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Physics

Physicists create new form of light

Newly observed optical state could enable quantum computing with photons

Try a quick experiment: Take two flashlights into a dark room and shine them so that their light beams cross. Notice anything peculiar? The anticlimactic answer is, probably not. That's because the individual photons that make up light do not interact. Instead, they pass each other by, like indifferent spirits in the night.

But what if light particles could be made to interact, attracting and repelling each other like atoms in ordinary matter? One tantalizing, albeit sci-fi possibility: lightsabers -- beams of light that can pull and push on each other, making for dazzling, epic confrontations. Or, in a more likely scenario, two beams of light could meet and merge into one single, luminous stream.

It may seem like such optical behavior would require bending the rules of physics, but in fact, scientists at MIT, Harvard University, and elsewhere have now demonstrated that photons can be made to interact -- an accomplishment that could open a path toward using photons in quantum computing, if not in lightsabers.

In a paper published today in the journal Science, the team, led by Vladan Vuletic, the Lester Wolfe Professor of Physics at MIT, and Professor Mikhail Lukin from Harvard University, reports that it has observed groups of three photons interacting and sticking together to form a completely new kind of photonic matter.

In controlled experiments, the researchers found that when they shone a weak laser beam through a cloud of ultracold rubidium atoms, rather than exiting the cloud as single, randomly spaced photons, the photons bound together in pairs or triplets, suggesting some interaction -- in this case, attraction -- taking place.

While photons normally have no mass and travel at 300,000 kilometers per second (the speed of light), the researchers found that the bound photons acquired a fraction of an electron's mass. These newly weighed-down light particles were also relatively sluggish, traveling about 100,000 times slower than normal noninteracting photons.

Vuletic says the results demonstrate that photons can attract, or entangle each other. If they can be made to interact in other ways, photons may be harnessed to perform extremely fast, incredibly complex quantum computations.

"The interaction of individual photons has been a very long dream for decades," Vuletic says.

Vuletic's co-authors include Qi-Yung Liang, Sergio Cantu, and Travis Nicholson from MIT, Lukin and Aditya Venkatramani of Harvard, Michael Gullans and Alexey Gorshkov of the University of Maryland, Jeff Thompson from Princeton University, and Cheng Ching of the University of Chicago.

In their new work, the researchers wondered whether interactions could take place between not only two photons, but more.

"For example, you can combine oxygen molecules to form O2 and O3 (ozone), but not O4, and for some molecules, you can't form even a three-particle molecule," Vuletic says. "So it was an open question: Can you add more photons to a molecule to make bigger and bigger things?"

To find out, the team used the same experimental approach they used to observe two-photon interactions. The process begins with cooling a cloud of rubidium atoms to ultracold temperatures, just a millionth of a degree above absolute zero. Cooling the atoms slows them to a near standstill. Through this cloud of immobilized atoms, the researchers then shine a weak laser beam -- so weak, in fact, that only a handful of photons travel through the cloud at any one time.

The researchers then measure the photons as they come out the other side of the atom cloud. In the new experiment, they found that the photons streamed out as pairs and triplets, rather than exiting the cloud at random intervals, as single photons having nothing to do with each other.

In addition to tracking the number and rate of photons, the team measured the phase of photons, before and after traveling through the atom cloud. A photon's phase indicates its frequency of oscillation.

"The phase tells you how strongly they're interacting, and the larger the phase, the stronger they are bound together," Venkatramani explains. The team observed that as three-photon particles exited the atom cloud simultaneously, their phase was shifted compared to what it was when the photons didn't interact at all and was three times larger than the phase shift of two-photon molecules. "This means these photons are not just each of them independently interacting, but they're all together interacting strongly."

Memorable encounters - The researchers then developed a hypothesis to explain what might have caused the photons to interact in the first place. Their model, based on physical principles, put forth the following scenario: As a single photon moves through the cloud of rubidium atoms, it briefly lands on a nearby atom before skipping to another atom, like a bee flitting between flowers, until it reaches the other end.

If another photon is simultaneously traveling through the cloud, it can also spend some time on a rubidium atom, forming a polariton -- a hybrid that is part photon, part atom. Then two polaritons can interact with each other via their atomic component. At the edge of the cloud, the atoms remain where they are, while the photons exit, still bound together. The researchers found that this same phenomenon can occur with three photons, forming an even stronger bond than the interactions between two photons.

"What was interesting was that these triplets formed at all," Vuletic says. "It was also not known whether they would be equally, less, or more strongly bound compared with photon pairs."

The entire interaction within the atom cloud occurs over a millionth of a second. And it is this interaction that triggers photons to remain bound together, even after they've left the cloud.

"What's neat about this is, when photons go through the medium, anything that happens in the medium, they 'remember' when they get out," Cantu says.

