New Depiction of Light Aids Telecommunications

New Depiction of Light Aids Telecommunications

  9:29:00 PM    Science Lover

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Physicists with the Institute of Ultrafast Spectroscopy and Lasers (IUSL) at The City College of New York have presented a new way to map spiraling light that could help harness untapped data channels in optical fibers. Increased bandwidth would ease the burden on fiber-optic telecommunications networks taxed by an ever-growing demand for audio, video and digital media. The new model, developed by graduate student Giovanni Milione, Professor Robert Alfano and colleagues, could even spur enhancements in quantum computing and other applications.

Higher Order Poincare Sphere model developed by physicists with the
Institute of Ultrafast Spectroscopy and Lasers tracks movement of
complex forms of light. (Credit: Image courtesy of City College of New
York)

"People now can detect (light in) the ground channel, but this gives us a way to detect and measure a higher number of channels," says Mr. Milione. With such heavy traffic funneled through a single channel, there is great interest in exploiting others that can be occupied by complex forms of light, he explains.

The team published their work in the July 25 issue of Physical Review Letters. Mr. Milione will present it at the Optical Society of America's "Frontiers in Optics 2011" conference, October 16-20 in San Jose, Calif.

Polarization is everything to a physicist tracking light in an optical fiber or laser. More than a way to cut glare with sunglasses, polarization refers to a specific direction and orientation of the light's movement and electric field -- when it isn't going every which way as it does when emanating from a light bulb, for example.

"Being able to follow polarization and other changes as light travels gives you insight into the material it travels through, " explains Milione. This helps control the light and can essentially give a fingerprint of the material being analyzed.

Detecting the polarization also lets users finely tune a laser. Such control can allow a laser to burn away one layer of material while leaving the other layers it passes through intact.

Until now, only the simplest form of light, the ground state, could be mapped and controlled. Multiple higher channels in an optical fiber, which could be occupied by more complex light, were left sitting idle.

A globe-shaped model, called the Poincaré Sphere, has long been used to map such simple light. This light has peaks and troughs, like waves on the ocean, and moves or vibrates in "plane waves." One maps how light intersects the sphere in the same way one pinpoints a location on Earth using longitude and latitude.

But complex light moves with both spin and orbital angular momentum, more or less like the movement of our moon as it spins on its axis and orbits Earth.




Such light twists like a tornado as it travels through space and takes the form of what are called vector beams and vortices. To map these vortices the researchers expanded the existing sphere to develop their Higher Order Poincaré Sphere (HOPS).

The team studies even more complex patterns of light, such as star-shaped forms. Their model uses the HOPS to reduce what could be pages of mathematics to single equations. These are the mathematical tools that will harness the complex light for use in technology.

"The sphere facilitates understanding, showing phase vortices are on poles and vector beams are on the equator," explains Milione. "It organizes the relationship between these vortices of light."

"This kind of organization on the higher level Poincaré Sphere could clear the path to a number of novel physics and engineering efforts such as quantum computing and optical transitions; could greatly expand the sensitivity of spectroscopy and the complexity of computer cryptography; and might further push the boundaries what can be 'seen'," said Dr. Alfano.

The research was funded in part by Corning Inc. and the Army Research Office. 

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New scientific research reveals diamonds aren't forever

New scientific research reveals diamonds aren't forever

  10:19:00 AM    Science Lover

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In a paper published in the US journal Optical Materials Express this week, Macquarie University researchers show that even the earth's hardest naturally occurring material, the diamond, is not forever.

Evaporation of diamond induced by an ultraviolent
laser beam. Image courtesy Richard Mildren

Associate Professor Richard Mildren and his colleagues from the Macquarie University Photonics Research Centre discovered that diamonds evaporate under exposure to light.

"Although this type of light-induced evaporation has been observed in some materials, this is the first time it's been shown to occur for diamond," Mildren said.

The diamonds were exposed to intense light pulses in the UV-C band (the harsh ultraviolet rays filtered out by the ozone layer), and small pits in the diamond surface were visible after only a few seconds. The rate of mass loss in the diamond fell notably for lower light levels but the etching process still continued - albeit at a slower and slower pace, Mildren said.



But before diamond lovers around the world start to panic, he is quick to note that the rate of evaporation is very small and not noticeable under normal conditions. In fact, even under very bright UV conditions, such as intense sunlight or under a UV tanning lamp, it would take approximately the age of the universe - about 10 billion years - to see an observable distance, he said.

The findings not only provide clues about the long-term stability of diamonds, but also have broad implications for future research.

"It's a very practical discovery and we are now looking at how we can exploit this," Mildren said.

"If we can make structures in the diamonds that enable us to control the position of the light within a very narrow filament in the diamond, that's the first step to making smaller and more efficient optical devices such as those used in quantum computing and high performance lasers."

The discovery may also have implications as far reaching as the prospects for finding diamonds on the surface of other planets, Mildren said.

More information: Mildren, R. P. et al. Opt. Mater. Express 1, 576-585 (2011).

Provided by Macquarie University

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Squeezed light from single atoms

Squeezed light from single atoms

  9:11:00 PM    Science Lover

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Max Planck Institute of Quantum Optics scientists generate amplitude-squeezed light fields using single atoms trapped inside optical cavities.

A single rubidium atom in a cavity squeezes the quantum
fluctuations of a weak laser beam, decreasing the fluctuations
of the amplitude at the expense of the phase.
The effect is exaggerated for clarity.

In classical optics light is usually described as a wave, but at the most fundamental quantum level this wave consists of discrete particles called photons. Over the time, physicists developed many tools to manipulate both the wave-like and the particle-like quantum properties of the light. For instance, they created single photon sources with single atoms, using their ability to absorb and emit photons one by one. A team around Professor Gerhard Rempe, Director at the Max Planck Institute of Quantum Optics (Garching near Munich) and head of the Quantum Dynamics Division, has now observed that the light emitted by a single atom may exhibit much richer dynamics (Nature 474, 623, June 30, 2011). Strongly interacting with light inside a cavity, the atom modifies the wave-like properties of the light field, reducing its amplitude or phase fluctuations below the level allowed for classical electromagnetic radiation. This is the very first observation of “squeezed” light produced by a single atom.

