QUROPE Quantum Information Processing and Communication in Europe

The results of local measurements on some composite quantum systems cannot be reproduced classically. This impossibility, known as quantum nonlocality, represents a milestone in the foundations of quantum theory. Quantum nonlocality is also a valuable resource for information-processing tasks, for example, quantum communication, quantum key distribution, quantum state estimation or randomness extraction. Still, deciding whether a quantum state is nonlocal remains a challenging problem.

The results of local measurements on some composite quantum systems cannot be reproduced classically. This impossibility, known as quantum nonlocality, represents a milestone in the foundations of quantum theory. Quantum nonlocality is also a valuable resource for information-processing tasks, for example, quantum communication, quantum key distribution, quantum state estimation or randomness extraction. Still, deciding whether a quantum state is nonlocal remains a challenging problem.

This highlight contains a small video and the accompanying poster displaying what is know as the AQUTE Flagchip, the wondrous box containing the state-of-the-art atomic chips that has been displayed in several ICT events (the last one having been ICT 2010 Digitally Driven in Brussels). Enjoy!

We establish a link between unitary relaxation dynamics after a quench in closed many-body systems and the entanglement in the energy eigenbasis. We find that even if reduced states equilibrate, they can have memory on the initial conditions even in certain models that are far from integrable. We show that in such situations the equilibrium states are still described by a maximum entropy or generalized Gibbs ensemble, regardless of whether a model is integrable or not, thereby contributing to a recent debate.

Ultracold atoms in optical lattices are a versatile tool to investigate fundamental properties of quantum many body systems. In particular, the high degree of control of experimental parameters has allowed the study of many interesting phenomena such as quantum phase transitions and quantum spin dynamics. Here we demonstrate how such control can be extended down to the most fundamental level of a single spin at a specific site of an optical lattice.

In this work, the authors study position-based cryptography in the quantum setting. On the negative side, they show that if adversaries are allowed to share an arbitrarily large entangled quantum state, no secure position-verification is possible at all. On the positive side, they show that if adversaries do not share any entangled quantum state but can compute arbitrary quantum operations, secure position-verification is achievable. In models where secure positioning is achievable, it has a number of interesting applications.

The signature of coherent coupling between two quantum states is an anticrossing in their energies as one is swept through the other. In single semiconductor quantum dots containing an electron–hole pair the eigenstates form a two-level system that can be used to demonstrate quantum effects in the solid state, but in all previous work these states were independent. Here we describe a technique to control the energetic splitting of these states using a vertical electric field, facilitating the observation of coherent coupling between them.

Science Magazine has named "The First Quantum Machine" the "Breakthrough of the Year".

Read the full story at Science Magazine.

Prof. Silberhorn leads the integrated quantum optics group in the Department of Physics.  

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