lunedì 17 dicembre 2007

Desktop Device Generates And Traps Rare Ultracold Molecules


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ScienceDaily (Dec. 17, 2007) — Physicists at the University of Rochester have combined an atom-chiller with a molecule trap, creating for the first time a device that can generate and trap huge numbers of elusive-yet-valuable ultracold polar molecules.
Scientists believe ultracold polar molecules will allow them to create exotic artificial crystals and stable quantum computers.
"The neat thing about this technology is that it's a very simple, but highly efficient method," says Jan Kleinert, a doctoral physics student at the University of Rochester and designer of the new device. "It lets us produce huge quantities of these ultracold polar molecules, which opens so many doors for us."
The Thin WIre electroStatic Trap, or TWIST, is the first electrostatic polar molecule trap that works simultaneously with a magneto-optical atom trap. This means Kleinert can use the lasers of the magneto-optical trap, or MOT, to chill atoms to just a few millionths of a degree above absolute zero, then force the atoms to group into molecules, and instantaneously hold them in place with the electrostatic TWIST trap.
Traditionally, a complex process of creating and trapping is required to produce these molecules, akin to repeatedly emptying and refilling the ice cube trays in your freezer, says Kleinert. A MOT with a TWIST, however, can create and store the chilled molecules in one place, instantly—more like a refrigerator with an automatic icemaker.
While polar molecules are literally as common as water, and dozens of laboratories around the world can cool atoms to such extreme temperatures, creating an ultracold polar molecule is difficult. Ultracold atoms can combine into molecules, but since only one type of atom can usually be cooled at once, the molecules it makes are electrically symmetric, not polar. Physicists have to either chill regular polar molecules, or chill several types of atoms at the same time and force them to join into molecules. Both processes are so complex that Kleinert says only four laboratories in the world do them, and the yield of ultracold polar molecules until now has been very low.
The TWIST, developed with Kleinert's advisor, Nicholas P. Bigelow, Lee A. DuBridge Professor of Physics at the University of Rochester, makes the complex process much more efficient, and thus makes available many more of these molecules.
The secret to the TWIST is the precise thickness of the tungsten wires that loop around the molecule-production area. In Kleinert's design, atoms are chilled with the lasers of a MOT, which drains away the atoms' energy, chilling them to nearly 460 degrees Fahrenheit below zero.
So far, this is exactly the same as the traditional method, but Kleinert surrounds his target area with tungsten loops that create an electric field. The field has no effect on the chilled atoms, but as the atoms are grouped into polar molecules by a process called photoassociation, the new polar molecules, with a positive charge on one side and a negative charge on the other, are affected by the field.
The electric field has a gradient, and due to some of the strange properties of the quantum world, polar molecules tend to "slide down" that gradient, collecting in the center of the field. As a result, says Kleinert, the TWIST collects and holds the low-field seeking polar molecules but lets other unaffected particles, such as atoms or other molecules, simply drift away.
Those tungsten loops have to be thick enough that they can withstand the electrostatic forces they generate, but thin enough that they don't block the MOT laser initiating the cooling. After months of trial and error and a lot of burned-out tungsten wire, Kleinert found that wires just the width of a hair provided the perfect balance.
"The coldest molecules so far have been produced from MOTs, but until the TWIST came along, electric field trapping and MOTs just didn't go together," says Kleinert. "Now we can accumulate these polar molecules continuously, without switching from creation to storage and back again."
With a good supply of ultracold polar molecules, computer scientists would have a new tool with which to tackle the creation of quantum computers, says Kleinert.
Quantum computer scientists are attracted to ultracold particles because their temperatures reduce decoherence, a phenomenon where your system decays from the carefully prepared quantum configuration you started with, to a classical physics state, which loses all the advantages quantum computers hold.
Ultracold polar molecules in particular are especially attractive because their strong polarity allows them to interact with each other over much larger distances than other atomic particles, and the stronger the interaction between particles, the faster a quantum computer can perform certain operations.
Ultracold polar molecules may even allow scientists to venture into an unknown quarter of the Standard Model of Physics—the size of the electron, says Kleinert. The answer to whether the electron has a definite size or is just a dimensionless point in space could support the Standard Model, or support one of the many alternate models. Trying to approximate the electron's size would likely require ultracold polar molecules, which can have 100 times the sensitivity of simple ultracold atoms. That difference could be enough to make a definitive measurement supporting or chipping away at the Standard Model altogether.
Adapted from materials provided by University of Rochester.