The “graininess” of the photons in a light wave causes small fluctuations of the wave’s amplitude and phase. For classical beams, the minimal amount of amplitude and phase fluctuations is equal. However, by creating interactions between the photons, one can “squeeze” the fluctuations of the amplitude below this so-called “shot noise” level at the expense of increasing the fluctuations of the phase, and vice-versa. Unfortunately, the photonic interactions inside standard optical media are very weak, and require bright light beams to be observed. Single atoms are promising candidates to enable such interactions at a few-photon level. Their ability to generate squeezed light has been predicted 30 years ago, but the amount of light they emit is very tiny and so far all attempts to set this idea into realization have failed. In the Quantum Dynamics Division at MPQ sophisticated methods for cooling, isolating and manipulating single atoms have been developed over many years, and made this observation possible.



A single rubidium atom is trapped inside a cavity made of two very reflective mirrors in a distance of about a tenth of a millimetre from each other. When weak laser light is injected into this cavity, the atom can interact with one photon many times, and forms a kind of artificial molecule with the photons of the light field. As a consequence, two photons can enter the system at the same time and become correlated. “According to the model of Bohr, a single atom emits exactly one single energy quantum, i.e., one photon. That means that the number of photons is exactly known, but the phase of the light is not defined”, Professor Gerhard Rempe explains. “But the two photons that are emitted by this strongly coupled atom are indistinguishable and oscillate together. Therefore this time the wave-like properties of the propagating light field are modified.”

When the physicists use a laser beam which is resonant with the excitation frequency of the atom, the measurements show a suppression of the phase fluctuations. If the laser light is resonant with the cavity, they observe a squeezing of the amplitude instead.

The latter situation is illustrated in the figure: The atom in the cavity turns a laser beam into light which has less amplitude and more phase fluctuations than the shot-noise limit. “Our experiment shows that the light emitted by single atoms is much more complex than in the simple view of Albert Einstein concerning photo-emission”, Dr. Karim Murr emphasizes. “The squeezing that we observe is due to the coherent interaction between the two photons emitted from the system. Our measurement is in excellent agreement with the predictions of quantum electrodynamics in the strong-coupling regime.” And Dr. Alexei Ourjoumtsev, who has been working on the experiment as a post doc, adds: “Usually single quantum objects are used to manipulate the particle-like properties of light. It is interesting to see that they can also modify its wave-like properties, and create observable squeezing with excitations beams containing only two photons on average”.

So far squeezed light has only been generated with systems containing many atoms, such as crystals, using very high intensity beams, i.e. many photons. For the first time now physicists have succeeded in generating this kind of non-classical radiation with single atoms and extremely weak light fields. The ability of a single atom to induce strong coherent interactions between propagating photons opens up new perspectives for photonic quantum logic with single emitters.

More information: A. Ourjoumtsev, A. Kubanek, M. Koch, C. Sames, P. W. H. Pinkse, G. Rempe, & K. Murr

Observation of squeezed light from one atom excited with two photons , Nature 474, 623, 30 June 2011. Provided by Max-Planck-Gesellschaft

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Tiny Ring Laser Accurately Detects and Counts Nanoparticles

Tiny Ring Laser Accurately Detects and Counts Nanoparticles

  8:05:00 PM    Science Lover

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A tiny doughnut-shaped laser is the latest marvel of silicon microminiaturization, but instead of manipulating bits it detects very small particles. Small particles play a big -- and largely unnoticed -- role in our everyday lives. Virus particles make us sick, salt particles trigger cloud formation, and soot particles sift deep into our lungs and make it harder to breathe.

Whispering-gallery microlasers can count 
and measure nano-scale synthetic or biological 
particles. As this conceptual illustration shows, 
a particle disturbs the lasing "mode" to split 
into two frequencies (shown here as two different 
colors) and the frequency split acts a ruler that 
allows the particle to be measured. The inset 
at the top right shows a particle landing on the 
microlaser (a torus supported by a pedestal). 
Lina He, a graduate student in electrical and 
systems engineering at Washington University 
in St. Louis, and her co-workers demonstrated 
that the microlasers can detect particles 10 
nanometers in radius. Their resolution limit is 
about one nanometer. (Credit: J. Zhu, L. He, 
S. K. Ozdemir, and L. Yang/WUSTL)

The sensor belongs to a category called whispering gallery resonators, which work like the famous whispering gallery in St. Paul's Cathedral in London, where someone on the one side of the dome can hear a message spoken to the wall by someone on the other side. Unlike the dome, which has resonances or sweet spots in the audible range, the sensor resonates at light frequencies.

Light traveling round the micro-laser is disturbed by a particle that lands on the ring, changing the light's frequency. The ring can count the touch-down of as many as 800 nanoparticles before the signals begin to be lost in the noise. By exciting more than one mode in the ring, scientists can double-check the accuracy of the count. And by changing the "gain medium," they can adapt the sensor for water rather than air.

Lan Yang, PhD, assistant professor of electrical and systems engineering at Washington University in St. Louis who leads the team that fabricated the new sensor, says that there is already lively interest in its commercialization in fields ranging from biology to aerosol science. The sensor is described and characterized in the June 26 online edition of Nature Nanotechnology.

Whispering gallery resonator becomes microlaser

A whispering gallery resonator supports "frequency degenerate modes" (modes, or patterns of excitation in the ring, with the same frequency, one traveling clockwise and the other counterclockwise around the ring.

