Internet, networks of connections between Hollywood actors, etc, are examples of complex networks, whose properties have been intensively studied in recent times. The small-world property (that everyone has a few-step connection to celebrities), for instance, is a prominent result derived in this field.
Illustration of a quantum random network
A group of scientists around Professor Cirac, Director at the Max Planck Institute of Quantum Optics (Garching near Munich) and Leader of the Theory Division, has now introduced complex networks in the microscopic, so called, quantum regime (Nature Physics, Advanced Online Publication, DOI:10.1038/NPHYS1665).
The scientists have proven that these quantum complex networks have surprising properties: even in a very weakly connected quantum network, performing some measurements and other simple quantum operations allows to generate arbitrary graphs of connections that are otherwise impossible in their classical counterparts.
The behaviour of networks has been widely explored in the context of classical statistical mechanics. Periodic networks, by definition, have a regular structure, in which each node is connected to a constant number of ‘geometrical’ neighbours. If one tries to enlarge these systems, their topology is not altered since the unit cell is just repeated ad aeternum. The construction of a random network is completely different: each node has a small probability of being connected to any other node. Depending on the connection probability and in the limit of infinite size, such networks exhibit some typical effects. For instance, if this probability is high enough, nearly all nodes will be part of one giant cluster; if it is too small only sparse groups of connected nodes will be present.
In a quantum network one link between neighbouring nodes is given by one pair of entangled qubits, for example atoms; in other words, one link in a quantum network represents the entanglement between two qubits. Therefore, a node possesses exactly one qubit for each neighbour, and since it can act on these qubits it is called a ‘station’. This holds for any kind of quantum networks. However, there are different ways of defining the entanglement between neighbouring qubits. Until now, quantum networks have been mostly modelled as periodically structured graphs, that is, lattices. In the work described here the scientists set the amount of entanglement between two nodes to be equal to the connection probability of the classical random graphs.
In the classical case, some specific subgraphs appear suddenly if one lets the connection probability scale with the size of the network: for very low probabilities only trivial connections (simple links) are present in the network, whereas for higher probabilities the subgraphs become more and more complex (e.g., triangles, squares, or stars). In quantum networks, on the other hand, a qualitatively different behaviour emerges: even for the lowest non-trivial connection probability, i.e., if the entanglement between the nodes is, at first sight, just sufficient to get simple connections, it is in fact possible to generate communication subgraphs of any complexity. This result mainly relies on the superposition principle and on the ability to coherently manipulate the qubits at the stations.
“In our article we want to point out that networks with a disordered structure and not periodic lattices have to be studied in the context of quantum communication”, says Sébastien Perseguers, who has worked on this topic in the frame of his doctoral thesis. “In fact, it is well known that real-world communication networks have a complex topology, and we may predict that this will also be the case for quantum networks. Furthermore, we want to emphasize the fact that the best results are obtained if one ‘thinks quantumly’ not only at the connection scale, but also from a global network perspective. In this respect, it is essential to deepen our knowledge of multipartite entanglement, that is, entanglement shared between more than two particles.” In the future the scientists are going to extend their model to networks of a richer structure, the so-called complex networks which describe a wide variety of systems in nature and society, and they expect to find many new and unexpected phenomena. Sébastien Perseguers/Olivia Meyer-StrengOriginal Publication:
Igniting a solar flare in the corona with lower-atmosphere kindling
29.03.2017 | New Jersey Institute of Technology
NASA spacecraft investigate clues in radiation belts
28.03.2017 | NASA/Goddard Space Flight Center
The Institute of Semiconductor Technology and the Institute of Physical and Theoretical Chemistry, both members of the Laboratory for Emerging Nanometrology (LENA), at Technische Universität Braunschweig are partners in a new European research project entitled ChipScope, which aims to develop a completely new and extremely small optical microscope capable of observing the interior of living cells in real time. A consortium of 7 partners from 5 countries will tackle this issue with very ambitious objectives during a four-year research program.
To demonstrate the usefulness of this new scientific tool, at the end of the project the developed chip-sized microscope will be used to observe in real-time...
Astronomers from Bonn and Tautenburg in Thuringia (Germany) used the 100-m radio telescope at Effelsberg to observe several galaxy clusters. At the edges of these large accumulations of dark matter, stellar systems (galaxies), hot gas, and charged particles, they found magnetic fields that are exceptionally ordered over distances of many million light years. This makes them the most extended magnetic fields in the universe known so far.
The results will be published on March 22 in the journal „Astronomy & Astrophysics“.
Galaxy clusters are the largest gravitationally bound structures in the universe. With a typical extent of about 10 million light years, i.e. 100 times the...
Researchers at the Goethe University Frankfurt, together with partners from the University of Tübingen in Germany and Queen Mary University as well as Francis Crick Institute from London (UK) have developed a novel technology to decipher the secret ubiquitin code.
Ubiquitin is a small protein that can be linked to other cellular proteins, thereby controlling and modulating their functions. The attachment occurs in many...
In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are creating...
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are...
20.03.2017 | Event News
14.03.2017 | Event News
07.03.2017 | Event News
29.03.2017 | Materials Sciences
29.03.2017 | Physics and Astronomy
29.03.2017 | Earth Sciences