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sabato 8 dicembre 2007

Ultrafast Optical Shutter Is Switched Entirely By Laser Light


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ScienceDaily (Dec. 7, 2007) — It's a rare case of all light and no heat: A new study reports that a laser can be used to switch a film of vanadium dioxide back and forth between reflective and transparent states without heating or cooling it.
It is one of the first cases that scientists have found where light can directly produce such a physical transition without changing the material's temperature.
It is also among the most recent examples of "coherent control," the use of coherent radiation like laser light to affect the behavior of atomic, molecular or electronic systems. The technique has been used to control photosynthesis and is being used in efforts to create quantum computers and other novel electronic and optical devices. The new discovery opens the possibility of a new generation of ultra-fast optical switches for communications.
The study, which was published in the Sept. 14 issue of Physical Review Letters, was conducted by a team of physicists from Vanderbilt University and the University of Konstanz in Germany headed by Richard Haglund of Vanderbilt and Alfred Leitenstorfer from Konstanz.
Vanadium dioxide's uncanny ability to switch back and forth between transparent and reflective states is well known. At temperatures below 154 degrees Fahrenheit, vanadium dioxide film is a transparent semiconductor. Heat it to just a few degrees higher, however, and it becomes a reflective metal. The semiconducting and metallic states actually have different crystalline structures. Among a number of possible applications, people have experimented with using vanadium dioxide film as the active ingredient in "thermochromic windows" that can block sunlight when the temperature soars and as microscopic thermometers that could be injected into the body.
In 2005, a research collaboration teaming Haglund and René Lopez (now at the University of North Carolina, Chapel Hill) with Andrea Cavalleri and Matteo Rini from the Lawrence Berkeley National Laboratory tested the vanadium dioxide transition with an ultra-fast laser that produced 120-femtosecond pulses. (A femtosecond is a quadrillionth of a second. At this time scale, an eye blink lasts almost forever. In the three-tenths of a second it takes to blink an eye, light can travel 56,000 miles. By contrast, it takes 100 femtoseconds to cross the width of a human hair.)
Using this laser, the researchers determined that VO2 film can flip from transparent to reflective in a remarkably short time: less than 100 femtoseconds. This was the fastest phase transition ever measured. However, the mechanism that allowed it to make such rapid transitions remained a matter of scientific debate.
Now, in a two-year collaboration with the Leitenstorfer group, the Vanderbilt researchers have used a laser with even shorter, 12-femtosecond pulses to "strobe" the vanadium dioxide transition with the fastest pulses ever used for this purpose. The result? "This transition takes place even faster than we thought possible," says Haglund. "It can shift from transparent to reflective and back to transparent again in less than 100 femtoseconds, making the transition more than twice as fast as we had thought."
In order to identify the driving mechanism for the rapid change of state in vanadium dioxide, Leitenstorfer's graduate student Carl Kübler developed a method that converts the near-infrared photons produced by their 12-femtosecond pulse laser into a broad spectrum of infrared wavelengths that bracket a well-known vibration in the vanadium dioxide crystal lattice. At the same time, the Vanderbilt researchers figured out how to grow VO2 film on a diamond substrate that is transparent to infrared light.
This allowed the researchers to show that the energy in the laser beam goes directly into the crystal lattice of the VO2, driving it to shift from its transparent, crystalline form to its more compact and symmetric metallic configuration.
The laser light doesn't produce this shift by heating the VO2 lattice until it melts, as the conventional wisdom about phase transitions suggested. Instead, the researchers found that the stream of photons directly drive the oxygen atoms from one position to another by a process that is rather like pumping a swing in time with its natural frequency.
"People have believed for a long time that what happened in this phase transition was that the electrons get excited and then, somehow or another, the crystal structure changes," says Haglund. "But it turns out that the change in crystal structure is associated with this coherent molecular vibration."
Such a rapid transition is only possible because the difference between the metallic and semiconductor geometries is extremely small. "You can think of the movement that results as a breathing motion of the oxygen 'cage' that surrounds the vanadium ions," says Haglund. "That makes it possible for the structure to change from the semiconducting to the metallic states. It's a little like taking a deep breath to get into last summer's clothes."
This mechanism also allows the researchers to trigger the transition without changing the film's temperature. "We can focus the laser beam on a transparent vanadium dioxide film and create a small reflective spot. We can switch it on and off in less than 100 femtoseconds provided we haven't dumped so much energy into the film that we've heated it up. However, the more laser energy you dump in the VO2, the longer it takes to return to the semiconducting state," Haglund says.
Henri Ehrke and Rupert Huber from the University of Konstanz, and Andrej Halabica from Vanderbilt University also collaborated in the study, which was funded by the National Science Foundation and the Alexander von Humboldt Foundation.
Adapted from materials provided by Vanderbilt University.

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