The mode fields have "evanescent tails" that penetrate the surface of the ring and probe the surrounding medium. When a particle lands on one of the "hot spots" it scatters energy from one of the modes into the other, and the modes adopt slightly different resonance frequencies. This is referred to as mode splitting.

In an earlier work, Yang team used mode splitting in a simple glass ring that functioned as a waveguide for light coupled into it from outside. Because the ring was passive, the external-laser had to be an expensive tunable laser so that it could scan a frequency range looking for the ring's resonances to measure mode splitting. (For more information on this sensor see "Tiny sensor takes measure of nanoparticles.")

The new sensor differs from earlier whispering gallery resonators in that it is itself a miniature laser rather than the resonating cavity of an external laser.

The new sensor is also glass but glass laced with atoms of the rare earth elements that serve as a "gain medium." The glass is doped with rare-earth atoms and when an external light source boosts enough of them into an excited state, the ring begins to lase at its own preferred frequency.

When a particle lands on the microlaser, a single lasing line splits into two slightly different frequencies.

A simple way of measuring the frequency splitting is to mix the split laser modes in a photodetector, which produces a "beat frequency" that corresponds to the frequency difference.

"The tiny sensors are mass produced by sol-gel method on silicon wafer, and it is easy to switch the gain medium" says Lina He, a graduate student and first author of the paper. "The resonators are made by mixing the rare-earth ions of choice into a solution of tetraethoxysilane, water and hydrochloric acid. The solution is heated until it becomes viscous and then spin-coated on a silicon wafer and annealed to remove solvents and complete the transition to amorphous glass. The thin film of glass is then etched to create silica disks supported underneath by silicon pillars. As a final step, the rough silica disks are reflowed into smooth toroids by laser annealing."

Active sensor outperforms passive one



"The light used for sensing is generated inside the resonator itself, and so it is purer than the light in the passive sensor," says Yang "When the light is not that pure, you might not be able to see small frequency changes. But the active sensor hits one frequency -- it has a really narrow linewidth -- and so it is much more sensitive."

The microlaser is orders of magnitude more sensitive than the passive resonator, she says. Its effective resolution limit is about one nanometer. One nanometer is to a meter, what a marble is to Earth.

Moreover, because the laser is now in the ring rather than coupled to it, the entire system is simpler and more self contained. "Now you just need a light source to excite the optical medium," says Yang, "and you can use a cheap laser diode for that instead of an expensive tunable laser."

Detecting many particles

The effect of a particle on a lasing mode depends on the particle's "polarizability," which is a function of its size and refractive index. To cover the possibilities, the Washington University team tested the micro-laser's performance with nanoparticles of various sizes made of various materials, including polystyrene (packing peanuts), virions (virus particles) and gold.

As particles enter the "mode volume" of the micro-laser one by one, the scientists can see a discrete upward or downward jump in the beat frequency. Each discrete jump signals the binding of a particle on the ring, and the number of the jumps reflects the number of particles.

Because the "resonator field" traps the particles on the resonator, once landed, they rarely drop off. But the team found they were able to count many particles before the losses induced by the particles made the laser linewidths so broad they couldn't detect changes in frequency splitting due to the latest arrival.

For example, they were able to detect and count as many as 816 gold nanoparticles using the same laser mode.

"When the line broadening is comparable to the change in splitting, then you're done," says Yang. "However, the whole resonator is fabricated on the chip, so you could just move on to the next resonator if necessary."

Doubling up for accuracy

The micro-laser can support more than one laser mode at a time. "By controlling the overlap of the pump light with the gain medium, you can excite more than one laser line," says Sahin Kaya Ozdemir, PhD, a research associate and co-author. "Then when a particle lands on the ring, each laser line will split into two, and generate a beat frequency. So you will have two beat frequencies instead of one."

That's an advantage, he explains, because the beat frequency depends in part on where the particle lands on the ring. If there is only one laser line and the particle falls between "hot spots" it might not be detected. The second beat frequency prevents these "false negatives," ensuring that every particle produces a detectable beat frequency.

Detecting particles in water

The microlasers intended to sense particles in air had been doped with erbium, a rare-earth element whose optical properties are well matched with those of air. In a final experiment designed to see whether this technique could be used to sense particles in water or blood, the team fabricated sensors that were doped with ytterbium rather than erbium.Ytterbium lases at wavelengths with low absorption of light by water

Yang's team has already begun working to make use of the enhanced sensitivity provided by the microlaser for studying various problems. In terms of applications, "the near-term use will be the monitoring of dynamic behaviors of particles in response to environmental and chemical changes at single particle resolution," says Yang.

The next step, the team see is to engineer the surface of these tiny microlasers to detect DNA and individual biological molecules. If the DNA is tagged with engineered nanoparticles, the micro-laser sensor can count individual DNA molecules or fragments of molecules.

Listening to Yang it is hard to escape the impression that you're hearing for the first time about an astonishing device that will one day be as ubiquitous -- and probably as underappreciated -- as the logic gates in our microwaves, cellphones and cars.

The Washington University in St. Louis team behind these results includes: L. He, W. Kim and J. Zhu, graduate students; S. K. Ozdemir, PhD, a research associate, and L. Yang, PhD, assistant professor in electrical and systems engineering.

This work is supported by National Science Foundation.

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Holograms in True Color

Holograms in True Color

  9:46:00 PM    Science Lover

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Researchers have developed a new way to create true-color holograms that can be viewed from any angle using ordinary white light. The advance could lead to a new generation of electronic devices, such as cell phones or miniature televisions that display three-dimensional (3D) images.

In living color. This three-dimensional, true-color
image of an apple was generated using a new
technique of making holograms (inset) that allows
the image to be viewed using ordinary white light.

