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Seminars
Seminar on Theoretical and Computational Physics:
 Title  Geometry of EinsteinPodolskyRosen correlations


Speaker  Assoc. Prof. Nguyen Huy Viet


Affiliation  Center for Computational Physics, Institute of Physics


Date  Tuesday, 11062019


Time  10:00 AM


Location  Meeting room, 6th floor, 2H Building, 18 Hoang Quoc Viet, Hanoi


Abstract  Spooky action at a distance" is Einstein’s famous phrase about one of the strangest phenomena in quantum physics: if measurements are performed on a particle at one place, certain properties of another particle at a distanceeven lightyears awaycan be changed instantly as if there must be a mysterious connection between them. In the words of Schrödinger, who regarded this phenomenon as being 'magic', we often say that one can 'steer' the state of a particle from a distance. While it is not possible to transfer information instantaneously with this steering, it still deeply disturbs the intuition of locality in physics, and has puzzled physicists for years. In fact, Einstein, together with Podolsky and Rosen (EPR), used this effect to argue against the completeness of quantum mechanics as a fundamental theory of nature in their famous paper in 1935 [1].
In EPR and Schrödinger’s analysis, only the very idealized situation where the paired particles are in the socalled pure states was considered, which in fact cannot be realized in experiment. Away from this ideal case, quantum effects are often weaker or even vanishing. Does the magic steering effect exist in realistic situations? And if yes, when can we observe this magic steering and when we cannot ? The answer to the first question came out a decade ago in a breakthrough discovery by Wiseman, Jones and Doherty, in which the notion of quantum steering was precisely defined so that one can still consider steering as a genuinely quantum mechanical effect for realistic quantum states [2]. The second question still remained unsolved even for the simplest case of two spin1/2 particles (also known as two qubits) despite the intense research in the last decade.
In this talk, after giving a brief introduction to three basic notions of quantum nonlocality, namely quantum entanglement, quantum steerability and Bell nonlocality [*], I will describe our recent attempt to answer the second question. By formulating the quantum steering problem as the inclusion of convex objects in geometry, we were able to solve completely the second question for the case of two spin1/2 particles [3]. Explicit formulae can be derived when the systems are in some special states, and for the most general states, the problem can be solved accurately in a simple manner with a classical computer. Closing up the quantum steering problem for two spin1/2 particles, our work also paves the way to characterize quantum steering for general systems. There are still challenges ahead, but one may be optimistic that this line of research sheds new light on various aspects of the mysterious quantum steering phenomenon in the near future.
[1] A. Einstein, B. Podolsky, and N. Rosen, "Can QuantumMechanical Description of Physical Reality be Considered Complete?" Phys. Rev. 47, 777–780 (1935)
[2] H. M. Wiseman, S. J. Jones, and A. C. Doherty, "Steering, Entanglement, Nonlocality, and the EinsteinPodolskyRosen Paradox". Phys. Rev. Lett. 98, 140402 (2007)
[3] H. C. Nguyen, H.V. Nguyen, and O. Gühne, "Geometry of EinsteinPodolskyRosen correlations", arXiv:1808.09349 (to appear in Phys. Rev. Lett. https://journals.aps.org/prl/accepted/5f071Yd6Lda1b469f06388601391253eafe2d141c)
[*] This part of the talk will be given in Vietnamese to facilitate the attention of students (if any) who might not be familiar with the topic. I can switch to English in the second part if it is more convenient for the attendees.


Host person  Trịnh Xuân Hoàng


