Physicists demonstrate rotated light images

(PhysOrg.com) -- In what might at first seem obvious, but isn't after further thinking, a group of physicists from the United States and Canada have demonstrated, for the first time, that images generated by light, can be rotated via a rotating medium. In a paper published in Science, physicists Sonja Franke-Arnold, Graham Gibson, Robert W. Boyd and Miles J. Padgett describe how they were able to replicate the effects of light shifting via a moving medium, in a spinning medium, opening the door to a possible new way of encoding transmitted images.

Scientists have long known that when a light is shone through certain moving material, that the light itself can be shifted along with it, due to the photons being absorbed and then released by the atoms in the medium. The effect has been demonstrated over the years and can be seen in the simplest of venues, such as light shining through a waterfall. Until now however, no one has shown that a similar effect might apply to a rotating medium.

The idea is that if a beam of light, projected in a certain shape, such as a square for example, were to be shone through a spinning medium, such as a round block of glass, the image would emerge on the other side, but not exactly opposite; it would be off, just a little bit, in the direction of the spin. The amount of shifting would of course depend on both the speed of revolution of the cylinder and on the medium used, as some, such as rubies are able to cause more of a drag, per se, on the light as it moves through, than others.

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Image rotation filmed for a light beam propagating through a 10cm length of ruby rod, spinning the rod first clockwise and then anti-clockwise (as viewed from the camera position). Video: Science, DOI: 10.1126/science.1203984

To test this theory, the team created a square beam of green light, which they then directed at a spinning cylinder made entirely of ruby. The light was sufficiently strong enough to shine all the way through the cylinder, creating a square image on the other side. To see if the image was being rotated as the cylinder spun, the exact location of the projected image was noted, then the cylinder was spun in the opposite direction, to see if it would then be in a different position; which of course, it was. With the cylinder spinning at 30 revolutions per second, they found that the projected image was rotated about a third of a degree. They also found that increasing the amount of light tended to increase the amount of rotation of the projected image, in some cases, by as much as ten degrees.

The research team note in their paper that they believe one application of this phenomenon could be its use in image encoding, just as current methods now include an image?s intensity.

More information: Rotary Photon Drag Enhanced by a Slow-Light Medium, Science 1 July 2011: Vol. 333 no. 6038 pp. 65-67 DOI: 10.1126/science.1203984

ABSTRACT

Transmission through a spinning window slightly rotates the polarization of the light, typically by a microradian. It has been predicted that the same mechanism should also rotate an image. Because this rotary photon drag has a contribution that is inversely proportional to the group velocity, the image rotation is expected to increase in a slow-light medium. Using a ruby window under conditions for coherent population oscillations, we induced an effective group index of about 1 million. The resulting rotation angle was large enough to be observed by the eye. This result shows that rotary photon drag applies to images as well as polarization. The possibility of switching between different rotation states may offer new opportunities for controlled image coding.

via PhysicsWorld

2010 PhysOrg.com

Physicists hit on mathematical description of superfluid dynamics

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A 2001 photo from the space shuttle shows a phenomenon called von Karman vortices in clouds downwind from Rashiri Island in the northern Sea of Japan. The vortices are similar to those that form in superfluids. (NASA)

(PhysOrg.com) -- It has been 100 years since the discovery of superconductivity, a state achieved when mercury was cooled, with the help of liquid helium, to nearly the coldest temperature achievable to form a superfluid that provides no resistance to electrons as they flow through it.

During that century, scientists have struggled to find a precise mathematical explanation of why and how this strange fluid behaves as it does. Liquid helium-4 itself becomes a superfluid when cooled to within a few degrees of absolute zero on the Kelvin scale (minus 273 Celsius or minus 460 Fahrenheit), and the resulting lack of viscosity allows it to seem to defy gravity, flowing up and over the sides of a container.

Now a team led by a University of Washington physicist, using the most powerful supercomputer available for open science, has devised a theoretical framework that explains the real-time behavior of superfluids that are made of fermions ? subatomic particles such as electrons, protons and neutrons that are basic building blocks of nature.