Credit: Science/AAAS

True 3D images can be created in several ways. In the 1960s, researchers generated the first holograms by firing a laser at an object and then using a photosensitive material to record the pattern of interference between light waves reflected off the object and those striking the material directly from the laser. This hologram, if later illuminated with the same wavelength of laser light, reproduced a 3D (but monochromatic) image of the object. One well-known hologram of that era captured in red laser light a chessboard on which pieces could be viewed from various angles as a viewer shifted position.

The so-called rainbow holograms now common on credit cards are generated differently, using white light reflected off a silvery backing through a plastic film that contains several images, each stored in a different color in its own layer. As the hologram is viewed from different angles, the shifting view of those colored layers with respect to one another provide a 3D perspective. In many cases, the image produced by these rainbow holograms isn’t a true-color representation of the object depicted.

Now, researchers report today in Science that they can create true-color holograms that can be viewed using only white light. Like the first holograms, the new technique uses lasers to generate an interference pattern, says Satoshi Kawata, a photonics physicist at Osaka University in Japan. To capture colors, Kawata and his colleagues illuminate the original object with three different lasers: red, blue, and green, the three primary colors of projected light. They store the hologram in a light-sensitive material coated with a thin layer of metal such as gold or silver, a veneer that contains free electrons that are easily excited when struck by radiation such as light waves.



To reproduce a 3D image, the researchers bathe the metal-sheathed material in ordinary white light, which contains all wavelengths of visible light (including red, blue, and green). That white light excites the free electrons; their resulting movements and oscillations (so-called surface plasmons) in turn give off light that regenerates the image—an image that combines the red, blue, and green versions of the hologram to generate a true-color representation of the original object. In their lab tests, Kawata and his colleagues created realistically hued holograms of an apple, a flower, a Japanese origami crane, and several other objects. For now, Kawata says, the new technique can produce only static holograms—no pint-sized Princess Leia pleading for help from Obi-Wan Kenobi just yet.

“It’s quite a scientific achievement,” says physicist Pierre-Alexandre Blanche of the University of Arizona in Tucson. The technique may be able to generate brighter images that can be seen through a broader range of viewing angles than holograms produced using other methods, adds media technologist V. Michael Bove Jr. of the Massachusetts Institute of Technology in Cambridge. The problem, he says, may be figuring out how to mass-produce images more cheaply than other techniques can.

Yet another issue may be scaling up the holograms to large size, says photonics physicist Nasser Peyghambarian of the University of Arizona. So far, the researchers apparently have created holograms only about the size of an index card. The prisms used to illuminate the holograms, which in the current scheme are mounted beneath or behind the metal-coated material, could easily become cumbersome in much larger displays, he contends.

Nevertheless, the notion of watching the Super Bowl on a coffee table with an embedded holographic display—complete with little linemen, wee wide receivers, even a tiny blimp floating a few feet above the potato chips and beer coasters—may someday become a reality to sports fans everywhere.

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Miniature Lasers Could Help Launch New Age of the Internet

Miniature Lasers Could Help Launch New Age of the Internet

  5:49:00 PM    Science Lover

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A new laser device created at the University of Central Florida could make high-speed computing faster and more reliable, opening the door to a new age of the Internet.

Sabine Freisem, a senior research scientist who 
has been collaborating with Deppe for the past eight 
years, works on lasers in their UCF lab. (Credit: UCF)

Professor Dennis Deppe's miniature laser diode emits more intense light than those currently used. The light emits at a single wavelength, making it ideal for use in compact disc players, laser pointers and optical mice for computers, in addition to high-speed data transmission.

Until now, the biggest challenge has been the failure rate of these tiny devices. They don't work very well when they face huge workloads; the stress makes them crack.

The smaller size and elimination of non-semiconductor materials means the new devices could potentially be used in heavy data transmission, which is critical in developing the next generation of the Internet. By incorporating laser diodes into cables in the future, massive amounts of data could be moved across great distances almost instantaneously. By using the tiny lasers in optical clocks, the precision of GPS and high-speed wireless data communications also would increase.

"The new laser diodes represent a sharp departure from past commercial devices in how they are made," Deppe said from his lab inside the College of Optics and Photonics. "The new devices show almost no change in operation under stress conditions that cause commercial devices to rapidly fail."

"At the speed at which the industry is moving, I wouldn't be surprised if in four to five years, when you go to Best Buy to buy cables for all your electronics, you'll be selecting cables with laser diodes embedded in them," he added.

Deppe and Sabine Freisem, a senior research scientist who has been collaborating with Deppe for the past eight years, presented their findings in January at the SPIE (formerly The International Society for Optical Engineering) Photonics West conference in San Francisco.

Deppe has spent 21 years researching semiconductor lasers, and he is considered an international expert in the area. sdPhotonics is working on the commercialization of many of his creations and has several ongoing contracts.

"This is definitely a milestone," Freisem said. "The implications for the future are huge."

But there is still one challenge that the team is working to resolve. The voltage necessary to make the laser diodes work more efficiently must be optimized

Deppe said once that problem is resolved, the uses for the laser diodes will multiply. They could be used in lasers in space to remove unwanted hair.

"We usually have no idea how often we use this technology in our everyday life already," Deppe said. "Most of us just don't think about it. With further development, it will only become more commonplace."

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Quantum Pen for Single Atoms Is a Big Step Toward Large-Scale Quantum Computing

Quantum Pen for Single Atoms Is a Big Step Toward Large-Scale Quantum Computing

  9:38:00 AM    Science Lover

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Physicists at the Max Planck Institute of Quantum Optics succeeded in manipulating atoms individually in a lattice of light and in arranging them in arbitrary patterns. These results are an important step towards large scale quantum computing and for the simulation of condensed matter systems.

