lunes, 19 de julio de 2010

Physicists Propose New Method for Quantum Computing




(PhysOrg.com) -- The new system, which can compute faster and more efficiently than previous quantum computers, may bring the technology closer to reality.

Illustration from a poster by Susanne Yelin, Elena Kuznetsova, and Robin Côté

Quantum computers can solve in a matter of moments problems that would take ordinary computers years to work out. But thus far, these computers exist only as state-of-the-art experimental setups in a few physics laboratories.

Now, Elena Kuznetsova, a post-doctoral researcher in UConn’s Department of Physics, has proposed a new type of quantum computer that could bring the technology one step closer to becoming a reality.

“The main excitement about quantum computers,” says Kuznetsova, “ comes from their potential ability to solve certain problems exponentially faster compared to classical computers, such as factoring a large number into its primes, which would allow us to break cryptographic codes. These problems cannot be solved using a classical computer in the foreseeable future.”

Quantum processors take advantage of the principles of quantum mechanics, in which objects behave differently at very small scales than matter does at larger scales. Usually, their processors encode information into either individual atoms or molecules made up of two atoms. But Kuznetsova and her research group have proposed the first viable system that uses both atoms and molecules, taking advantage of the benefits of each. This system could be capable of computing faster and more efficiently than previous quantum processors.

Kuznetsova and her colleagues in physics, including graduate student Marko Gacesca and professors Susanne Yelin and Robin Côté, report their results in the March 2010 issue of Physical Review A.

As Yelin explains, there are several components to quantum computing. The first challenge is to create a system that you can control well enough to perform your computing, and another is to fashion a device that will report the results without damaging the system. The most advanced quantum computing to date is performed by neutral atoms, which, says Yelin, physicists have spent decades mastering and can now control to a very fine degree. These neutral atoms have no electric charge, and are therefore very difficult to get to interact with one another. This difficulty slows down the rate of computations.

In recent years, however, scientists discovered that polar molecules — which contain two atoms with equal and opposite charges — could lead to faster processing in quantum systems because the presence of these contrasting charges encourages the molecules to interact strongly with one other.

But this difference in molecular behavior is at once a great solution and a big problem, says Yelin. To be useful, these hyperactive molecules need to be cooled to only a few millionths of a degree above absolute zero, which slows them down and allows scientists to control them.

“Molecules in quantum states are very fragile,” Yelin says. “You heat them up, they’re gone. You bring them too close to each other, they’re gone. You look at them the wrong way, and they’re gone.”



Illustration from a poster by Susanne Yelin, Elena Kuznetsova, and Robin Côté.


The fragility of these molecules also poses another problem: when they’re used to report results from a quantum processor, scientists often lose control of them and the very data they’ve been trying to compute is destroyed. Until now, researchers hadn’t come up with a good way to read data out of these molecules.

In their recent paper, Kuznetsova’s group devised a way to separate the molecules into their component parts so that the processor’s results can be read from the more easily controllable individual atoms. By using lasers, says Kuznetsova, they were able to break down the molecules without compromising the data encoded in them.

“We let the molecule interact with a laser light with a very specific wavelength, or color,” she says. “This excites the molecule into another excited state, from which we can, with another laser light, break it down into two atoms. It’s a nondestructive, efficient way to keep the information and read it.”

Kuznetsova says that each portion of their concept is practically feasible using current experimental methods, but only for a generalized ensemble of qubits — as units of quantum information are known — all at once. The next step toward building a computer with polar molecules, says Yelin, is to create a system in which the qubits can be controlled individually.

Yelin admits that her work can sometimes seem supernatural to non-specialists, but she says she wouldn’t have it any other way.

“On first glance, these interactions of light and particles looks like magic,” she says. “These polar molecules are the pinnacle of quantum optics.”

Obtenido de: http://www.physorg.com/news195837003.html

Tirso Ramírez

CRF

'Quantum computer' a stage closer with silicon breakthrough

The remarkable ability of an electron to exist in two places at once has been controlled in the most common electronic material - silicon - for the first time. The research findings - published in Nature by a UK-Dutch team from the University of Surrey, UCL (University College) London, Heriot-Watt University in Edinburgh, and the FOM Institute for Plasma Physics near Utrecht - marks a significant step towards the making of an affordable "quantum computer".


The electron orbits a phosphorus atom embedded in the silicon lattice, shown in silver. The undisturbed electron density distribution, calculated from the quantum mechanical equations of motion is shown in yellow. A laser pulse can modify the electron’s state so that it has the density distribution shown in green. Our first laser pulse, arriving from the left, puts the electron into a superposition of both states, which we control with a second pulse, also from the left, to give a pulse which we detect, emerging to the right. The characteristics of this "echo" pulse tell us about the superposition we have made. Credit: UCL

According to the research paper in Nature the scientists have created a simple version of Schrodinger's cat - which is paradoxically simultaneously both dead and alive - in the cheap and simple material out of which ordinary computer chips are made.

"This is a real breakthrough for modern electronics and has huge potential for the future," explained Professor Ben Murdin, Photonics Group Leader at the University of Surrey. "Lasers have had an ever increasing impact on technology, especially for the transmission of processed information between computers, and this development illustrates their potential power for processing information inside the computer itself. In our case we used a far-infrared, very short, high intensity pulse from the Dutch FELIX laser to put an electron orbiting within silicon into two states at once - a so-called quantum superposition state. We then demonstrated that the superposition state could be controlled so that the electrons emit a burst of light at a well-defined time after the superposition was created. The burst of light is called a photon echo; and its observation proved we have full control over the quantum state of the atoms."

And the development of a silicon based "quantum computer" may be only just over the horizon. "Quantum computers can solve some problems much more efficiently than conventional computers - and they will be particularly useful for security because they can quickly crack existing codes and create un-crackable codes," Professor Murdin continued. "The next generation of devices must make use of these superpositions to do quantum computations. Crucially our work shows that some of the quantum engineering already demonstrated by atomic physicists in very sophisticated instruments called cold atom traps, can be implemented in the type of silicon chip used in making the much more common transistor."

Professor Gabriel Aeppli, Director of the London Centre for Nanotechnology added that the findings were highly significant to academia and business alike. "Next to iron and ice, silicon is the most important inorganic crystalline solid because of our tremendous ability to control electrical conduction via chemical and electrical means," he explained. "Our work adds control of quantum superpositions to the silicon toolbox."

Obtenido de: http://www.physorg.com/news196516338.html
Tirso Ramírez
CRF


How 'spooky' quantum mechanical laws may affect everyday objects (Update)

(PhysOrg.com) -- In a study published in the July 1 issue of the journal Nature, Dartmouth researchers describe one example of the microscopic quantum world influencing--even dominating, they say--the behavior of something in the macroscopic classical world.



An optical micrograph of one of the samples measured by the research team is shown here. The electrical contacts are at the top and bottom. The gold gates used to form the QPC tunnel barrier are also labeled. Credit: Joel Stettenheim, Dartmouth College

"One major question in physics has to do with the connection between the microscopic and macroscopic worlds," said Alex Rimberg, associate professor of physics at Dartmouth College.