Such superfluids are found in neutron stars, which rotate between one and 1,000 times a second. These stars, also called pulsars, have 50 percent greater mass than the sun but are packed so densely that one can occupy an area only about the size of a city such as Seattle, said Aurel Bulgac, a UW physics professor and lead author of a paper in the June 10 edition of Science that details the work.

As a neutron star rotates, the superfluid on the surface behaves quite differently than a liquid would on the surface of the Earth. As the rotational speed increases the fluid opens a series of small vortices. As the vortices assemble into triangular patterns, the triangles build a lattice structure within the superfluid.

"When you reach the correct speed, you'll create one vortex in the middle," Bulgac said. "And as you increase the speed, you will increase the number of vortices. But it always occurs in steps."

Similar behavior can be recreated in a laboratory using a vacuum chamber and a laser beam to create a high-intensity electrical field that will cool a small sample, perhaps 1 million atoms, to temperatures near absolute zero. A "laser spoon" then can stir the superfluid fast enough to create vortices.

In trying to understand the odd behavior, scientists have attempted to devise descriptive equations, as they might to describe the swirling action in a cup of coffee as it is stirred, Bulgac said. But to describe the action in a superfluid made of fermions, a nearly limitless number of equations is needed. Each describes what happens if just one variable ? such as velocity, temperature or density ? is changed. Because the variables are linked, if one changes others will change as well.

The challenge, Bulgac said, was to formulate the proper mathematical problem and then find a computer that could work through the problem as the number of variable changes reached 1 trillion or more. To reach its solution, the team in the last year used the JaguarPF computer at Oak Ridge National Laboratory in Tennessee, one of the largest supercomputers in the world, for the equivalent of 70 million hours, which would require almost 8,000 years on a single-core personal computer (JaguarPF has nearly a quarter-million cores).

"This tells you the complexity of these calculations and how difficult this is," he said.

The researchers also found through their calculations that by increasing the speed at which the fluid was stirred, eventually it would lose its superfluid properties ? though not as soon as had been previously hypothesized. Video representations of the results of the massive numerical simulations are at http://www.phys.washington.edu/groups/qmbnt/UFG.

The work means that researchers can "to some extent" study the properties of a neutron star using computer simulations, Bulgac said. It also opens new directions of research in cold-atom physics.

"This is a pretty major step forward in studying these dynamic processes," he said.

Provided by University of Washington (news : web)

Beam line 13 fuels discovery fever for fundamental physicists

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Serpil Kucuker Dogan and Matthew Musgrave work on a helium-3 cell that is used to measure the angle at which the neutron beam strikes the liquid hydrogen sample.

(PhysOrg.com) -- The simplest, most sensible "Big Bang" universe, theoretical physicists believe, would be one in which equal numbers of particles and antiparticles are formed in pairs. As the universe cools, most of these particles would encounter their antiparticles, and they would annihilate.

"In many ways, the most reasonable universe would be one in which there is no matter," says the University of Tennessee's Geoff Greene. "But that is manifestly not the universe we see. So something is wrong with the simple picture, and it is not understood why the universe actually has matter, instead of no matter, which makes more sense." This question, and others like it, are at the heart of the science that will be addressed at the Fundamental Physics Beam Line now being commissioned at SNS.

Beam line 13 is a cooperative venture between Basic Energy Sciences at DOE, which granted a beam line to nuclear physics, and the Nuclear Physics Program Office, which supported the construction of the FNPB and supports operation of the experiments.

Beam line 13 has an atypical user program. As with other beam lines, selection of approved experiments is made by a proposal-driven process, with the key criterion being scientific merit as determined by peer review. But at FNPB, a single experiment doesn't necessarily run for a few days, as most do at SNS and HFIR. Instead, it may run continuously for several years.

There may be as many as 100 collaborators. "They may come for an extended stay. They may send students. But each experiment may take a year or years to construct, a year or years to collect the data, and then it's taken down and something else of similar scope will be put in place," Greene explains.

There are two classes of experiment that the scientists will undertake. One is to determine the fundamental properties of the neutron itself. The other will investigate the interaction of the neutron in very simple nuclear systems.