With the help of a laser beam, the scientists could address single atoms in the lattice of light and change their spin state. In this way they succeeded in having total control over the single atoms and in "writing" arbitrary two-dimensional patterns. (Credit: Image courtesy of Max Planck Institute of Quantum Optics)

Physicists around the world are searching for the best way to realize a quantum computer. Now scientists of the team around Stefan Kuhr and Immanuel Bloch at the Max Planck Institute of Quantum Optics (Garching/Munich) took a decisive step in this direction. They can now address and change the spin of single atoms with laser light and arrange them in arbitrary patterns. In this way, the physicists strung the atoms along a line and could directly observe their tunneling dynamics in a “racing duel” of the atoms. A register of hundreds of addressable quantum particles could serve for storing and processing of quantum information in a quantum computer.

In the present experiment, the scientists loaded laser-cooled rubidium atoms into an artificial crystal of light. These so-called optical lattices are generated by superimposing several laser beams. The atoms are kept in the lattice of light in a way similar to marbles being contained in the hollows of an egg carton.

A few months ago, the team of Stefan Kuhr and Immanuel Bloch showed that each site of the optical lattice can be filled with exactly one atom. With the help of a microscope, the scientists visualized the array atom by atom and thereby verified the shell-like structure of this “Mott insulator.” Now the scientists succeeded in individually addressing the atoms in the lattice and in changing their respective energy state. Using the microscope, they focused a laser beam down to a diameter of about 600 nanometers, which is just above the lattice spacing, and directed it at individual atoms with high precision.

The laser beam slightly deforms the electron shell of the addressed (targeted) atom and thereby changes the energy difference between its two spin states. Atoms with a spin – i.e. an intrinsic angular momentum – behave like little magnetic needles that can align in two opposite directions. If the atoms are irradiated with microwaves that are in resonance with the modified spin transition, only the addressed atoms absorb a microwave photon, which causes their spin to flip. All other atoms in the lattice remain unaffected by the microwave field.

The scientists demonstrated the high fidelity of this addressing scheme in a series of experiments. For this purpose, the spins of all atoms along a line were flipped one after the other, by moving the addressing laser from lattice site to lattice site. After removing all atoms with a flipped spin from the trap, the addressed atoms are visible as holes, which can easily be counted. In this way, the physicists deduced that the addressing worked in 95% of the cases. Atoms at the neighboring sites are not influenced by the addressing laser. The method provides the possibility to generate arbitrary distributions of atoms in the lattice.

Starting from an arrangement of 16 atoms that were strung together on neighboring lattice sites like a necklace of beads, the scientists studied what happens when the height of the lattice is ramped down so far that the particles are allowed to “tunnel” according to the rules of quantum mechanics. They move from one lattice site to the other, even if their energy is not sufficient to cross the barrier between the lattice wells. “As soon as the height of the lattice has reached the point where tunneling is possible, the particles start running as if they took part in a horse-race”, doctoral candidate Christof Weitenberg describes. “By taking snapshots of the atoms in the lattice at different times after the "starting signal", we could directly observe the quantum mechanical tunneling-effect of single massive particles in an optical lattice for the first time.”

The new addressing technique allows many interesting studies of the dynamics of collective quantum states, as they appear in solid state systems. It also opens new perspectives in quantum information processing. “A Mott isolator with exactly one atom per lattice site acts as a natural quantum register with a few hundred quantum bits, the ideal starting point for scalable quantum information processing,” as Stefan Kuhr explains. “We have shown that we can individually address single atoms. In order for the atom to suit as a quantum bit, we need to generate coherent superpositions of its two spin states. A further step is to realize elementary logical operations between two selected atoms in the lattice, so-called quantum gates.”

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Researchers Create 'Quantum Cats' Made of Light

Researchers Create 'Quantum Cats' Made of Light

  9:18:00 PM    Science Lover

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Researchers at the National Institute of Standards and Technology (NIST) have created "quantum cats" made of photons (particles of light), boosting prospects for manipulating light in new ways to enhance precision measurements as well as computing and communications based on quantum physics.

These colorized plots of electric field values indicate how closely the NIST "quantum cats" (left) compare with theoretical predictions for a cat state (right). The purple spots and alternating blue contrast regions in the center of the images indicate the light is in the appropriate quantum state. (Credit: Gerrits/NIST)

The NIST experiments, described in a forthcoming paper, repeatedly produced light pulses that each possessed two exactly opposite properties -- specifically, opposite phases, as if the peaks of the light waves were superimposed on the troughs. Physicists call this an optical Schrödinger's cat. NIST's quantum cat is the first to be made by detecting three photons at once and is one of the largest and most well-defined cat states ever made from light. (Larger cat states have been created in different systems by other research groups, including one at NIST.)

A "cat state" is a curiosity of the quantum world, where particles can exist in "superpositions" of two opposite properties simultaneously. Cat state is a reference to German physicist Erwin Schrödinger's famed 1935 theoretical notion of a cat that is both alive and dead simultaneously.

"This is a new state of light, predicted in quantum optics for a long time," says NIST research associate Thomas Gerrits, lead author of the paper. "The technologies that enable us to get these really good results are ultrafast lasers, knowledge of the type of light needed to create the cat state, and photon detectors that can actually count individual photons."

The NIST team created their optical cat state by using an ultrafast laser pulse to excite special crystals to create a form of light known as a squeezed vacuum, which contains only even numbers of photons. A specific number of photons were subtracted from the squeezed vacuum using a device called a beam splitter. The photons were identified with a NIST sensor that efficiently detects and counts individual photons. Depending on the number of subtracted photons, the remaining light is in a state that is a good approximation of a quantum cat says Gerrits -- the best that can be achieved because nobody has been able to create a "real" one, by, for instance, the quantum equivalent to superimposing two weak laser beams with opposite phases.

NIST conducts research on novel states of light because they may enhance measurement techniques such as interferometry, used to measure distance based on the interference of two light beams. The research also may contribute to quantum computing -- which may someday solve some problems that are intractable today -- and quantum communications, the most secure method known for protecting the privacy of a communications channel. Larger quantum cats of light are needed for accurate information processing.