In the microscopic world, tiny sub-atomic particles such as photons and electrons, obey the sometimes bizarre laws of quantum mechanics. Meanwhile objects in the macroscopic world, generally anything visible with the naked eye, conform to the laws of classical physics discovered by Newton in the 17th century.

But a little more than 300 years after Newton, Einstein proved that light consists of tiny "packets" of energy, called quanta. This discovery marked the beginning of quantum theory, though it took decades of further work by several great scientific minds to finally settle on the modern theory of quantum mechanics.

One of the strangest laws of quantum mechanics is the Uncertainty Principle, first noted by German physicist and Nobel Laureate Werner Heisenberg in 1927. Heisenberg realized that when trying to locate a fast-moving particle, such as an electron, it was impossible to pin down both its position and its momentum at the same time.

"To do a measurement, an experiment has to interact with whatever is being measured," explained Rimberg. "But interaction means ultimately that you must exert a force on what you're measuring. If you're trying to measure the position of an object, any measurement will make the object move in an unpredictable and random way. This tendency to randomly affect what you are measuring is called "backaction."

Einstein could never accept this idea--that the act of measurement changes the object being measured--on philosophical grounds, and fought it until his dying breath. But the uncertainty principle is now known to be true for all quantum-level interactions.

What is not yet known is how the quantum and classical worlds relate. "What we don't understand, really, is how classical behavior emerges from quantum behavior as systems become larger and larger," Rimberg said. "We also don't really understand how large an influence quantum mechanics can have on the classical world we live in."

Making it real

Rimberg and colleague Miles Blencowe, both supported by grants from the National Science Foundation (NSF), have now led a team of researchers in demonstrating quantum mechanical events affecting the classical world.

The scientists didn't start out to accomplish any such thing, according to Rimberg. Instead, they were trying to measure fast changes in charge at nanometer scales.

To do this, they first created tiny semiconductor crystals, similar to a computer chip, each about 3 millimeters (about 1/10 of an inch) across. They deposited gold electrical gates running over the crystal, leaving a tiny break of only about a few hundred micrometers in the middle of the chip. This break is called a "quantum point contact," or QPC.

By hooking the chip up to an electrical circuit, electrons flow through metal contacts until they hit the QPC. And that's where they started to see one of quantum mechanics' quirks.

"You can think of the QPC as a tunnel barrier, sort of a wall for electrons," Rimberg explained. "When the wall is sufficiently high, the electrons do not have enough energy to go over it. If electrons were classical objects, that would be the end of the story. But since electrons obey the laws of quantum mechanics, instead of going over the barrier they can also "quantum tunnel" through it."

Thus, when a stream of electrons in an electrical current approaches the QPC, each electron in the stream randomly "chooses" to reflect backward off the barrier or go through it.

"This random process introduces noise into the electrical current, caused by random fluctuations in the number of electrons going through at any time," said Rimberg. "Because this noise is generated quantum mechanically, it is sometimes referred to as quantum noise."

Measuring quantum noise

For this experiment, the scientists used semiconductor crystals made of gallium arsenide, which happen to exhibit a property called piezoelectricity. The term "piezoelectric" means that an electrical current traveling through the crystal causes a mechanical or physical movement of the crystal itself, similar to the way a sponge expands when water hits it.

Piezoelectric crystals are sometimes called resonators, because they can resonate, or vibrate, in reaction to electrical signals. These resonators can move in different ways-stretching or bending, depending on the frequency of the signal and the shape of the crystal.

"The three-dimensional vibration of a resonator crystal is exactly like the vibration you get if you strike a tuning fork, or run a wet finger about the rim of a wine glass," Rimberg explained. "The glass (or tuning fork) starts to hum with a musical note; that's because there is a particular kind of vibrational pattern, determined by its geometry, that the atoms in the wine glass collectively take part in."

In the same way, the electrons bouncing off the QPC "wall" apply a random "backaction" force to the crystal, Rimberg said. In this case, the backaction force just happened to vibrate the crystal at one of its favorite frequencies. When the researchers measured the electrical current versus frequency and found strong peaks indicating that the backaction was creating a feedback loop, it took them by surprise.

"Neither I nor anyone else anticipated the spectral features that show the samples are vibrating," Rimberg said. "It took us quite a bit of time and effort to convince ourselves that it was a real effect, and yet more time and effort to figure out what it was."

Uncertainty in action

"In our case, the current running through the QPC gives information about the position of the semiconducting crystal that the QPC lives in," Blencowe said. "But because of the quantum noise in the current, at any given time there are random fluctuations in the number of electrons (on the order of 10,000 or so) on either side of the QPC."

And because these electrons have an electrical charge, they exert a piezoelectric force on the crystal, making it move. "The remarkable thing is that only 10,000 or so electrons are able to make all 1020 (100 quintillion) atoms in the crystal move at once," said Blencowe.

"The difference in size between the two parts of the system is really extreme," Blencowe explained. "To give a sense of perspective, imagine that the 10,000 electrons correspond to something small but macroscopic, like a flea. To complete the analogy, the crystal would have to be the size of Mt. Everest. If we imagine the flea jumping on Mt. Everest to make it move, then the resulting vibrations would be on the order of meters!"

"Our work is a direct example of the microscopic quantum world influencing, and even dominating the behavior of something in the macroscopic classical world," Rimberg said. "The motion of the semiconducting crystal is not dominated by something classical like thermal motion, but instead by the random quantum fluctuations in the number of tunneling electrons."

And in this case, Rimberg pointed out, the macroscopic world also influences the quantum world, because the crystal's vibrations cause the electrons to tunnel in large bunches.

In future research the team could take several possible directions. "First, we will actually use the QPC for charge detection, as we had intended to all along," Rimberg said. "Second, we'll continue to look at questions regarding the quantum-classical transition, but with resonators that are smaller than these crystals--things that are in the murky borderland between the much better understood quantum and classical regimes." This borderland is sometimes known as the "mesoscopic" scale.

"The study of these kinds of systems advances fundamental knowledge and also addresses some very practical questions, including: What are the fundamental limits of measurement? And what is the most sensitive measurement device that can be made?" said Daryl Hess, a program manager in NSF's Division of Materials Research.

"Questions of this kind become more pressing as our science and technology shrink to ever smaller scales with the vision of devices, electronic and mechanical, that are perhaps only a few atoms in one or more dimensions," added Hess. "At these scales, devices could show some aspects that appear squarely in the world of quantum mechanics and others that appear to be squarely in the world of classical mechanics."

Obtenido de: http://www.physorg.com/news197120339.html

Tirso Ramírez

CRF

Novel ion trap with optical fiber could link atoms and light in quantum networks

Physicists at the National Institute of Standards and Technology have demonstrated an ion trap with a built-in optical fiber that collects light emitted by single ions (electrically charged atoms), allowing quantum information stored in the ions to be measured. The advance could simplify quantum computer design and serve as a step toward swapping information between matter and light in future quantum networks.