The first experiment at beam line 13, which is now in place, is of the second type: A very simple nuclear reaction is studied to investigate what happens when a proton captures a polarized neutron?a neutron with an oriented spin. In this interaction a gamma ray is emitted. Is it emitted randomly, in any direction, or is there a slight preference for the direction of the emitted gamma ray to lie along the spin axis of the neutron?

"Why do we care about this? Because only one of the four forces of nature?weak, strong, electromagnetic, and gravitation?is known to violate parity, to be 'handed'," Greene says. That is the weak force, which is "left-handed," and which is normally studied in particle decays. But very little is actually known about the operation of the weak force between pairs of nucleons (neutrons and protons, the particles within the nucleus of an atom). David Bowman and Seppo Penttila of the ORNL Physics Division are the principal investigators on this first experiment, which is a collaboration between ORNL, Los Alamos National Laboratory, and the Universities of Tennessee, Virginia, Manitoba (Canada), Arizona State, Kentucky, Michigan and others.

The target for the SNS neutron beam in this experiment is a sample of liquid hydrogen; this is effectively a target of protons, since each hydrogen atom has a single proton as its nucleus. The SNS pulsed neutron beam is fired at the target, which is surrounded by gamma ray detectors. The neutrons are polarized and are either "spin-up" or "spin-down." SNS provides 60 neutron pulses per second, and the researchers select the incoming beam's spin orientation to give an alternating orientation at the target. They then check whether the detectors see a corresponding alternating pattern of emitted gamma rays (less, more, less, etc.), correlated with the direction of the incident neutrons' spins.

Unless they observe a large sample of such incident beam spin reversals (more than 100 million), they won't see a discernible 'handedness' in the direction the gamma rays emitted from the nucleus because the "weak" force is so very weak relative to the dominant nuclear force (the "strong interaction"). Greene compares this to the flipping of a coin. To see if it is a 'fair coin,' it must be flipped a great many times to determine if there is a statistically significant bias between heads and tails. Once the direction of gamma ray emission is measured with sufficient accuracy, "we expect to see something. It is predicted at somewhere between 1 part in 108 and 1 part in 107. That means we capture on the order of 1016 neutrons. That, of course, is why we are at SNS-the most intense pulsed neutron source in the world."

The early results of this experiment, conducted at Los Alamos, were recently published in Physical Review C.

A second experiment is in preparation to look at one of the fundamental properties of the neutron-its electric dipole moment.

Here, physicists want to determine whether the neutron is uniformly electrically neutral or its positive and negative charges are actually displaced slightly with respect to one another. "If it has such an electric dipole moment, that has a very profound implication. Because to have an electric dipole moment would require a violation of time reversal symmetry."

In physics, symmetry under time reversal (T) tests whether physical laws can distinguish between forward and backwards directions of the passage of time (the direction is sometimes referred to as the "arrow" of time). To a good approximation the laws of physics are symmetric (invariant, unchanged) under T.

If the neutron electric dipole is not zero, "that could shed light on a really fundamental interesting question, which is, why does the universe have matter at all," Greene says, "for most theories that seek to explain the matter-anti-matter asymmetry require a violation of time reversal symmetry."

Construction of FNPB began in 2002. The instrument team first opened the shutter for testing in 2008. The current neutron/proton capture experiment took its first beam in December 2010.

Provided by Oak Ridge National Laboratory (news : web)

When matter melts: Physicists map phase changes in quark-gluon plasma

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An ordinary proton or neutron (foreground) is formed of three quarks bound together by gluons, carriers of the color force. Above a critical temperature, protons and neutrons and other forms of hadronic matter "melt" into a hot, dense soup of free quarks and gluons (background), the quark-gluon plasma. Credit: Lawrence Berkeley National Laboratory

In its infancy, when the universe was a few millionths of a second old, the elemental constituents of matter moved freely in a hot, dense soup of quarks and gluons. As the universe expanded, this quark?gluon plasma quickly cooled, and protons and neutrons and other forms of normal matter "froze out": the quarks became bound together by the exchange of gluons, the carriers of the color force.