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Extreme Darkness: Carbon Nanotube Forest Covers Ultra-Dark Detector

Extreme Darkness: Carbon Nanotube Forest Covers Ultra-Dark Detector

  10:46:00 AM    Science Lover

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Harnessing darkness for practical use, researchers at the National Institute of Standards and Technology (NIST) have developed a laser power detector coated with the world's darkest material -- a forest of carbon nanotubes that reflects almost no light across the visible and part of the infrared spectrum.

This is a colorized micrograph of the world's darkest 
material -- a sparse "forest" of fine carbon nanotubes -- 
coating a NIST laser power detector. Image shows a 
region approximately 25 micrometers across. 
(Credit: Aric Sanders, NIST)

NIST will use the new ultra-dark detector, described in a new paper in Nano Letters, to make precision laser power measurements for advanced technologies such as optical communications, laser-based manufacturing, solar energy conversion, and industrial and satellite-borne sensors.

Inspired by a 2008 paper by Rensselaer Polytechnic Institute (RPI) on "the darkest man-made material ever," the NIST team used a sparse array of fine nanotubes as a coating for a thermal detector, a device used to measure laser power. A co-author at Stony Brook University in New York grew the nanotube coating. The coating absorbs laser light and converts it to heat, which is registered in pyroelectric material (lithium tantalate in this case). The rise in temperature generates a current, which is measured to determine the power of the laser. The blacker the coating, the more efficiently it absorbs light instead of reflecting it, and the more accurate the measurements.

The new NIST detector uniformly reflects less than 0.1 percent of light at wavelengths from deep violet at 400 nanometers (nm) to near infrared at 4 micrometers (μm) and less than 1 percent of light in the infrared spectrum from 4 to 14 μm. The results are similar to those reported for the RPI material and in a 2009 paper by a Japanese group. The NIST work is unique in that the nanotubes were grown on pyroelectric material, whereas the other groups grew them on silicon. NIST researchers plan to extend the calibrated operating range of their device to 50 or even 100 micrometer wavelengths, to perhaps provide a standard for terahertz radiation power.

NIST previously made detector coatings from a variety of materials, including flat nanotube mats. The new coating is a vertical forest of multiwalled nanotubes, each less than 10 nanometers in diameter and about 160 micrometers long. The deep hollows may help trap light, and the random pattern diffuses any reflected light in various directions. Measuring how much light was reflected across a broad spectrum was technically demanding; the NIST team spent hundreds of hours using five different methods to measure the vanishingly low reflectance with adequate precision. Three of the five methods involved comparisons of the nanotube-coated detector to a calibrated standard.

Carbon nanotubes offer ideal properties for thermal detector coatings, in part because they are efficient heat conductors. Nickel phosphorous, for example, reflects less light at some wavelengths, but does not conduct heat as well. The new carbon nanotube materials also are darker than NIST's various Standard Reference Materials for black color developed years ago to calibrate instruments.

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World’s Smallest Microlaser Could Revolutionize Chip Technology

World’s Smallest Microlaser Could Revolutionize Chip Technology

  6:00:00 AM    Science Lover

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The centerpiece of the new microlaser is the electric resonator, 
consisting of two semi-circular capacitors that are connected 
via an inductor (here, a scanning electron microscope image). 
The color intensity represents the strength of the electrical field; 
the color itself, the respective polarity.
(Credit: Photo: ETH Zurich)

ETH-Zurich physicists have developed a new kind of laser that shatters the boundaries of possibility: it is by far the smallest electrically pumped laser in the world and one day could revolutionize chip technology.

It took a good one and a half years from the idea to its inception; a time when Christoph Walther, a PhD student in the Quantum Optoelectronics Group at ETH Zurich, spent days and nights in the FIRST lab. This was because ETH Zurich's state-of-the-art clean-room facility provided him with the ideal conditions to set a new record in laser technology: the physicist teamed up with four colleagues and developed the smallest electrically pumped laser in the world to date.

Much smaller than the wavelength

It's 30 micrometers long -- that's 30 millionths of a meter -- eight micrometers high and has a wavelength of 200 micrometers. This makes the laser considerably smaller than the wavelength of the light it emits -- a scientific first. After all, lasers normally can't be smaller than their wavelength, the reason being that in conventional lasers light waves cause an optic resonator to oscillate -- much like acoustic waves do to the soundbox of a guitar. In doing so, the light waves basically "travel" back and forth between two mirrors. The principle only works if the mirrors are larger than the wavelength of the laser. Consequently, normal lasers are limited in terms of their size.

Other researchers have endeavored to push the boundaries; "But by developing a completely new laser concept we were able to go quite a way below the limit," says Christoph Walther.

Inspired by electronics

In developing their laser concept, Christoph Walther and some of his team mates under his supervisor Jérôme Faist, professor and head of ETH Zurich's Institute of Quantum Electronics, were inspired by electronics. "Instead of the usual optic resonators, we use an electrical resonant circuit made up of an inductor and two capacitors," explains Walther. The light is effectively "captured" in it and induced into self-sustaining electromagnetic oscillations on the spot using an optical amplifier.

"This means the size of the resonator is no longer limited by the wavelength of the light and can in principle -- and that's what makes it so special -- be scaled down to whatever size you want." This prospect especially makes the microlaser interesting for chip manufacturers -- as an optic alternative to the transistors. "If we manage to approximate the transistors in terms of size using the microlasers, one day they could be used to build electro-optic chips with an extremely high concentration of electronic and optic components," says Christoph Walther. These could one day considerably speed up the exchange of data on microprocessors.

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Turning Noise Into Vision: New Way to Reveal Images of Hidden Objects

Turning Noise Into Vision: New Way to Reveal Images of Hidden Objects

  2:06:00 AM    Science Lover

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A new technique for revealing images of hidden objects may one day allow pilots to peer through fog and doctors to see more precisely into the human body without surgery.

