Physicists have manipulated light with large artificial atoms, so-called quantum dots. Before, this has only been done so well with actual atoms. It is an important step towards light-based quantum technology. Publication on August 30th in Nature Communications.
a presentation, when you point a laser pointer at the screen, an immense amount of light particles race through the air at a billion kilometers per hour. They don’t travel in a continuous flow, but in packages containing varying numbers of particles. Sometimes as much as four so-called photons pass by, and other times none at all. You won’t notice this during your presentation, but for light-based quantum technology it is crucial that scientists have control over the number of photons per package.
In theory you can manipulate photons with real individual atoms, but because of their small size it is extremely hard to work with them. Now, Leiden physicists have discovered that the same principle goes for large artificial atoms—so-called quantum dots—that are much easier to handle. In fact, they managed to filter light beams with one photon per package out of a laser. ‘Another big advantage of quantum dots is that the system already works within nanoseconds,’ says first author Henk Snijders. ‘With atomic systems you need microseconds, so a thousand times longer. This way, we can manipulate photons much faster.’
The ultimate goal for the research group led by Prof. Dirk Bouwmeester is to entangle many photons using quantum dots. This is essential for example in techniques like quantum cryptography. Snijders: ‘This research shows that we are already able to manipulate individual photons with our system. And the beauty is that in principle we don’t need large experimental setups. We can just integrate our quantum dots in small microchips.’
Catalyst research aims to make gasoline less polluting. It turns out that during experiments it is actually necessary to protect catalysts from air itself. Publication in The Journal of Physical Chemistry C on August 26.
catalysts that nowadays gasoline is much less polluting than earlier. Crude oil contains the harmful substance sulfur, which refineries filter out in the process of turning oil into gasoline. They add hydrogen and for example the catalyst NiMoS2. The hydrogen removes a sulfur component of NiMoS2, giving the catalyst room to collect the sulfur from the oil.
To further improve the process, scientists research substances like NiMoS2. A small adaptation in the chemical composition could make it a more efficient catalyst. In such experiments it is important to know how to keep the studied sample free from external influences.
A group of physicists led by Joost Frenken (Leiden University) and Patricia Kooyman (University of Cape Town) together with TU Eindhoven have now shown that exposure to air is very harmful. ‘Oxygen molecules from the air oxidize the NiMoS2 catalyst particles, so that further studying the sample essentially produces no relevant information,’ says first author Marien Bremmer. ‘We noticed that the oxidation occurred extremely fast at first, but slowed down in the long-term. This indicates the formation of a shielding oxide ring.’
The research group used a high-resolution transmission electron microscope (HRTEM) to see that after 24 hours already twenty percent of each NiMoS2 particle is covered with oxygen. They describe the study in The Journal of Physical Chemistry C.
After 24 hours, a clean NiMoS2 particle (yellow-red) is already 20% covered with oxygen (blue spheres). In the subsequent period this process settles down, but a month later the coverage still has significantly increased.
The LCN2 community is proud to present the Leiden Networks Day: a one-day symposium open to all researchers interested in networks, from Leiden, the Netherlands and beyond. The event will feature a number of excellent international speakers and is free More info
Friday September 23, 2016, plenary conference room, Poortgebouw
9:30 - 10:00 Arrival
10:00 - 10:05 Opening by Han de Winde, vice-dean of the Faculty of Science
10:05 - 10:15 Introduction by Frank den Hollander
10:15 - 11:00
Pierluigi Crescenzi (University of Florence)
A large graph mining round trip: From theory to practice and back
11:00 - 11:30 Break
11:30 - 12:15
Janos Kertesz (Central European University)
Social contagion: A truly complex phenomenon
12:15 - 13:00
Kees Stam (Free University Amsterdam)
Complex brain networks: from observations to models
13:00 - 14:15
(free if you are registered)
14:15 - 15:00
Mariangeles Serrano (Universitat de Barcelona)
Network geometry and gravity models in complex networks
15:00 - 15:45
Stefan Thurner (Medical University of Vienna)
How financial multilayer networks create systemic risk - and how to manage it
15:45 - 16:15 Break
16:15 - 17:00
Alessandro Vespignani (Northeastern University)
From spreading processes in networks to infectious disease forecast
There will be plenty of opportunity to meet with researchers interested in network science throughout the day. A lunch, refreshments and afternoon drinks and snacks will be provided.
Be sure to note September 23 in your calendar and register by clicking this link.
Nature publishes an article on a paradoxical discovery in superconductivity. Leiden physicist Jan Zaanen writes a News & Views article about this in the same issue of August 19th.
Superconductivity is a bizarre but useful physical phenomenon. By More info
cooling a material down to below a critical temperature, its electrical resistance suddenly disappears completely. That way, you can easily send electricity through a wire without any loss of energy. This comes in very handy for example in windmills or MRI scanners.
News & Views
The cooling however poses a problem, which finally does cost energy. That is why physicists are on the hunt for a material with superconductive behavior at not-too-low temperatures. In a News & Views article in Nature, Leiden physicist Jan Zaanen describes how a new discovery leads to an interesting paradox.
The discovery, made by Ivan Božović from Yale University, has to do with copper oxides. In principle these don’t conduct any current. Their electrons have too strong of an interaction and retain each other, like cars in a traffic jam. But taking away some electrons gives the rest some room to manoeuver. They do this in pairs. These electron pairs can actually move so well that superconductivity occurs, even though the temperature is relatively high.
By removing too many electrons, a surplus of empty spots arises. Electrons now have difficulty finding each other to form pairs, and the value for the critical temperature drops. Everything seems to indicate that the famous Bardeen-Cooper-Schrieffer (BCS) theory from 1957 is applicable in this case. This theory describes quantum physics of conventional superconductors very precisely. BCS counter-intuitively predicts that all electrons take part in superconductivity, even if the critical temperature value is extremely low. However, Božović sees the number of participating electrons diminish proportionally to the critical temperature value. Like Zaanen describes, this as an apparent paradox which cannot be explained with our current understanding of quantum physics.
L. van Haandel, G.M. Bremmer, E.J.M. Hensen, Th. Weber (2016) Influence of sulfiding agent and pressure on structure and performance of CoMo/Al2O3 hydrodesulfurization catalyst, Journal of Catalysis, 10, 342, 27-39. [DOI][pdf]
G.M. Bremmer, L. van Haandel, E.J.M. Hensen, J.W.M. Frenken, P.J. Kooyman (2016) The instability of NiMoS2 and CoMoS2 HDS Catalyst at ambient conditions: A quasi-in-situ HRTEM and XPS Study, J. Phys. Chem. C. [DOI][pdf]
6 Sept, 13:45, Academy building, Rapenburg 73
Thesis Defense Ke Liu - IL: Gauge Theory and Nematic Order. The Rich Landscape of Orientational Phase Transition. Promotor: Prof.dr. J. Zaanen
14 Sept, 18:00, Sitterzaal 032
Colloquium Ehrenfestii William Irvine (Chicago): TBA
20 Sept, 15:00, Academy building, Rapenburg 73
Thesis Defense Brian M. Tarasinski - IL: On Periodically Driven Quantum Systems Promotor: Prof.dr. C.W.J. Beenakker; co-promotor: Dr. J.K. Asbóth (Hungarian Acad. of Sciences)