"The theory that describes the color force is called quantum chromodynamics, or QCD," says Nu Xu of the U.S. Department of Energy's Lawrence Berkeley National Laboratory, the spokesperson for the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at DOE's Brookhaven National Laboratory. "QCD has been extremely successful at explaining interactions of quarks and gluons at short distances, such as high-energy proton and antiproton collisions at Fermi National Accelerator Laboratory. But in bulk collections of matter ? including the quark-gluon plasma ? at longer distances or smaller momentum transfer, an approach called lattice gauge theory has to be used."

Until recently, lattice QCD calculations of hot, dense, bulk matter could not be tested against experiment. Beginning in 2000, however, RHIC was able to recreate the extreme conditions of the early universe in miniature, by colliding massive gold nuclei (heavy ions) at high energies.

Experimentalists at RHIC, working with theorist Sourendu Gupta of India's Tata Institute of Fundamental Research, have recently compared lattice-theory predictions about the nature of the quark-gluon plasma with certain STAR experimental results for the first time. In so doing they have established the temperature boundary where ordinary matter and quark matter cross over and change phase. Their results appear in the journal Science.

Phase diagrams

The aim of both the theoretical and experimental work is to explore and fix key points in the phase diagram for quantum chromodynamics. Phase diagrams are maps, showing, for example, how changes in pressure and temperature determine the phases of water, whether ice, liquid, or vapor. A phase diagram of QCD would map the distribution of ordinary matter (known as hadronic matter), the quark-gluon plasma, and other possible phases of QCD such as color superconductivity.

"Plotting a QCD phase diagram requires both theory calculations and experimental effort with heavy-ion collisions," says Xu, who is a member of Berkeley Lab's Nuclear Science Division and an author of the Science paper. Experimental studies require powerful accelerators like RHIC on Long Island or the Large Hadron Collider at CERN in Geneva, while calculations of QCD using lattice gauge theory require the world's biggest and fastest supercomputers. Direct comparisons can achieve more than either approach alone.

One of the basic requirements of any phase diagram is to establish its scale. A phase diagram of water might be based on the Celsius temperature scale, defined by the boiling point of water under normal pressure (i.e., at sea level). Although the boiling point changes with pressure ? at higher altitudes water boils at lower temperatures ? these changes are measured against a fixed value.

The scale of the QCD phase diagram is defined by a transition temperature at the zero value of "baryon chemical potential." Baryon chemical potential measures the imbalance between matter and antimatter, and zero indicates perfect balance.

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The "current conjecture" for the QCD phase diagram. The boundary between the normal (hadronic) low-temperature phase and the high-temperature quark-gluon plasma phase is marked in black. The square box on the solid line indicates the yet-to-be-found critical point where phases can co-exist; RHIC is the only heavy-ion collider whose energy can be tuned across the region where it is likely to lie. Neutrons and protons and other ordinary matter particles (including antimatter particles) are detected after they "freeze out" of fireballs caused by heavy-ion collisions like those at RHIC, indicated by the dotted line. To the right is a possible region of "color superconductivity." Credit: Gupta et al.

Through extensive calculations and actual data from the STAR experiment, the team was indeed able to establish the QCD transition temperature. Before they could do so, however, they first had to realize an equally significant result, showing that the highly dynamical systems of RHIC's gold-gold collisions, in which the quark-gluon plasma winks in and out of existence, in fact achieve thermal equilibrium. Here's where theory and experiment worked hand in hand.

"The fireballs that result when gold nuclei collide are all different, highly dynamic, and last an extremely short time," says Hans Georg Ritter, head of the Relativistic Nuclear Collisions program in Berkeley Lab's Nuclear Science Division and an author of the Science paper. Yet because differences in values of the kind observed by STAR are related to fluctuations in thermodynamic values predicted by lattice gauge theory, says Ritter, "by comparing our results to the predictions of theory, we have shown that what we measure is in fact consistent with the fireballs reaching thermal equilibrium. This is an important achievement."

The scientists were now able to proceed with confidence in establishing the scale of the QCD phase diagram. After a careful comparison between experimental data and the results from the lattice gauge theory calculations, the scientists concluded that the transition temperature (expressed in units of energy) is 175 MeV (175 million electron volts).