Developed by Princeton engineers, the method relies on the surprising ability to clarify an image using rays of light that would typically make the image unrecognizable, such as those scattered by clouds, human tissue or murky water.

By adjusting an electrical voltage across a crystal of 
nonlinear material,the researchers recovered an image 
of lines and numbers that originally was hidden in noise 
(upper left). As they tuned the system (from left to right 
across each row from top to bottom), the image "stole" 
energy from the noise, first appearing and then degrading 
as they adjusted past the optimal voltage.
(Credit: Jason Fleischer/Dmitry Dylov)

In their experiments, the researchers restored an obscured image into a clear pattern of numbers and lines. The process was akin to improving poor TV reception using the distorted, or "noisy," part of the broadcast signal.

"Normally, noise is considered a bad thing," said Jason Fleischer, an assistant professor of electrical engineering at Princeton. "But sometimes noise and signal can interact, and the energy from the noise can be used to amplify the signal. For weak signals, such as distant or dark images, actually adding noise can improve their quality."

He said the ability to boost signals this way could potentially improve a broad range of signal technologies, including the sonograms doctors use to visualize fetuses and the radar systems pilots use to navigate through storms and turbulence. The method also potentially could be applied in technologies such as night vision goggles, inspection of underwater structures such as levies and bridge supports, and in steganography, the practice of masking signals for security purposes.

The findings were reported online March 14 in Nature Photonics.

In their experiments, Fleischer and co-author Dmitry Dylov, an electrical engineering graduate student, passed a laser beam through a small piece of glass engraved with numbers and lines, similar to the charts used during eye exams. The beam carried the image of the numbers and lines to a receiver connected to a video monitor, which displayed the pattern.

The researchers then placed a translucent piece of plastic similar to cellophane tape between the glass plate and the receiver. The tape-like material scattered the laser light before it arrived at the receiver, making the visual signal so noisy that the number and line pattern became indecipherable on the monitor, similar to the way smoke or fog might obstruct a person's view.

The crucial portion of the experiment came when Fleischer and Dylov placed another object in the path of the laser beam. Just in front of the receiver, they mounted a crystal of strontium barium niobate (SBN), a material that belongs to a class of substances known as "nonlinear" for their ability to alter the behavior of light in strange ways. In this case, the nonlinear crystal mixed different parts of the picture, allowing signal and noise to interact.

By adjusting an electrical voltage across the piece of SBN, the researchers were able to tune in a clear image on the monitor. The SBN gathered the rays that had been scattered by the translucent plastic and used that energy to clarify the weak image of the lines and numbers.

"We used noise to feed signals," Dylov said. "It's as if you took a picture of a person in the dark, and we made the person brighter and the background darker so you could see them. The contrast makes the person stand out."

The technique, known as "stochastic resonance," only works for the right amount of noise, as too much can overwhelm the signal. It has been observed in a variety of fields, ranging from neuroscience to energy harvesting, but never has been used this way for imaging.

Based on the results of their experiment, Fleischer and Dylov developed a new theory for how noisy signals move through nonlinear materials, which combines ideas from the fields of statistical physics, information theory and optics.

The research was funded by the National Science Foundation, the U.S. Department of Energy and the U.S. Air Force.

Their theory provides a general foundation for nonlinear communication that can be applied to a wide range of technologies. The researchers plan to incorporate other signal processing techniques to further improve the clarity of the images they generate and to apply the concepts they developed to biomedical imaging devices, including those that use sound and ultrasound instead of light.

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Synthetic Magnetic Fields Trick Neutral Atoms Into Acting as If Electrically Charged

Synthetic Magnetic Fields Trick Neutral Atoms Into Acting as If Electrically Charged

  12:27:00 AM    Science Lover

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Achieving an important new capability in ultracold atomic gases, researchers at the Joint Quantum Institute, a collaboration of the National Institute of Standards and Technology (NIST) and the University of Maryland, have created "synthetic" magnetic fields for ultracold gas atoms, in effect "tricking" neutral atoms into acting as if they are electrically charged particles subjected to a real magnetic field.

A pair of laser beams (red arrows) impinges upon an ultracold gas 

cloud of rubidum atoms (green oval) to create synthetic magnetic fields 

(labeled Beff).(Inset) The beams, combined with an external magnetic 

field (not shown) cause the atoms to "feel" a rotational force; the 

swirling atoms create vortices in the gas. (Credit: JQI)

The demonstration, described in the latest issue of the journal Nature, not only paves the way for exploring the complex natural phenomena involving charged particles in magnetic fields, but may also contribute to an exotic new form of quantum computing.

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Lasers Generate Underwater Sound: Potential For Naval And Commercial Underwater Acoustic Applications

Lasers Generate Underwater Sound: Potential For Naval And Commercial Underwater Acoustic Applications

  9:34:00 PM    Science Lover

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Scientists at the Naval Research Laboratory are developing a new technology for use in underwater acoustics. The new technology uses flashes of laser light to remotely create underwater sound. The new acoustic source has the potential to expand and improve both Naval and commercial underwater acoustic applications, including undersea communications, navigation, and acoustic imaging.

Scattered light from a 532 nm laser pulse can be seen as it enters the water in the Salt Water Tank Facility, and ionizes a small volume of water for acoustic generation. Air bubblers and controlled water and air temperatures can create ocean-like conditions in the laboratory. (Credit: Image courtesy of Naval Research Laboratory)

Dr. Ted Jones, a physicist in the Plasma Physics Division, is leading a team of researchers from the Plasma Physics, Acoustics, and Marine Geosciences Divisions in developing this acoustic source.

Efficient conversion of light into sound can be achieved by concentrating the light sufficiently to ionize a small amount of water, which then absorbs laser energy and superheats. The result is a small explosion of steam, which can generate a 220 decibel pulse of sound. Optical properties of water can be manipulated with very intense laser light to act like a focusing lens, allowing nonlinear self-focusing (NSF) to take place.