This means that photons that have interacted with each other, in this case through an attraction between them, can be thought of as strongly correlated, or entangled -- a key property for any quantum computing bit.

"Photons can travel very fast over long distances, and people have been using light to transmit information, such as in optical fibers," Vuletic says. "If photons can influence one another, then if you can entangle these photons, and we've done that, you can use them to distribute quantum information in an interesting and useful way."

Going forward, the team will look for ways to coerce other interactions such as repulsion, where photons may scatter off each other like billiard balls.

"It's completely novel in the sense that we don't even know sometimes qualitatively what to expect," Vuletic says. "With repulsion of photons, can they be such that they form a regular pattern, like a crystal of light? Or will something else happen? It's very uncharted territory."

Scientists discover almost 100 new exoplanets

Based on data from NASA's K2 mission an international team of scientists have just confirmed nearly 100 new exoplanets, planets located outside our solar system. This brings the total number of new exoplanets found with the K2 mission up to almost 300.

"We started out analyzing 275 candidates of which 149 were validated as real exoplanets. In turn 95 of these planets have proved to be discoveries," said American Ph.D. student Andrew Mayo at the National Space Institute (DTU Space) at the Technical University of Denmark.

"This research has been underway since the first K2 data release in 2014."

Mayo is the main author of the work being presented in the Astronomical Journal.

The research has been conducted partly as a senior project during his undergraduate studies at Harvard College. It has also involved a team of international colleagues from institutions such as NASA, Caltech, UC Berkeley, the University of Copenhagen, and the University of Tokyo.

The Kepler spacecraft was launched in 2009 to hunt for exoplanets in a single patch of sky, but in 2013 a mechanical failure crippled the telescope. However, astronomers and engineers devised a way to repurpose and save the space telescope by changing its field of view periodically. This solution paved the way for the follow up K2 mission, which is still ongoing as the spacecraft searches for exoplanet transits.

These transits can be found by registering dips in light caused by the shadow of an exoplanet as it crosses in front of its host star. These dips are indications of exoplanets which must then be examined much closer to validate the candidates that are exoplanets.

The field of exoplanets is relatively young. The first planet orbiting a star similar to our own Sun was detected only in 1995. Today some 3,600 exoplanets have been found, ranging from rocky Earth-sized planets to large gas giants like Jupiter.

It's difficult to distinguish which signals are actually coming from exoplanets. Mayo and his colleagues analyzed hundreds of signals of potential exoplanets thoroughly to determine which signals were created by exoplanets and which were caused by other sources.

"We found that some of the signals were caused by multiple star systems or noise from the spacecraft. But we also detected planets that range from sub-Earth-sized to the size of Jupiter and larger," said Mayo.

One of the planets detected was orbiting a very bright star.

"We validated a planet on a 10-day orbit around a star called HD 212657, which is now the brightest star found by either the Kepler or K2 missions to host a validated planet. Planets around bright stars are important because astronomers can learn a lot about them from ground-based observatories," said Mayo.

"Exoplanets are a fascinating field of space science. As more planets are discovered, astronomers will develop a much better picture of the nature of exoplanets which in turn will allow us to place our solar system into a galactic context".

The Kepler space telescope has made huge contributions to the field of exoplanets both in its original mission and its successor K2 mission. So far these missions have provided over 5,100 exoplanet candidates that can now be examined more closely.

With new, upcoming space missions like the James Webb Space Telescope and the Transiting Exoplanet Survey Satellite, astronomers will take exciting new steps toward characterizing and studying exoplanets like the rocky, habitable, Earth-sized planets that might be capable of supporting life.

Science Daily


Converting heat into electricity with pencil and paper

Thermoelectric materials can use thermal differences to generate electricity. Now there is an inexpensive and environmentally friendly way of producing them with the simplest of components: a normal pencil, photocopy paper, and conductive paint are sufficient to convert a temperature difference into electricity via the thermoelectric effect.

The thermoelectric effect is nothing new -- it was discovered almost 200 years ago by Thomas J. Seebeck. If two different metals are brought together, then an electrical voltage can develop if one metal is warmer than the other. This effect allows the residual heat to be partially converted into electrical energy. Residual heat is a by-product of almost all technological and natural processes, such as in power plants and every household appliance, and the human body as well. It is one of the largest underutilized energy sources in the world -- and usually goes completely unused.

Unfortunately, as useful an effect as it is, it is tiny in ordinary metals. This is because metals not only have high electrical conductivity but high thermal conductivity as well, so that differences in temperature disappear immediately. Thermoelectric materials need to have low thermal conductivity despite their high electrical conductivity. Thermoelectric devices made of inorganic semiconductor materials such as bismuth telluride are already being used today in specific technological applications. However, such material systems are expensive, and their use only pays off in certain situations. Flexible, non-toxic, organic materials based on carbon nanostructures, for example, are also being investigated for use in the human body.