This is a diagram of a NIST ion trap that incorporates an optical fiber to collect light emitted by the ions (electrically charged atoms). Individual electrodes used to trap an ion 30 to 50 micrometers above the surface are shown in different colors surrounding a 50-micrometer-wide hole where light is collected and deposited in a fiber attached below. Credit: A. VanDevender/NIST


Described in a forthcoming issue of Physical Review Letters,* the new device is a 1-millimeter-square ion trap with a built-in optical fiber. The authors use ions as quantum bits (qubits) to store information in experimental quantum computing, which may someday solve certain problems that are intractable today. An ion can be adjustably positioned 80 to 100 micrometers from an optical fiber, which detects the ion's fluorescence signals indicating the qubit's information content.

"The design is helpful because of the tight coupling between the ion and the fiber, and also because it's small, so you can get a lot of fibers on a chip," says first author Aaron VanDevender, a NIST postdoctoral researcher.

NIST scientists demonstrated the new device using magnesium ions. Light emitted by an ion passes through a hole in an electrode and is collected in the fiber below the electrode surface (see image). By contrast, conventional ion traps use large external lenses typically located 5 centimeters away from the ions—about 500 times farther than the fiber—to collect the fluorescence light. Optical fibers may handle large numbers of ions more easily than the bulky optical systems, because multiple fibers may eventually be attached to a single ion trap.

The fiber method currently captures less light than the lens system but is adequate for detecting quantum information because ions are extremely bright, producing millions of photons (individual particles of light) per second, VanDevender says. The authors expect to boost efficiency by shaping the fiber tip and using anti-reflection coating on surfaces. The new trap design is intended as a prototype for eventually pairing single ions with single photons, to make an interface enabling matter qubits to swap information with photon qubits in a quantum computing and communications network.

Photons are used as qubits in quantum communications, the most secure method known for ensuring the privacy of a communications channel. In a quantum network, the information encoded in the "spins" of individual ions could be transferred to, for example, electric field orientations of individual photons for transport to other processing regions of the network.

Obtenido de: http://www.physorg.com/news197806669.html
Tirso Ramírez
CRF

Tunable quantum cascade laser

A team at the Central Research Laboratory at Hamamatsu Photonics K.K. in Japan has come up with, and tested, a tunable quantum cascade laser design that demonstrates broad optical gain. Their work is published in Applied Physics Letters: “High-performance, homogenous broad-gain quantum cascade lasers based on dual-upper-state design.”

“Usually, quantum cascade laser designs have only one upper state, except for superlattice active region,” Kazuue Fujita, the lead author on the paper, tells PhysOrg.com in an email interview. In this case, though, there is an additional upper state added to the design of the laser. “The additional upper state is created by a first quantum well adjacent to the injection barrier. This state corresponds to a lowest energy state in the first quantum well.”

Fujita goes on to explain that the regular upper state, seen in most conventional quantum cascade lasers, is designed to be nearly the same energy at operating condition as the state in the first quantum well. “Electrons are injected into the higher upper state via resonant tunneling from the previous injector. And then, electrons populated in the both upper states transit from both upper states to a lower state.” Both of these transitions contribute to optical gain.

This new laser design would have a number of advantages over current quantum cascade laser designs insists Fujita. This laser design allows for broadband tuning. On top of that, there is weak dependence on voltage by these lasers, and they are not sensitive to temperature change. “Slow efficiency at threshold is observed to be nearly constant over the wide range. Also I-L characteristics show super-linear behavior. These distinctive features have never been observed in a quantum cascade laser so far,” Fujita explains. “I think this design concept holds large potentialities.”

Fujita says that the design has actually been tested. “We have already fabricated and measured many lasers with the design. The lasers demonstrate very good performances. In addition, high temperature, continuous wave operation of the laser has also been achieved.”

In terms of application, Fujita sees a great deal of usefulness, especially in terms of spectroscopy. Trace gas sensing is considered one of the more likely applications, since wide tunability is desired. “The external cavity quantum cascade laser with this design may operate very stable due to its low dependences on voltage and temperature,” he says. Due to its tunability, this quantum cascade laser design could also increase the cost efficiency of some applications. “This design can lead to high-performance broadband tuning. Therefore, the laser allows a reduction in the number of lasers in a spectroscopic analysis system.”

Obtenido de: http://www.physorg.com/news197714902.html
Tirso Ramírez
CRF

Quantum non-demolition measurement allows physicists to count photons without destroying them

(PhysOrg.com) -- In a way, the quantum world seems to know when it's being watched. When physicists make measurements on photons and other quantum-scale particles, the measurements always disturb the system in some way. Although an ideal disturbance should still enable physicists to make multiple measurements and get the same result twice, most real measurements cause a greater disturbance than this ideal minimum, and prohibit physicists from making repeated measurements. In a recent study, physicists have demonstrated a new way to make one of the ideal measurements - called quantum non-demolition (QND) measurements - allowing physicists to detect single particles repeatedly without destroying them.

The physicists performed a quantum non-demolition measurement, illustrated in this circuit schematic, that could detect single photons without destroying them. The technique allows repeated measurements to be made that give the same result. Image credit: B.R. Johnson, et al. ©2010 Macmillan Publishers Limited.


The concept of QND measurements has been around since the beginning of quantum mechanics, and physicists have demonstrated different QND measurement techniques since the ‘70s. In the latest technique, developed by a team of physicists from Yale University, Princeton University, and the University of Waterloo, the scientists have shown how to measure the number of photons inside a microwave cavity in a way that preserves the photon state 90% of the time; in other words, the method is 90% QND. The physicists explain that, unlike previously reported QND methods, the new technique is strongly selective to chosen photon number states, which could make it useful for applications such as monitoring the state of a photon-based memory in a quantum computer.


In their experiments, the physicists wanted to find out how many photons were in a microwave cavity. To do this without disturbing the system, they coupled a superconducting qubit to a cavity. This cavity stored the photons long enough for them to be measured - or “interrogated” - by using a set of controlled-NOT (CNOT) operations to encode information about the cavity state onto the qubit state. Then the qubit and storage cavity were decoupled, and the qubit state was read out. Because the qubit state now depends on the number of photons in the cavity, measuring the qubit reveals the number of photons.

“Our method takes advantage of the ability to engineer interactions between cavities and qubits in superconducting circuits to make the qubit energies strongly depend on the number of photons in the cavity,” coauthor Blake Johnson of Yale University told PhysOrg.com. “We have made this effect large enough to build a new qubit-photon logic gate which allows us to perform conditional qubit operations based on the cavity state. This type of logic gate is not only applicable to photon readout, but also to some proposals for engineering interactions between photons by using a qubit as a mediator.”

In the new design, the photon read out time is faster than the photon decay time. This timing difference allows the physicists to measure any qubit state several times during the lifetime of photons in the storage cavity. A single interrogation process takes about 550 nanoseconds, which includes the 50-nanosecond to initialize the qubit state. As expected with a high-quality QND method, the results of repeated interrogations are essentially indistinguishable from the first. In contrast, as Johnson explained, a typical quantum measurement would destroy one photon every time, so that repeated interrogations would give different results.