In addition, the slightly different colors of the laser, which travel at different speeds in water due to group velocity dispersion (GVD), can be arranged so that the pulse also compresses in time as it travels through water, further concentrating the light. By using a combination of GVD and NSF, controlled underwater compression of optical pulses can be attained.

The driving laser pulse has the ability to travel through both air and water, so that a compact laser on either an underwater or airborne platform can be used for remote acoustic generation. Since GVD and NSF effects are much stronger in water than air, a properly tailored laser has the ability to travel many hundreds of meters through air, remaining relatively unchanged, then quickly compress upon entry into the water. Atmospheric laser propagation is useful for applications where airborne lasers produce underwater acoustic signals without any required hardware in the water, such as undersea communications from aircraft.

Also, commercially available, high-repetition-rate pulsed lasers, steered by a rapidly movable mirror, can generate arbitrary arrays of phased acoustic sources. On a compact underwater platform with an acoustic receiver, such a setup can rapidly generate oblique-angle acoustic scattering data, for imaging and identifying underwater objects. This would be a significant addition to traditional direct backscattering acoustic data.

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World's Smallest Semiconductor Laser Heralds New Era In Optical Science

World's Smallest Semiconductor Laser Heralds New Era In Optical Science

  1:26:00 AM    Science Lover

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Researchers at the University of California, Berkeley, have reached a new milestone in laser physics by creating the world's smallest semiconductor laser, capable of generating visible light in a space smaller than a single protein molecule.

The schematic on the left illustrates light being compressed and sustained in the 5 nanometer gap -- smaller than a protein molecule -- between a nanowire and underlying silver surface. To the right is an electron microscope image of the hybrid design shown in the schematic. (Credit: Courtesy of Xiang Zhang Lab, UC Berkeley)

This breakthrough, described in an advanced online publication of the journal Nature on Aug. 30, breaks new ground in the field of optics. The UC Berkeley team not only successfully squeezed light into such a tight space, but found a novel way to keep that light energy from dissipating as it moved along, thereby achieving laser action.

"This work shatters traditional notions of laser limits, and makes a major advance toward applications in the biomedical, communications and computing fields," said Xiang Zhang, professor of mechanical engineering and director of UC Berkeley's Nanoscale Science and Engineering Center, which is funded by the National Science Foundation (NSF), and head of the research team behind this work.

The achievement helps enable the development of such innovations as nanolasers that can probe, manipulate and characterize DNA molecules; optics-based telecommunications many times faster than current technology; and optical computing in which light replaces electronic circuitry with a corresponding leap in speed and processing power.

While it is traditionally accepted that an electromagnetic wave - including laser light - cannot be focused beyond the size of half its wavelength, research teams around the world have found a way to compress light down to dozens of nanometers by binding it to the electrons that oscillate collectively at the surface of metals. This interaction between light and oscillating electrons is known as surface plasmons.

Scientists have been racing to construct surface plasmon lasers that can sustain and utilize these tiny optical excitations. However, the resistance inherent in metals causes these surface plasmons to dissipate almost immediately after being generated, posing a critical challenge to achieving the buildup of the electromagnetic field necessary for lasing.

Zhang and his research team took a novel approach to stem the loss of light energy by pairing a cadmium sulfide nanowire - 1,000 times thinner than a human hair - with a silver surface separated by an insulating gap of only 5 nanometers, the size of a single protein molecule. In this structure, the gap region stores light within an area 20 times smaller than its wavelength. Because light energy is largely stored in this tiny non-metallic gap, loss is significantly diminished.

With the loss finally under control through this unique "hybrid" design, the researchers could then work on amplifying the light.

"When you are working at such small scales, you do not have much space to play around with," said Rupert Oulton, the research associate in Zhang's lab who first theorized this approach last year and the study's co-lead author. "In our design, the nanowire acts as both a confinement mechanism and an amplifier. It's pulling double duty."

Trapping and sustaining light in radically tight quarters creates such extreme conditions that the very interaction of light and matter is strongly altered, the study authors explained. An increase in the spontaneous emission rate of light is a telltale sign of this altered interaction; in this study, the researchers measured a six-fold increase in the spontaneous emission rate of light in a gap size of 5 nanometers.

Recently, researchers from Norfolk State University reported lasing action of gold spheres in a dye-filled, glasslike shell immersed in a solution. The dye coupled to the gold spheres could generate surface plasmons when exposed to light.

The UC Berkeley researchers used semiconductor materials and fabrication technologies that are commonly employed in modern electronics manufacturing. By engineering hybrid surface plasmons in the tiny gap between semiconductors and metals, they were able to sustain the strongly confined light long enough that its oscillations stabilized into the coherent state that is a key characteristic of a laser.

"What is particularly exciting about the plasmonic lasers we demonstrated here is that they are solid state and fully compatible with semiconductor manufacturing, so they can be electrically pumped and fully integrated at chip-scale," said Volker Sorger, a Ph.D. student in Zhang's lab and study co-lead author.

"Plasmon lasers represent an exciting class of coherent light sources capable of extremely small confinement," said Zhang. "This work can bridge the worlds of electronics and optics at truly molecular length scales."

Scientists hope to eventually shrink light down to the size of an electron's wavelength, which is about a nanometer, or one-billionth of a meter, so that the two can work together on equal footing.

"The advantages of optics over electronics are multifold," added Thomas Zentgraf, a post-doctoral fellow in Zhang's lab and another co-lead author of the Nature paper. "For example, devices will be more power efficient at the same time they offer increased speed or bandwidth."

In addition to the three co-lead authors, other co-authors of the paper are Renmin Ma and Lun Dai from Peking University, and Christopher Gladden and Guy Bartal from Zhang's research group.

This work is supported by the U.S. Air Force Office of Scientific Research and the NSF.

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