HB pencil and co-polymer varnish

A team led by Prof. Norbert Nickel at the HZB has shown that the effect can be obtained much more simply: using a standard HB-grade pencil, they covered over a small area in pencil on ordinary photocopy paper. As a second material, they applied a transparent, conductive co-polymer paint (PEDOT: PSS) onto the surface.

What transpires is that the pencil traces on the paper deliver a voltage comparable to other far more expensive nanocomposites that are currently used for flexible thermoelectric elements. And this voltage could be increased tenfold by adding some indium selenide to the graphite from the pencil.

Poor heat transport explained.

The researchers investigated graphite and co-polymer coating films using a scanning electron microscope and spectroscopic methods (Raman scattering) at HZB. "The results were surprising for us as well," explains Nickel. "But we have now found an explanation of why this works so well: the pencil deposit left on the paper forms a surface characterized by unordered graphite flakes, some graphene, and clay. While this only slightly reduces the electrical conductivity, heat is transported much less effectively."

Outlook: Flexible Components printed right on paper

These simple constituents might be able to be used in the future to print thermoelectric components onto paper that is extremely inexpensive, environmentally friendly, and non-toxic. Such tiny and flexible components could also be used directly on the body and could use body heat to operate small devices or sensors.

Helmholtz-Zentrum Berlin


Chemical power

Chemical cluster could transform energy storage for large electrical grids

To power entire communities with clean energy, such as solar and wind power, a reliable backup storage system is needed to provide energy when the wind isn't blowing and the sun isn't out.

One possibility is to use any excess solar- and wind-based energy to charge solutions of chemicals that can subsequently be stored for use when sunshine and wind are scarce. During these down times, chemical solutions of opposite charge can be pumped across solid electrodes, thus creating an electron exchange that provides power to the electrical grid.

The key to this technology, called a redox flow battery, is finding chemicals that can not only "carry" sufficient charge, but also be stored without degrading for long periods, thereby maximizing power generation and minimizing the costs of replenishing the system.

Researchers at the University of Rochester and University at Buffalo believe they have found a promising compound that could transform the energy storage landscape.

In a paper published in Chemical Science, an open access journal of the Royal Society of Chemistry, the researchers describe modifying a metal-oxide cluster, which has promising electroactive properties, so that it is nearly twice as effective as the unmodified cluster for electrochemical energy storage in a redox flow battery.

The research was led by the lab of Ellen Matson, PhD, University of Rochester assistant professor of chemistry. Matson's team partnered with Timothy Cook, PhD, assistant professor of chemistry in the UB College of Arts and Sciences, to develop and study the cluster.

"Energy storage applications with polyoxometalates are pretty rare in the literature. There are maybe one or two examples prior to ours, and they didn't really maximize the potential of these systems," says first author Lauren VanGelder, a third-year PhD student in Matson's lab and a UB graduate who received her BS in chemistry and biomedical sciences.

"This is really an untapped area of molecular development," Matson adds.

The cluster was first developed in the lab of German chemist Johann Spandl, and studied for its magnetic properties. Tests conducted by VanGelder showed that the compound could store charge in a redox flow battery, "but was not as stable as we had hoped."

However, by making what Matson describes as "a simple molecular modification" -- replacing the compound's methanol-derived methoxide groups with ethanol-based ethoxide ligands -- the team was able to expand the potential window during which the cluster was stable, doubling the amount of electrical energy that could be stored in the battery.

Cook's team -- including fourth-year PhD candidate Anjula Kosswattaarachchi -- contributed to the research by carrying out tests that enabled the scientists to determine how stable different cluster compounds were.

"We carried out a series of experiments to evaluate the electrochemical properties of the clusters," Cook says. "Specifically, we were interested in seeing if the clusters were stable over the course of minutes, hours, and days. We also constructed a prototype battery where we charged and discharged the clusters, keeping track of how many electrons we could transfer and seeing if all of the energy we stored could be recovered, as one would expect of a good battery.

"These experiments let us calculate the efficiency of the device in a very exact way, letting us compare one system to another. Because of these studies, we were able to make molecular changes to the cluster and then determine exactly what properties were effected."

Says Matson: "What's really cool about this work is the way we can generate the ethoxide and methoxide clusters by using methanol and ethanol. Both of these reagents are inexpensive, readily available and safe to use. The metal and oxygen atoms that compose the remainder of the cluster are earth-abundant elements. The straightforward, efficient synthesis of this system is a totally new direction in charge-carrier development that, we believe, will set a new standard in the field."

Matson and Cook's research groups have applied for a National Science Foundation grant as part of an ongoing collaboration to further refine the clusters for use in commercial redox flow batteries.

A University of Rochester Furth Fund Award that Matson received last year enabled the lab to purchase electrochemical equipment needed for the study. Patrick Forrestal of the Matson lab also contributed to the study.

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