Thus the team could develop a "conjectural" phase diagram that showed the boundary between the low-temperature hadronic phase of ordinary matter and the high-temperature quark-gluon phase.

In search of the critical point

Lattice QCD also predicts the existence of a "critical point." In a QCD phase diagram the critical point marks the end of a line showing where the two phases cross over, one into the other. By changing the energy, for example, the baryon chemical potential (balance of matter and antimatter) can be adjusted.

Among the world's heavy-ion colliders, only RHIC can tune the energy of the collisions through the region of the QCD phase diagram where the critical point is most likely to be found ? from an energy of 200 billion electrons volts per pair of nucleons (protons or neutrons) down to 5 billion electron volts per nucleon pair.

Says Ritter, "Establishing the existence of a QCD critical point would be much more significant than setting the scale." In 2010, RHIC started a program to search for the QCD critical point.

Xu says, "In this paper, we compared experimental data with lattice calculations directly, something never done before. This is a real step forward and allows us to establish the scale of the QCD phase diagram. Thus begins an era of precision measurements for heavy-ion physics."

Provided by Lawrence Berkeley National Laboratory (news : web)

Physicists observe 'campfire effect' in blinking nanorod semiconductors

When semiconductor nanorods are exposed to light, they blink in a seemingly random pattern. By clustering nanorods together, physicists at the University of Pennsylvania have shown that their combined "on" time is increased dramatically providing new insight into this mysterious blinking behavior.

The research was conducted by associate professor Marija Drndic's group, including graduate student Siying Wang and postdoctorial fellows Claudia Querner and Tali Dadosh, all of the Department of Physics and Astronomy in Penn's School of Arts and Sciences. They collaborated with Catherine Crouch of Swarthmore College and Dmitry Novikov of New York University's School of Medicine.

Their research was published in the journal Nature Communications.

When provided with energy, whether in the form of light, electricity or certain chemicals, many semiconductors emit light. This principle is at work in light-emitting diodes, or LEDs, which are found in any number of consumer electronics.

At the macro scale, this electroluminescence is consistent; LED light bulbs, for example, can shine for years with a fraction of the energy used by even compact-fluorescent bulbs. But when semiconductors are shrunk down to nanometer size, instead of shining steadily, they turn "on" and "off" in an unpredictable fashion, switching between emitting light and being dark for variable lengths of time. For the decade since this was observed, many research groups around the world have sought to uncover the mechanism of this phenomenon, which is still not completely understood.

"Blinking has been studied in many different nanoscale materials for over a decade, as it is surprising and intriguing, but it's the statistics of the blinking that are so unusual," Drndic said. "These nanorods can be 'on' and 'off' for all scales of time, from a microsecond to hours. That's why we worked with Dmitry Novikov, who studies stochastic phenomena in physical and biological systems. These unusual Levi statistics arise when many factors compete with each other at different time scales, resulting in a rather complex behavior, with examples ranging from earthquakes to biological processes to stock market fluctuations."

Drndic and her research team, through a combination of imaging techniques, have shown that clustering these nanorod semiconductors greatly increases their total "on" time in a kind of "campfire effect." Adding a rod to the cluster has a multiplying effect on the "on" period of the group.

"If you put nanorods together, if each one blinks in rare short bursts, you would think the maximum 'on' time for the group will not be much bigger than that for one nanorod, since their bursts mostly don't overlap," Novikov said. "What we see are greatly prolonged 'on' bursts when nanorods are very close together, as if they help each other to keep shining, or 'burning.'"

Drndic's group demonstrated this by depositing cadmium selenide nanorods onto a substrate, shining a blue laser on them, then taking video under an optical microscope to observe the red light the nanorods then emitted. While that technique provided data on how long each cluster was "on," the team needed to use transmission electron microscopy, or TEM, to distinguish each individual, 5-nanometer rod and measure the size of each cluster.

A set of gold gridlines allowed the researchers to label and locate individual nanorod clusters. Wang then accurately overlaid about a thousand stitched-together TEM images with the luminescence data that she took with the optical microscope. The researchers observed the "campfire effect" in clusters as small as two and as large as 110, when the cluster effectively took on macroscale properties and stopped blinking entirely.

