World's smallest radio stations: Two molecules communicate via single photons

 We know since the dawn of modern physics that although events in our everyday life can be described by classical physics, the interaction of light and matter is down deep governed by the laws of quantum mechanics. Despite this century-old wisdom, accessing truly quantum mechanical situations remains nontrivial, fascinating and noteworthy even in the laboratory. Recently, interest in this area has been boosted beyond academic curiosity because of the potential for more efficient and novel forms of information processing.

In one of the most basic proposals, a single atom or molecule acts as a quantum bit that processes signals that have been delivered via single photons. In the past twenty years scientists have shown that single molecules can be detected and single photons can be generated. However, excitation of a molecule with a photon had remained elusive because the probability that a molecule sees and absorbs a photon is very small. As a result, billions of photons per second are usually impinged on a molecule to obtain a signal from it.

One common way to get around this difficulty in atomic physics has been to build a cavity around the atom so that a photon remains trapped for long enough times to yield a favorable interaction probability. Scientists at ETH Zurich and Max Planck Institute for the Science of Light in Erlangen have now shown that one can even interact a flying photon with a single molecule. Among many challenges in the way of performing such an experiment is the realization of a suitable source of single photons, which have the proper frequency and bandwidth. Although one can purchase lasers at different colors and specifications, sources of single photons are not available on the market.

So a team of scientists led by Professor Vahid Sandoghdar made its own. To do this, they took advantage of the fact that when an atom or molecule absorbs a photon it makes a transition to a so-called excited state. After a few nanoseconds (one thousand millionth of a second) this state decays to its initial ground state and emits exactly one photon. In their experiment, the group used two samples containing fluorescent molecules embedded in organic crystals and cooled them to about 1.5 K (-272 °C). Single molecules in each sample were detected by a combination of spectral and spatial selection.

To generate single photons, a single molecule was excited in the “source” sample. When the excited state of the molecule decayed the emitted photons were collected and tightly focused onto the “target” sample at a distance of a few meters. To ensure that a molecule in that sample “sees” the incoming photons, the team had to make sure that they have the same frequency. Furthermore, the precious single photons had to interact with the target molecule in an efficient manner. A molecule is about one nanometer is size (100000 times smaller than the diameter of a human hair) but the focus of a light beam cannot be smaller than a few hundred nanometers.

This usually means that most of the incoming light goes around the molecule, i.e. without them seeing each other. However, if the incoming photons are resonant with the quantum mechanical transition of the molecule, the latter acts as a disk that is comparable to the area of the focused light. In this process the molecule acts as an antenna that grabs the light waves in its vicinity. The results of the study published in Physical Review Letters provide the first example of long-distance communication between two quantum optical antennas in analogy to the 19th century experiments of Hertz and Marconi with radio antennas. In those early efforts, dipolar oscillators were used as transmitting and receiving antennas.

In the current experiment, two single molecules mimic that scenario at optical frequencies and via a nonclassical optical channel, namely a single-photon stream. This opens many doors for further exciting experiments in which single photons act as carriers of quantum information to be processed by single emitters.

The experimental work was performed at ETH Zurich before the group of Prof. Sandoghdar moved to the newly founded Max Planck Institute for the Science of Light in Erlangen in 2011.

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The above story is reprinted from materials provided by ETH Zürich, via AlphaGalileo.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Y. L. A. Rezus, S. G. Walt, R. Lettow, A. Renn, G. Zumofen, S. Götzinger, and V. Sandoghdar. Single-Photon Spectroscopy of a Single Molecule. Physical Review Letters, 108, 093601; Feb 27, 2012

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The green look for electric vehicle charging stations

The network of electric-vehicle (EV) charging stations in Germany is still relatively sparse, but their number is growing rapidly. The majority of roadside charging points take the form of steel-clad pillars. A group of researchers has set out to develop an alternative design based on environmentally compatible materials.

Our cityscapes will soon be dominated by a new feature: EV charging points. At present they usually feature a steel- or aluminum-clad housing. Researchers at the Fraunhofer Institute for Mechanics of Materials IWM in Halle want to improve their green credentials. In collaboration with industrial partner Bosecker Verteilerbau Sachsen GmbH, they are developing an alternative solution based on eco-friendly materials. Their idea is to replace the steel cladding that protects cables, power outlets and electronic switchgear with honeycomb panels made of a wood-plastic composite (WPC). At present, the main application for this type of reconstituted wood product is weather-resistant decking for patios.

WPC is a natural fiber composite made up of 70 parts of cellulosic wood fiber derived from sustainable resources to 30 parts of thermoplastic polypropylene. Its advantages, apart from the high proportion of sustainable raw materials, are that it is 100% recyclable and contains no tropical timber. Wood-plastic composites can be repeatedly recycled into new products and have a neutral carbon footprint. As Sven Wüstenhagen, one of the IWM researchers in Halle, explains: "Trees extract huge quantities of carbon dioxide from the atmosphere as they grow, and sequester carbon in their ligneous fibers. It is therefore probable that the use of WPC in this new application will result in lower CO2 emissions compared with the use of steel."

Another advantage of the composite material, according to Wüstenhagen, is that its production is more energy-efficient than that of steel or other metal cladding materials. WPC is produced using an extrusion process that involves melting a mixture of wood fibers and thermoplastic resin under high pressure and at high temperature and feeding the resulting viscous product into a continuous mold. With modern processing technologies, the fibers can be added to the mixture in their natural state, without first being transformed into granulate, thus eliminating an energy-intensive intermediate stage and preserving the quality of the fibers. Because wood has a high thermal sensitivity, it has to be processed at temperatures below 200 degrees Celsius.

The housings are manufactured in the form of modular components that can be clipped together as required to create a wide variety of different designs, thus allowing them to blend in with the surrounding architecture. Their modular structure also enables the composite panels to be removed easily during repairs. Industrial design expert Wüstenhagen is already thinking about other possible new applications for the WPC components: "They could be used, for instance, to construct street furniture such as park benches or bus shelters. That's one of our next objectives. Another of our ideas is to integrate functional elements such as cable holders and cable management systems in the components for EV charging stations. This is a viable proposition because WPC can be formed into almost any shape, unlike the metal sheeting used in currently available housings."

Nonetheless, the WPC cladding has to live up to some very demanding requirements. It must be shatterproof and sufficiently elastic to withstand impact without damage, and it must be capable of resisting wide variations in temperature, high levels of humidity and prolonged UV exposure. The researchers are therefore testing samples of the material in a climate chamber to assess its resistance to extreme temperature conditions and determine which additives or types of coating provide the best weather protection. The IWM experts have almost completed their first prototype of the new WPC housing and are about to start outdoor testing. Sven Wüstenhagen and his team are confident that it won't be long before the first "all-green" EV charging stations appear on our streets.

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The above story is reprinted from materials provided by Fraunhofer-Gesellschaft.

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