“A typical photon detector, like a CCD or photo-multiplier tube, absorbs photons,” Johnson said. “These detectors don’t work for microwaves because the energy of a microwave photon is too small to generate charges. However, with a setup similar to the one used in our paper, one could measure the photon state by transferring the photon energy into the qubit. This method would destroy exactly one photon every time. In contrast, our detector does not transfer any energy. Instead, we attempt to add energy to the qubit from an external source in such a way that the success or failure of these attempts reveals information about the cavity state. You might worry that this added energy might leak into the cavity and changed the photon number, but we have checked that this does not, in fact, happen.”

Achieving QND measurements of photons, while challenging, could be very useful for the development of quantum information technologies, which require complete control of quantum measurements. As the physicists note in their study, recent progress in manipulating microwave photons in superconducting circuits has increased the demand for a QND detector that operates in the gigahertz frequency range (like the one demonstrated here). In addition, the physicists predict that further research could make it possible to observe quantum jumps of light in a circuit, among other things.

“QND detection in general is interesting because it is the only way that quantum mechanics allows to extract information from a system without modifying its state, and then allowing feedback and manipulation of the same,” Johnson said. “The applications are interesting because if one could implement feedback of a quantum system, one could imagine using these systems for quantum simulation and quantum computation, harnessing quantum mechanics toward the goal of practical application.”

Obtenido de: http://www.physorg.com/news197873165.html
Tirso Ramírez
CRF

Seeking a bridge to the quantum world

Science fiction has nothing over quantum physics when it comes to presenting us with a labyrinthine world that can twist your mind into knots when you try to make sense of it.

A team of Arizona State University researchers, however, believe they’ve opened a door to a clearer view of how the common, everyday world we experience through our senses emerges from the ethereal quantum world.

Physicists call our familiar everyday environment the classical world. That’s the world in which we and the things around us appear to have measurable characteristics such as mass, height, color, weight, texture and shape.

The quantum world is the world of the elemental building block of matter – atoms. Atoms are combinations of neutrons and protons and electrons bound to a nucleus by electrical attraction.

But most of an atom – more than 99 percent of it – is empty space filled with invisible energy.

So from a quantum-world view, we and the things around us are mostly empty space. The way we experience ourselves and other things in the classical world is really just “a figment of our imaginations shaped by our senses,” said ASU Regents’ Professor David Ferry.

Akin to evolution

For more than a century, scientists and engineers have struggled to come to a satisfactory conclusion about the missing link that bridges the classical and quantum worlds and enables a transition from that world of mostly empty space to the familiar environment we experience through our senses.

One proposed scenario based on these questions was investigated in a dissertation written by Adam Burke to earn his doctorate in electrical engineering in 2009 from ASU’s Ira A. Fulton Schools of Engineering.

To try working out an answer to some of the questions, Burke teamed with Ferry, a professor in the School of Electrical, Computer and Energy Engineering; Tim Day, who recently earned his doctorate in electrical engineering from the school; physicist Richard Akis, an associate research professor in the school; Gil Speyer, an assistant research scientist for the engineering schools’ High Performance Computing Initiative; and Brian Bennett, a materials scientist with the Naval Research Laboratory.

The resultis an article published recently in the research journal Physical Review Letters and featured on PhysOrg.com, a science, technology and research news website. It describes the transition from quantum to classical world as a “decoherence” process that involves a kind of evolutionary progression somewhat analogous to Charles Darwin’s concept of natural selection.

Darwinian processes

The authors built on two theories called decoherence and quantum Darwinism, both proposed by Los Alamos National Laboratory researcher Wojciech Zurek.

The decoherence concept holds that many quantum states “collapse” into a “broad diaspora,” or dispersion, while interacting with the environment. Through a selection process, other quantum states arrive at a final stable state, called a pointer state, which is “fit enough” (think “survival of the fittest” in Darwinian terms) to be transmitted through the environment without collapsing.

These single states with the lowest energy can then make high-energy copies of themselves that can be described by the Darwinian process and observed on the macroscopic scale in the classical world.

The experiments arose from using advanced scanning gate microscopy to obtain images of what are called quantum dots.

Entering ‘forbidden space’

Burke, now doing research in a post-doctoral program at the University of New South Wales in Sydney, Australia, explains it like this:

Imagine the quantum dot as a billiard table in which the quantum point contacts are the two openings through which a ball could enter or leave the dot, and the interior walls of the dot act as bumpers.

If there were no friction on the table, a billiard ball with an initial trajectory would bounce off of these walls until eventually finding an exit and leaving the dot (this is the decoherence part).

Or it might find a trajectory that does not couple to the openings and would therefore be a surviving pointer state, what is called a diamond state.

One difference between the classical physics of billiard balls and the quantum physics of electrons is that an electron can tunnel through “forbidden phase space” to enter this diamond state, whereas a billiard ball entering from outside the dot would not find itself able to reach this diamond trajectory.

Bouncing around quantum dots

It is this isolated classical trajectory, and the buildup of an electron wave functions' amplitude along that trajectory, that is referred to as a scarred wave function.

To experimentally measure these scars, imagine that we can’t see inside the walls of our billiard table, but we can count the billiard balls exiting the table. This is what is normally measured with the conductance of the quantum dot and its environment.

“We measure the current through the dot, the numbers of ‘billiard balls’ passing through it per second, to try to see how this changes when we move our probe around the ‘billiard table,’ ” Ferry said.

Furthermore, there is the probe of the scanning gate microscope, which applies a small electric field. This can be pictured as a small circular bumper on the billiard table that can be moved around within the dot.

This small "bumper" is rastered left to right, top to bottom over the area of interest. If a ball is traveling along this diamond pattern it is perturbed by the bumper when it rasters into the trajectory.

Think of rastering like the way a television image works, with a pattern of scanning lines that cover the area on which the image is projected, or a set or horizontal lines composed of individual pixels that are used to form an image on a computer screen.

When this happens, the ball bounces off the perturbation, and takes a new course within the dot until finally coupling out one of the openings to be measured. The change in the ball’s motion appears as a change in the conductance – the number of balls going through the openings in a given time.

Smoking gun

Ferry explained: “With scanning gate microscopy, we monitor where these changes occur within the scans, and hopefully this gives us a map of the scarred wave functions corresponding to the pointer states.”

Quantum mechanically, he said, a new electron will tunnel right into the diamond state, so the measurement can continue until the whole area is mapped.

The data that came from the team’s experiment supports Zurek's theories of decoherence and quantum Darwinism, Burke said.

Ferry said these findings are just one step in a process that is open to conjecture, but they point toward a “smoking gun” for the existence of this quantum Darwinism and a new view in the search for evidence of how the quantum-to-classical world transition actually occurs.

If you can wrap your mind around all this, he said, “You open the door to a deeper understanding of what is really going on” at the core of physical reality.

Obtenido de: http://asunews.asu.edu/20100630_davidferryquantum

Tirso Ramírez

CRF

sábado, 17 de julio de 2010

Physicists capture first images of atomic spin

ATHENS, Ohio (April 26, 2010) – Though scientists argue that the emerging technology of spintronics may trump conventional electronics for building the next generation of faster, smaller, more efficient computers and high-tech devices, no one has actually seen the spin—a quantum mechanical property of electrons—in individual atoms until now.
In a study published as an Advance Online Publication in the journal Nature Nanotechnology on Sunday, physicists at Ohio University and the University of Hamburg in Germany present the first images of spin in action.