While the exact mechanism that causes this prolonged luminescence can't yet be pinpointed, Drndic's team's findings support the idea that interactions between electrons in the cluster are at the root of the effect.

"By moving from one end of a nanorod to the other, or otherwise changing position, we hypothesize that electrons in one rod can influence those in neighboring rods in ways that enhance the other rods' ability to give off light," Crouch said. "We hope our findings will give insight into these nanoscale interactions, as well as helping guide future work to understand blinking in single nanoparticles."

As nanorods can be an order of magnitude smaller than a cell, but can emit a signal that can be relatively easily seen under a microscope, they have been long considered as potential biomarkers. Their inconsistent pattern of illumination, however, has limited their usefulness.

"Biologists use semiconductor nanocrystals as fluorescent labels. One significant disadvantage is that they blink," Drndic said. "If the emission time could be extended to many minutes it makes them much more usable. With further development of the synthesis, perhaps clusters could be designed as improved labels."

Future research will use more ordered nanorod assemblies and controlled inter-particle separations to further study the details of particle interactions.

Quantum dots are not dots: physicists

Quantum dots are not dots: physicists

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Quantum dots are solid-state "artificial atoms" that are made up of thousands of atoms (yellow spheres) embedded in a semiconductor (blue spheres). Despite this complexity, the photon emission properties of quantum dots were hitherto believed to be like traditional atoms, where a point-emitter description is sufficient. Due to their mesoscopic dimensions, however, the point-emitter description is revealed to break down by comparing photon emission from quantum dots with opposite orientations relative to a metallic mirror.

Researchers from the Quantum Photonics Group at DTU Fotonik in collaboration with the Niels Bohr Institute, University of Copenhagen surprise the scientific world with the discovery that light emission from solid-state photon emitters, the so-called quantum dots, is fundamentally different than hitherto believed. The new insight may find important applications as a way to improve efficiency of quantum information devices. Their findings are published on December 19th 2010 in Nature Physics.

Today it is possible to fabricate and tailor highly efficient light sources that emit a single photon at a time, which constitutes the fundamental unit of light. Such emitters are referred to as quantum dots and consist of thousands of atoms. Despite the expectations reflected in this terminology, quantum dots cannot be described as point sources of light, which leads to the surprising conclusion: quantum dots are not dots!

This new insight was realized by experimentally recording photon emission from quantum dots positioned close to a metallic mirror. Point sources of light have the same properties whether or not they are flipped upside down, and this was expected to be the case for quantum dots as well. However, this fundamental symmetry was found to be violated in the experiments at DTU where a very pronounced dependence of the photon emission on the orientation of the quantum dots was observed.

The experimental findings are in excellent agreement with a new theory of light-matter interaction developed by DTU-researchers in collaboration with Anders S. Sørensen from the Niels Bohr Institute. The theory takes the spatial extent of quantum dots into account.

At the metal mirror surface, highly confined optical surface modes exist; the so-called plasmons. Plasmonics is a very active and promising research field, and the strong confinement of photons, available in plasmonics, may have applications for quantum information science or solar energy harvesting. The strong confinement of plasmons also implies that photon emission from quantum dots can be strongly altered, and that quantum dots can excite plasmons with very large probability. The present work demonstrates that the excitation of plasmons can be even more efficient than previously thought. Thus the fact that quantum dots are extended over areas much larger than atomic dimensions implies that they can interact more efficiently with plasmons.

The work may pave the way for new nanophotonic devices that exploit the spatial extent of quantum dots as a novel resource. The new effect is expected to be important also in other research areas than plasmonics, including photonic crystals, cavity quantum electrodynamics, and light harvesting.

Quantum dots are not dots: physicists

Quantum dots are not dots: physicists

Enlarge

Quantum dots are solid-state "artificial atoms" that are made up of thousands of atoms (yellow spheres) embedded in a semiconductor (blue spheres). Despite this complexity, the photon emission properties of quantum dots were hitherto believed to be like traditional atoms, where a point-emitter description is sufficient. Due to their mesoscopic dimensions, however, the point-emitter description is revealed to break down by comparing photon emission from quantum dots with opposite orientations relative to a metallic mirror.

