep. 3, 1627 (2013).