The researchers used a custom-built microscope with an iron-coated tip to manipulate cobalt atoms on a plate of manganese. Through scanning tunneling microscopy, the team repositioned individual cobalt atoms on a surface that changed the direction of the electrons’ spin. Images captured by the scientists showed that the atoms appeared as a single protrusion if the spin direction was upward, and as double protrusions with equal heights when the spin direction was downward.

The study suggests that scientists can observe and manipulate spin, a finding that may impact future development of nanoscale magnetic storage, quantum computers and spintronic devices.

“Different directions in spin can mean different states for data storage,” said Saw-Wai Hla, an associate professor of physics and astronomy in Ohio University’s Nanoscale and Quantum Phenomena Institute and one of the primary investigators on the study. “The memory devices of current computers involve tens of thousands of atoms. In the future, we may be able to use one atom and change the power of the computer by the thousands.”

Unlike electronic devices, which give off heat, spintronic-based devices are expected to experience less power dissipation.

The experiments were conducted in an ultra-high vacuum at the low temperature of 10 Kelvin, with the use of liquid helium. Researchers will need to observe the phenomenon at room temperature before it can be used in computer hard drives.

But the new study suggests a path to that application, said study lead author Andre Kubetzka of the University of Hamburg. To image spin direction, the team not only used a new technique but also a manganese surface with a spin that, in turn, allowed the scientists to manipulate the spin of the cobalt atoms under study.

“The combination of atom manipulation and spin sensitivity gives a new perspective of constructing atomic-scale structures and investigating their magnetic properties,” Kubetzka said.

The research, which was carried out at the University of Hamburg, was supported by the German Research Foundation and a Partnership for International Collaboration and Education (PIRE) grant from National Science Foundation.

The research is the result of a collaboration among three research teams: a spin-polarized scanning tunneling microscopy group of Professor Roland Wiesendanger led by Kubetzka at the University of Hamburg, Germany; Hla, an expert in atom manipulation at Ohio University; and two theorists, Professor Stefan Heinze and Paolo Ferriani, now at the Christian-Albrechts-Universität Kiel, in Germany.

Obtenido de: http://www.ohio.edu/research/communications/spin.cfm
Tirso Ramírez
CRF

FRICTION DIFFERENCES OFFER NEW MEANS FOR MANIPULATING NANOTUBES

Nanotubes and nanowires are promising building blocks for future integrated nanoelectronic and photonic circuits, nanosensors, interconnects and electro-mechanical nanodevices. But some fundamental issues remain to be resolved—among them, how to position and manipulate the tiny tubes.


Publishing in the journal Nature Materials, researchers from four different institutions report measuring different friction forces when a carbon nanotube slides along its axis compared to when it slides perpendicular to its axis. This friction difference has its origins in soft lateral distortion of the tubes when they slide in the transverse direction.

The findings not only could provide a better understanding of fundamental friction issues, but from a more practical standpoint, offer a new tool for assembling nanotubes into devices and clarify the forces acting on them. Asymmetries in the friction could potentially also be used in sorting nanotubes according to their chirality, a property that is now difficult to measure with other means.

When an atomic force microscope (AFM) tip is scanned transversely across a multi-walled carbon nanotube, the amount of friction measured is twice as much as when the same tube is scanned longitudinally, along the length of the tube. The researchers attribute this difference to what they call “hindered rolling”—additional effort required to overcome the nanotube’s tendency to roll as the AFM tip strokes across it rather than along it.

“Because the energy required to move in one direction is twice as much as required to move in the other direction, this could be an easy way to control the assembly of carbon nanotubes for nanoelectronics, sensors and other applications,” said Elisa Riedo, co-author of the study and an associate professor in the School of Physics at the Georgia Institute of Technology. “To assemble nanotubes on a surface, you need to know how they interact and what force is needed to move them.”

The combined theoretical and experimental study was supported by the U.S. Department of Energy. Other institutions contributing to the project include the International Centre for Theoretical Physics, International School for Advanced Studies and CNR Democritos Laboratory—all in Trieste, Italy—and the University of Hamburg in Germany. The paper was published in advance online on September 13 by the journal Nature Materials.

Carbon nanotubes have exceptional thermal, mechanical and electrical properties that have generated considerable interest since they were first reported in 1991. Though friction has been studied before in nanotubes, this research is the first to provide detailed information about the frictional forces at work in both the longitudinal and transverse directions when the tubes interact with an AFM tip.

Friction is one of the oldest problems in physics and one of the most important to everyday life. It is estimated that the losses in the U.S. economy due to friction total about 6 percent of the gross national product. Friction is even more important to micro-electromechanical systems (MEMS) and nanoscale devices because these smaller systems are more affected by surface forces than large systems.

“As systems become smaller and smaller, it becomes more important to understand how to address friction,” said Riedo. “Surface forces can prevent micro and nano systems from operating at all.”

Experimentally, the researchers scanned an atomic force microscope tip longitudinally along a series of multiwalled carbon nanotubes held stationary on a substrate. They also conducted a series of similar scans in the transverse direction. The researchers applied a consistent force on the AFM tip in both scanning directions, and relied on powerful Van der Waals forces to hold tubes in place on the substrate.

“When you scan a nanotube transversely, you are probing something very different,” said Riedo. “You are also probing additional dissipation modes due to a kind of swaying motion in which energy is also dissipated through movement of the nanotube as it alters its cross section.”

The experiment showed that greater forces were required to move the tip in the transverse direction. Using molecular dynamics simulations, collaborators Erio Tosatti and Xiaohua Zhang at the International Centre for Theoretical Physics, International School for Advanced Studies and CNR Democritos Laboratory analyzed the phenomenon to understand what was happening.

“In principle, there seems to be no reason why the frictional forces required to move the AFM tip would be different in one direction,” Riedo noted. “But the theory confirmed that this ‘hindered rolling’ and soft mode movement of the nanotubes are the sources of the higher friction when the tip moves transversely.”

Because the nanotube-tip system is so simple, it offers an ideal platform for studying basic friction principles, which are important to all moving systems.

“This kind of system gives you the opportunity to explore friction using an ideal experiment so you can really probe the energy dissipation mechanism,” Riedo explained. “The system is so simple that you can distinguish between the dissipation mechanisms, which you can’t usually do well in macro-scale systems.”

Based on the molecular dynamics simulations, Riedo and Tosatti believe that the friction anisotropy will be very different in chiral nanotubes versus non-chiral—left-to-right symmetric—nanotubes.

“Because of the chirality, the tip moves in a screw-like fashion, creating hindered rolling even for longitudinal sliding,” Tosatti said. “Thus, the new measuring technique may suggest a simple way to sort the nanotubes; among the next steps in the research will be to show experimentally that this can be done.”

In addition to the researchers already mentioned, co-authors for this paper include Christian Klinke at the Institute of Physical Chemistry at the University of Hamburg, and Marcel Lucas and Ismael Palaci at Georgia Tech.