Researchers from the Quantum Photonics Group at DTU Fotonik in collaboration with the Niels Bohr Institute, University of Copenhagen surprise the scientific world with the discovery that light emission from solid-state photon emitters, the so-called quantum dots, is fundamentally different than hitherto believed. The new insight may find important applications as a way to improve efficiency of quantum information devices. Their findings are published on December 19th 2010 in Nature Physics.

Today it is possible to fabricate and tailor highly efficient light sources that emit a single photon at a time, which constitutes the fundamental unit of light. Such emitters are referred to as quantum dots and consist of thousands of atoms. Despite the expectations reflected in this terminology, quantum dots cannot be described as point sources of light, which leads to the surprising conclusion: quantum dots are not dots!

This new insight was realized by experimentally recording photon emission from quantum dots positioned close to a metallic mirror. Point sources of light have the same properties whether or not they are flipped upside down, and this was expected to be the case for quantum dots as well. However, this fundamental symmetry was found to be violated in the experiments at DTU where a very pronounced dependence of the photon emission on the orientation of the quantum dots was observed.

The experimental findings are in excellent agreement with a new theory of light-matter interaction developed by DTU-researchers in collaboration with Anders S. Sørensen from the Niels Bohr Institute. The theory takes the spatial extent of quantum dots into account.

At the metal mirror surface, highly confined optical surface modes exist; the so-called plasmons. Plasmonics is a very active and promising research field, and the strong confinement of photons, available in plasmonics, may have applications for quantum information science or solar energy harvesting. The strong confinement of plasmons also implies that photon emission from quantum dots can be strongly altered, and that quantum dots can excite plasmons with very large probability. The present work demonstrates that the excitation of plasmons can be even more efficient than previously thought. Thus the fact that quantum dots are extended over areas much larger than atomic dimensions implies that they can interact more efficiently with plasmons.

The work may pave the way for new nanophotonic devices that exploit the spatial extent of quantum dots as a novel resource. The new effect is expected to be important also in other research areas than plasmonics, including photonic crystals, cavity quantum electrodynamics, and light harvesting.

Physicists use graphene to decode DNA

Physicists use graphene to decode DNA

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This is the cover of Physics World. Credit: Physics World

Genome sequencing will have a profound effect on our understanding of genetic biology and could usher in a day when doctor and patient are able to review individual genome sequences to fully personalise medical treatment.

As the X PRIZE FOUNDATION begins to receive nominations for its $10m prize for the first privately funded company that can accurately sequence 100 genomes in 10 days for less than $10,000 per genome, the science writer Philip Ball looks at the latest advances towards success in December Physics World's lead feature.

The baton, once firmly in the hands of chemists and biologists, has been grabbed by physicists around the world since the mid-1990s when David Deamer from the University of California, Santa Cruz imagined threading a DNA strand through a tiny pore -- reading out the chemical bases strung along the strand as it passes through. His idea was that in a salt solution, the number of dissolved ions passing through the pore would vary depending on which base was sitting in the pore.

Over the past decade, scientists have sought means to use Deamer's technique with far greater control of the pore and the movement of DNA through the pore, while also contemplating how the technique can be turned into a handy device that could be used in doctors' surgeries worldwide.

Initial thoughts were towards the use of a silicon-nitride nanopore but researchers have found the material a little too thick, meaning that more than one nucleotide -- the structural units that make up DNA -- can be in the pore at any one time.

Now, however, graphene -- one-atom thick sheets of carbon that led to this year's Nobel Prize for Physics -- is generating huge excitement as a possible DNA sequencing material following the work of three independent research groups earlier this year.

The teams -- based at the universities of Delft, Pennsylvania and Harvard -- have each drawn DNA through a nanopore drilled into graphene. As the materials is so much thinner than silicon nitride, the teams are reported to believe that graphene may be a "game changer".

Whether for the physicists it's the lure of a $10m prize, the joy of basic research, or the satisfaction of designing a technique that could revolutionize medicine, it looks like graphene -- already dubbed a "wonder material on account it being ultrathin, ultrastrong and a great electrical conductor -- could be adding one more string to its already powerful bow.