May 13th, 2020 at 2:00 pm
Speaker: Gláucia Murta,
from Heinrich-Heine-Universität Düsseldorf, Germany
Title: Device-independent quantum cryptography in the multipartite scenario
Going beyond the simple two-party scenario of quantum key distribution, we consider N parties who wish to certify security against a potential eavesdropper in a cryptographic task. Moreover, we consider the very adversarial scenario in which the parties make no assumption about the underlying quantum system or the internal working of their measurement devices. This is the device-independent scenario. In the device-independent scenario, security is certified by the violation of a Bell inequality. In this talk I will present our recent results on bounds on Eve’s uncertainty as a function of the violation of the multipartite MABK Bell inequality. I will discuss the implication of these results to cryptographic tasks, such as randomness expansion and conference key agreement. Finally, I discuss the challenges and possibilities to extend our results to other Bell inequalities, which can lead to better cryptographic protocols.
Link to the paper: https://arxiv.org/abs/2004.14263
Join us via ZOOM: https://zoom.us/j/703988067?pwd=UXVvOExYaVpoeFdXenFGaFl3dEo5Zz09
April 22, 2020 at 2:00 pm
Seminar given by Patryk Lipka-Bartosik
University of Bristol, UK
Title: Operational advantages provided by nonclassical teleportation
The standard benchmark for teleportation is the average fidelity of teleportation and according to this benchmark not all states are useful for teleportation. It was recently shown, however, that all entangled states lead to nonclassical teleportation, with there being no classical scheme able to reproduce the states teleported to Bob. Here we study the operational significance of this result. On the one hand, we demonstrate that every state is useful for teleportation if a generalization of the average fidelity of teleportation is considered which concerns teleporting quantum correlations. On the other hand, we show the strength of a particular entangled state and entangled measurement for teleportation—as quantified by the robustness of teleportation—precisely characterizes their ability to offer an advantage in the task of subchannel discrimination with side information. This connection allows us to prove that every entangled state outperforms all separable states when acting as a quantum memory in this discrimination task. Finally, within the context of a resource theory of teleportation, we show that the two operational tasks considered provide complete sets of monotones for two partial orders based on the notion of teleportation simulation, one classical and one quantum.
Link to the paper: https://arxiv.org/abs/1908.05107
March 27, 2020 at 12:00
Seminar given by Steven Bass
Kitzbühel Centre for Physics, Austria / Jagiellonian University in Krakow, Poland
Title: The Cosmological Constant Puzzle – Symmetries of Quantum Fluctuations
The cosmological constant in Einstein’s equations of General Relativity is a prime candidate to describe the dark energy that drives the accelerating expansion of the Universe and which contributes 69% of its energy budget. The cosmological constant measures the energy density of the vacuum perceived by gravitation. Experimentally, it is characterised by a tiny energy scale 0.002 eV. How should we understand this ? The quantum vacuum is described by particle physics where the mass scales that enter are very much larger. If one naively sums the zero-point energies of quantum fluctuations up to the energies where we do collider experiments at CERN then the cosmological constant comes out 10^60 times too large. Here we argue that the tiny value of the cosmological constant may be telling us something deep about the origin of symmetry in the subatomic world. The gauge symmetries which describe particle interactions may be emergent. The presentation will be given at Colloquium level and suitable for good Masters students.