“Understanding the basic mechanism of friction in carbon nanotubes will help us in designing devices with them in the future,” Riedo added. “We have shown an anisotropy in the friction coefficient of carbon nanotubes in the transverse and longitudinal directions, which has its origin in the soft lateral distortion of tubes when the tip-tube contact is moving in the transverse direction. Our findings could help in developing better strategies for chirality sorting, large-scale self-assembling of nanotubes on surfaces, and designing nanotube adhesives and nanotube-polymer composite materials.”

Obtenido de: http://www.scitech-news.com/search/label/Nanotechnology
Tirso Ramírez
CRF

PARTICLE CHAMELEON CAUGHT IN THE ACT OF CHANGING

Researchers on the OPERA experiment at the INFN1’s Gran Sasso laboratory in Italy announced the first direct observation of a tau particle in a muon neutrino beam sent through the Earth from CERN2, 730km away. This is a significant result, providing the final missing piece of a puzzle that has been challenging science since the 1960s, and giving tantalizing hints of new physics to come.

The neutrino puzzle began with a pioneering and ultimately Nobel Prize winning experiment conducted by US scientist Ray Davis beginning in the 1960s. He observed far fewer neutrinos arriving at the Earth from the Sun than solar models predicted: either solar models were wrong, or something was happening to the neutrinos on their way. A possible solution to the puzzle was provided in 1969 by the theorists Bruno Pontecorvo and Vladimir Gribov, who first suggested that chameleon-like oscillatory changes between different types of neutrinos could be responsible for the apparent neutrino deficit.

Several experiments since have observed the disappearance of muon-neutrinos, confirming the oscillation hypothesis, but until now no observations of the appearance of a tau-neutrino in a pure muon-neutrino beam have been observed: this is the first time that the neutrino chameleon has been caught in the act of changing from muon-type to tau-type.

Antonio Ereditato, Spokesperson of the OPERA collaboration described the development as: “an important result which rewards the entire OPERA collaboration for its years of commitment and which confirms that we have made sound experimental choices. We are confident that this first event will be followed by others that will fully demonstrate the appearance of neutrino oscillation”.

“The OPERA experiment has reached its first goal: the detection of a tau neutrino obtained from the transformation of a muon neutrino, which occurred during the journey from Geneva to the Gran Sasso Laboratory,” added Lucia Votano, Director Gran Sasso laboratories. “This important result comes after a decade of intense work performed by the Collaboration, with the support of the Laboratory, and it again confirms that LNGS is a leading laboratory in Astroparticle Physics”.

The OPERA result follows seven years of preparation and over three years of beam provided by CERN. During that time, billions of billions of muon-neutrinos have been sent from CERN to Gran Sasso, taking just 2.4 milliseconds to make the trip. The rarity of neutrino oscillation, coupled with the fact that neutrinos interact very weakly with matter makes this kind of experiment extremely subtle to conduct. CERN’s neutrino beam was first switched on in 2006, and since then researchers on the OPERA experiment have been carefully sifting their data for evidence of the appearance of tau particles, the telltale sign that a muon-neutrino has oscillated into a tau-neutrino. Patience of this kind is a virtue in particle physics research, as INFN President Roberto Petronzio explained:

“This success is due to the tenacity and inventiveness of the physicists of the international community, who designed a particle beam especially for this experiment,” said Petronzio. “In this way, the original design of Gran Sasso has been crowned with success. In fact, when constructed, the laboratories were oriented so that they could receive particle beams from CERN”.

At CERN, neutrinos are generated from collisions of an accelerated beam of protons with a target. When protons hit the target, particles called pions and kaons are produced. They quickly decay, giving rise to neutrinos. Unlike charged particles, neutrinos are not sensitive to the electromagnetic fields usually used by physicists to change the trajectories of particle beams. Neutrinos can pass through matter without interacting with it; they keep the same direction of motion they have from their birth. Hence, as soon as they are produced, they maintain a straight path, passing through the Earth's crust. For this reason, it is extremely important that from the very beginning the beam points exactly towards the laboratories at Gran Sasso.

“This is an important step for neutrino physics,” said CERN Director General Rolf Heuer. “My congratulations go to the OPERA experiment and the Gran Sasso Laboratories, as well as the accelerator departments at CERN. We’re all looking forward to unveiling the new physics this result presages.”

While closing a chapter on understanding the nature of neutrinos, the observation of neutrino oscillations is strong evidence for new physics. In the theories that physicists use to explain the behaviour of fundamental particles, which is known as the Standard Model, neutrinos have no mass. For neutrinos to be able to oscillate, however, they must have mass: something must be missing from the Standard Model. Despite its success in describing the particles that make up the visible Universe and their interactions, physicists have long known that there is much the Standard Model does not explain. One possibility is the existence of other, so-far unobserved types of neutrinos that could shed light on Dark Matter, which is believed to make up about a quarter of the Universe’s mass.

Obtenido de: http://www.scitech-news.com/2010/06/particle-chameleon-caught-in-act-of.html
Tirso Ramírez
CRF

All-in-one nanoparticle: A Swiss Army knife for nanomedicine

Nanoparticles are being developed to perform a wide range of medical uses -- imaging tumors, carrying drugs, delivering pulses of heat. Rather than settling for just one of these, researchers at the University of Washington have combined two nanoparticles in one tiny package.

The result is the first structure that creates a multipurpose nanotechnology tool for medical imaging and therapy. The structure is described in a paper published online this week in the journal Nature Nanotechnology.A quantum dot (red) encapsulated in a gold shell, combining two useful nanoparticles in one package. The total structure measures less than 20 nanometers across

"This is the first time that a semiconductor and metal nanoparticles have been combined in a way that preserves the function of each individual component," said lead author Xiaohu Gao, a UW assistant professor of bioengineering.

The current focus is on medical applications, but the researchers said multifunctional nanoparticles could also be used in energy research, for example in solar cells.

Quantum dots are fluorescent balls of semiconductor material just a few nanometers across, a small fraction of the wavelength of visible light (a nanometer is 1-millionth of a millimeter). At this tiny scale, quantum dots' unique optical properties cause them to emit light of different colors depending on their size. The dots are being developed for medical imaging, solar cells and light-emitting diodes.

Glowing gold nanoparticles have been used since ancient times in stained glass; more recently they are being developed for delivering drugs, for treating arthritis and for a type of medical imaging that uses infrared light. Gold also reradiates infrared heat and so could be used in medical therapies to cook nearby cells.

But combine a quantum dot and a gold nanoparticle, and the effects disappear. The electrical fields of the particles interfere with one another and so neither behaves as it would on its own. The two have been successfully combined on a surface, but never in a single particle.

The paper describes a manufacturing technique that uses proteins to surround a quantum dot core with a thin gold shell held at 3 nanometers distance, so the two components' optical and electrical fields do not interfere with one another. The quantum dot likely would be used for fluorescent imaging. The gold sphere could be used for scattering-based imaging, which works better than fluorescence in some situations, as well as for delivering heat therapy.

The manufacturing technique developed by Gao and co-author Yongdong Jin, a UW postdoctoral researcher, is general and could apply to other nanoparticle combinations, they said.

"We picked a tough case," Gao said. "It is widely known that gold or any other metal will quench quantum dot fluorescence, eliminating the quantum dot's purpose."

Gao and Jin avoided this problem by building a thin gold sphere that surrounds but never touches the quantum dot. They carefully controlled the separation between the gold shell and the nanoparticle core by using chains of polymer, polyethylene glycol. The distance between the quantum dot core and charged gold ion is determined by the length of the polymer chain and can be increased with nanometer precision by adding links to the chain. On the outside layer they added short amino acids called polyhistidines, which bind to charged gold atoms.

Gao compares the completed structure to a golden egg, where the quantum dot is the yolk, the gold is the shell, and polymers fill up the space of the egg white.

Using ions allowed the researchers to build a 2- to 3-nanometer gold shell that's thin enough to allow about half of the quantum dot's fluorescence to pass through.

"All the traditional techniques use premade gold nanoparticles instead of gold ions,"Gao said. "Gold nanoparticles are 3 to 5 nanometers in diameter, and with factoring in roughness the thinnest coating you can build is 5-6 nanometers. Gold ions are much, much smaller.”

The total diameter of the combined particle is roughly 15-20 nanometers, small enough to be able to slip into a cell.

Incorporating gold provides a well-established binding site to attach biological molecules that target particular cells, such as tumor cells. Gold could also potentially amplify the quantum dot's fluorescence by five to 10 times, as it has in other cases.

The gold sphere offers one further benefit. Gold is biocompatible, is medically approved and does not biodegrade. A gold shell could thus provide a durable non-toxic container for nanoparticles being used in the body, Gao said.

The research was supported by the National Institutes of Health, the National Science Foundation, the Seattle Foundation and the UW's Department of Bioengineering.

Obtenido de: http://uwnews.org/article.asp?articleID=51016
Tirso Ramírez
CRF

Answer from 'Dusty Shelf' Aids Quest to See Matter as it Was Just After Big Bang

Scientists trying to recreate conditions that existed just a few millionths of a second after the big bang that started the universe have run into a mysterious problem -- some of the reactions they are getting don't mesh with what they thought they were supposed to see.

Now, two University of Washington physicists have dusted off a quantum mechanics technique usually associated with low-energy physics and applied it to results from experiments at Brookhaven National Laboratory on New York's Long Island that produce high-energy collisions between gold nuclei. The result is data much more in line with what theorists expected from the experiments, said John Cramer, a UW physics professor. That means physicists at Brookhaven probably have actually succeeded in creating quark-gluon plasma, a state of matter that has not existed since a microsecond after the big bang that began the universe.

As it turns out, the model the physicists were using was missing some pieces, say Cramer and Gerald Miller, also a UW physics professor, whose findings will be published this month in Physical Review Letters, a journal of the American Physical Society.

"We think we've solved the puzzle by identifying important phenomena that were left out of the model," Cramer said.

Since 2000, scientists have been using the Relativistic Heavy Ion Collider at Brookhaven to collide gold nuclei with each other at nearly the speed of light. They are trying to get subatomic particles called quarks and gluons to separate from the nuclei and form a superheated quark-gluon plasma, 40 billion times hotter than room temperature.

Physicists used a technique called Hanbury Brown-Twiss Interferometry, originally used by astronomers to measure the size of stars, to learn the size and duration of a fireball produced in the collision of two gold nuclei. The technique focuses on momentum differences between pairs of pions, the particles produced in the fireball.

Before the collider experiment began, scientists expected a quark-gluon plasma to fuel a large and long-lasting fireball. Instead, the interferometry data showed a fireball similar in size and duration to those seen at much lower energies. Researchers also expected to see pions pushed out of the plasma gradually, but instead they seem to explode out all at once.

"We expected to bring the nuclear liquid to a boil and produce a steam of quark-gluon plasma," Cramer said. "Instead, the boiler seems to be blowing up in our faces."

While other evidence suggested that the collider experiment had created a quark-gluon plasma, the interferometry data pointed away from that possibility. To solve the puzzle, Cramer and Miller used a phenomenon called chiral symmetry restoration, which predicts that subatomic particles will change in mass and size depending on their environment -- in a hot, dense plasma as opposed to a vacuum, for instance.

By adding that process to the model, they found that pions in the plasma have to expend a large amount of energy to escape, as if they were stuck in a deep hole and had to climb out. That is because chiral symmetry gives pions a low mass when they are inside the plasma but a much higher mass once outside. The scientists also allowed for some pions to disappear completely, to transform into some other type of particle as they emerge from the plasma.

The result reconciles all the evidence from the collider experiments, supporting the possibility that a quark-gluon plasma actually has been created.

"We have taken a quantum mechanics technique, called the nuclear optical model, from an old and dusty shelf and applied it to puzzling new physics results," Miller said. "It's really a scientific detective story."

The work, supported by U.S. Department of Energy grants, adds to the general understanding of what happened in the first microseconds after the big bang, he said, "and what we bring to bear is a better microscope, the microscope of quantum mechanics."

Cramer noted that adding chiral symmetry restoration to the picture achieved results very close to what computer models told scientists to expect, and did so without forcing the experimental data to fit preconceived standards.

"A microsecond after the big bang, there was a state of matter that no one was able to investigate until very recently," he said. "We are still learning, but our understanding is growing."

For more information, contact Cramer at (206) 616-4635, (206) 543-9194 or cramer@phys.washington.edu, or Miller at (206) 543-2995 or miller@phys.washington.edu

Obtenido de: http://www.bnl.gov/bnlweb/pubaf/pr/PR_display.asp?prID=05-X3
Tirso Ramírez
CRF

domingo, 11 de julio de 2010

Dartmouth researchers propose new way to reproduce a black hole

Despite their popularity in the science fiction genre, there is much to be learned about black holes, the mysterious regions in space once thought to be absent of light. In a paper published in the August 20 issue of Physical Review Letters, the flagship journal of the American Physical Society, Dartmouth researchers propose a new way of creating a reproduction black hole in the laboratory on a much-tinier scale than their celestial counterparts.

Miles Blencowe (photo by Joseph Mehling '69)

The new method to create a tiny quantum sized black hole would allow researchers to better understand what physicist Stephen Hawking proposed more than 35 years ago: black holes are not totally void of activity; they emit photons, which is now known as Hawking radiation.

"Hawking famously showed that black holes radiate energy according to a thermal spectrum," said Paul Nation, an author on the paper and a graduate student at Dartmouth. "His calculations relied on assumptions about the physics of ultra-high energies and quantum gravity. Because we can't yet take measurements from real black holes, we need a way to recreate this phenomenon in the lab in order to study it, to validate it."

In this paper, the researchers show that a magnetic field-pulsed microwave transmission line containing an array of superconducting quantum interference devices, or SQUIDs, not only reproduces physics analogous to that of a radiating black hole, but does so in a system where the high energy and quantum mechanical properties are well understood and can be directly controlled in the laboratory. The paper states, "Thus, in principle, this setup enables the exploration of analogue quantum gravitational effects."

"We can also manipulate the strength of the applied magnetic field so that the SQUID array can be used to probe black hole radiation beyond what was considered by Hawking," said Miles Blencowe, another author on the paper and a professor of physics and astronomy at Dartmouth.

This is not the first proposed imitation black hole, says Nation. Other proposed analogue schemes have considered using supersonic fluid flows, ultracold bose-einstein condensates and nonlinear fiber optic cables. However, the predicted Hawking radiation in these schemes is incredibly weak or otherwise masked by commonplace radiation due to unavoidable heating of the device, making the Hawking radiation very difficult to detect. "In addition to being able to study analogue quantum gravity effects, the new, SQUID-based proposal may be a more straightforward method to detect the Hawking radiation," says Blencowe.

In addition to Nation and Blencowe, other authors on the paper include Alexander Rimberg at Dartmouth and Eyal Buks at Technion in Haifa, Israel.

Obtenido de: http://www.dartmouth.edu/~news/releases/2009/08/21a.html
Tirso Ramírez
CRF

Conjuntos de Electrones Moviéndose Como Si Hubiera un Campo Magnético

Un equipo internacional de científicos dirigido por un grupo de la Universidad de Princeton ha descubierto recientemente que, en la superficie de ciertos materiales, los conjuntos de electrones se mueven de maneras que imitan la presencia de un campo magnético donde no hay ninguno presente. El hallazgo representa la constatación de uno de los más exóticos fenómenos cuánticos macroscópicos en la física de la materia condensada.
La investigación podría llevar a importantes avances en la construcción de un nuevo tipo de computadora cuántica que tendría la flexibilidad suficiente como para operar a temperaturas moderadas, en vez de a las bajas temperaturas que son un requisito ineludible para los diseños actuales.

Anteriormente, los científicos podían observar movimientos similares de los electrones sólo bajo fuertes campos magnéticos y bajas temperaturas, en lo que se conoce como Efecto Hall Cuántico, que condujo a la concesión de dos Premios Nobel de Física, en 1985 y 1998.

Sin embargo, unos teóricos de la Universidad de Pensilvania y la Universidad de California en Berkeley propusieron que en los límites de ciertos materiales tridimensionales, el espín de los electrones individuales y la dirección en que se mueven, estarían alineados directamente con los electrones correspondientes sin necesitar de altos campos magnéticos o temperaturas muy bajas. Los investigadores también teorizaron que para que esto sucediera, los electrones necesitaban moverse a velocidades sumamente altas.

Ahora, Zahid Hasan, profesor de física en la Universidad de Princeton, y sus colegas, han conseguido observar los espines sincronizados de muchos electrones moviéndose en un material exótico, un cristal de antimonio enlazado con bismuto.

"Este resultado es bastante asombroso porque estamos viendo a los electrones comportarse de una manera que es muy similar al modo en que lo hacen cuando está presente un campo magnético fuerte, pero no había ninguno en nuestro experimento", explica Hasan, quien dirigió la colaboración internacional con científicos de EE.UU., Suiza y Alemania.

Obtenido de: http://www.amazings.com/ciencia/noticias/250309b.html
Tirso Ramírez
CRF

La Lámpara Incandescente Más Pequeña del Mundo

Con el fin de explorar la frontera entre la termodinámica y la mecánica cuántica (dos teorías fundamentales de la física aparentemente incompatibles hasta ahora), un equipo del Departamento de Física y Astronomía de la Universidad de California en Los Ángeles (UCLA) ha creado la lámpara incandescente más pequeña del mundo.
El equipo lo dirige Chris Regan, del Instituto de Nanosistemas de California, en la UCLA, e incluye a Yuwei Fan, Scott Singer y Ray Bergstrom.

La termodinámica, la "joya de la corona" de los físicos del siglo XIX, se aplica a los sistemas con muchas partículas.

Por su parte, la mecánica cuántica, desarrollada en el siglo XX, funciona mejor cuando se trata de sistemas con sólo unas pocas partículas.

El equipo de la UCLA está utilizando su pequeña lámpara para estudiar la ley de la radiación del cuerpo negro, desarrollada por el célebre físico Max Planck en el año 1900, utilizando principios que ahora se consideran propios de ambas teorías.

La lámpara incandescente utiliza un filamento confeccionado con un único nanotubo de carbono que sólo tiene 100 átomos de ancho. A simple vista, el filamento resulta completamente invisible cuando la lámpara está apagada, pero parece un diminuto punto de luz cuando está encendida. Incluso con el mejor microscopio óptico, lo único que se puede determinar es que el tamaño del nanotubo es superior a cero. Para tener una idea de la verdadera estructura del filamento, el equipo utiliza un microscopio electrónico con resolución atómica.

Con menos de 20 millones de átomos, el filamento de nanotubo es lo bastante grande como para aplicarle las suposiciones estadísticas de la termodinámica, y lo suficientemente pequeño como para ser considerado un sistema molecular, o sea un sistema propio de la mecánica cuántica.

Obtenido de: http://www.amazings.com/ciencia/noticias/050609c.html
Tirso Ramírez
CRF

Hacia el Control de las Fuerzas de Casimir

Un grupo de investigadores del MIT ha encontrado una manera de calcular los efectos de las fuerzas de Casimir, ofreciendo con ello una forma de impedir que los componentes de las micromáquinas se peguen entre sí.
Descubiertas en 1948, las de Casimir son fuerzas cuánticas complicadas que sólo afectan a los objetos que están estrechamente cercanos entre sí. Son tan sutiles que en la mayor parte de este periodo de seis décadas transcurrido desde su descubrimiento, los ingenieros las han ignorado sin problemas.

Sin embargo, con la llegada de la era de los dispositivos electromecánicos diminutos, como los acelerómetros en algunos teléfonos móviles o los microespejos en los proyectores digitales, las fuerzas de Casimir se han revelado como creadoras de conflictos ya que pueden producir que las diminutas partes móviles de las micromáquinas se peguen entre sí.

Alexander McCauley, Alejandro Rodríguez y John Joannopoulos han dado con una manera de resolver las ecuaciones de las fuerzas de Casimir para casi cualquier número de objetos con cualquier forma concebible.

Estos investigadores del MIT han desarrollado una nueva y potente herramienta para calcular los efectos de las fuerzas de Casimir, con ramificaciones tanto para la física básica como para el diseño de sistemas microelectromecánicos (MEMS).

Uno de los descubrimientos más recientes de los investigadores usando la nueva herramienta, fue una manera de colocar objetos diminutos para que las fuerzas de Casimir, que ordinariamente ejercen atracción, pasen a tener el efecto contrario, repeliendo a los objetos.

Si los ingenieros pueden diseñar MEMS para que las fuerzas de Casimir impidan que sus partes móviles se peguen entre sí, en vez de hacer que se peguen, se podría disminuir substancialmente la proporción de fallos de los MEMS existentes.

La aplicación del nuevo concepto también podría ayudar a producir nuevos y económicos dispositivos MEMS, como diminutos sensores médicos o científicos, e incluso dispositivos microfluídicos que permitan que puedan ser realizados en paralelo centenares de experimentos químicos o biológicos.

Obtenido de: http://www.amazings.com/ciencia/noticias/230610c.html
Tirso Ramírez
CRF