The Nature of Microscopic World: Behind Quantum Mechanics

As one turns the scale to the atom level, the behavior of a particle becomes very unpredictable. Quantum mechanics has a better description of microscopic properties than classical mechanics for this scale. It brings us the most precise and successful numerical predictions in the history of science. But a contradiction accompanied with the development of quantum mechanics was brought to light, and has been queried from realists. It is certain that the probabilistic interpretation contravenes the law of causality, and is unable to delineate the fundamental physical process of the universe. According to quantum mechanics, measurements of some properties, such as a particle's momentum for example, can yield a range of possible results with varying probabilities. In other words, the objective physical process, once physicists took it for granted to possess definite properties that suitable observations can reveal, is no longer adaptable in the microscopic world.

The most profound conceptual difficulties of quantum mechanics are those that originated from conflicting non-causations. The compelling results unearth the incompleteness of quantum mechanics revealed by the EPR experiment [1] and the double-slit experiment, where the former lies in the inadmissible awareness of definite position and momentum at the same time; while the latter designates the wave-particle duality, which is the most unusual character of quantum particles. Moreover, this is just the outset of this battle to uncover the covert facts behind quantum mechanics. More quantum strangeness is rooted in the relevance of causation at a microscopic level, such as the uncertainty principle, entanglement, tunneling effect and so on, which have been proposed since. These phenomena profoundly violate the standpoint of human experience to our orderly, causal universe. It seems that the usage of quantum mechanics has been widely accepted while ignoring its essential features. This metaphysical quantum theory perfectly interprets experimental results, however, addresses nothing which approaches the realistic description with regards to the nature of quantum world, and yet becomes the most wizardly of theories.

The need for a more complete theory is revealed as our understanding of the physical universe has deepened profoundly, and, in particular, as the desiderative exploration of the very beginning of our universe has been carried out. One of the possible theories is the “hidden variable theory” proposed by D. Bohm [2]. An insinuation of invisible variables revises empirical grounds and preserves the causality on the subject of quantum behavior based on a wave conception. It provides a possible sketch of the parentage of “multi-path” methodology, which is an alternative perspective on the issue of a particle’s wave-like property proposed by R. Feynman [3]. His standpoint concurs exactly with all that went before from the numerical predication point of view, but presented in a causal way. It concludes that there are a lot of trajectories between two fixed points, and becomes only one trajectory, the classical one, at the macroscopic level. However, this causal manner emerges from a contradiction of quantum theory, and has not yet been fully understood from the anthropocentric viewpoint. In terms of physical laws in the objective physical process, it appears that an invisible physical strength acts on a particle and draws forth all possible trajectories. Obviously, more questions arise from the deterministic viewpoint to this hidden variable theory, such as: “What is matter wave exactly?” and “Why is there an invisible part which does generate an effect on particles?” and so on. Those puzzling questions could be straightened out if we could visualize this invisible portion in some way. Therefore, the primary research to be addressed next is how to convert hidden variables into realistic physical quantities and to provide a concrete depiction that can bring a convincing theory forth, which would account for all wired quantum phenomena and describe every property of the quantum world.

The main purpose of this article is to explore the process of visualizing hidden variables, and to represent a complete theory within microscopic physics. One can imagine that there is a bee in a house with no window. The bee has a special power allowing it to pass through walls to go outside. Of course, we cannot see the bee while it stays outside the house and we are inside. It can only be seen after passing through the wall and coming back into the house again. Hence, what we can observe is that the bee appears all of a sudden and disappears later if it passes in and out of the house. In such a condition, we have no idea about when it will come back to the house for the reason that we cannot see anything outside the house, but what we can do is to estimate the probability of being stung by the bee according to the position we are in inside the house. This is the probabilistic interpretation proposed by quantum mechanics to describe a quantum system.

On the other hand, let us replace all the walls of the house by transparent glass then there will be no doubt that we can see everything outside the house as we stay inside. Now, we still can observe where the bee is and even how it moves after it is passing through the glass wall to go outside. There is no problem for us to predict its flight path, position, velocity and heading. In other words, we can be told when and where the bee comes back into the house in a deterministic way, without estimating the stinging probability. A contrast can be made here that transparent walls symbolize the visualization process; it can reveal motions of the bee outside the house in the former case in an objective physical process. Consequently, a continuous and deterministic interpretation of the quantum world can arise if we can find some method of replacing the invisible border.

It is straightforward to think of extending dimension to bring transparent walls into existence. To deliberate on the imperceptible part to the sense at quantum level, a rational speculation of complex domain could strike a bargain, in which its imaginary part can represent the invisible world. In fact, it is not an unrestrained attempt originated from an intuition only. The complex concept was objective in Schrodinger’s equation and can be made aware of by the appearance of the imaginary factor “i”, and has been permitted as a genuine mathematical tool. In fact, the ignoring of the imaginary sign in wave mechanics can be attributed to practical experimental results, which cause people’s attention to delve into the atom scale. Owing to the limited observable dimension, imaginary features of nature which could dissolve the consequence of experiments has been eliminated by empiricists. However, no thorough canvass can be addressed if our understanding of nature contains a one-legged version of the full view. This is the main reason why quantum mechanics is an incomplete theory, for its grounds for existence lie on observation that has been criticized from the philosophical aspect and the causality of its nature.

In pioneering work approaching causal quantum physics, a remarkable achievement based on the complex concept has been proposed by C. D. Yang [4]. In his study of the physical process in a complex domain, astonishing results have been acquired. They reveal that Schrodinger’s equation is a deformation of the Hamilton-Jacobi equation when considering the existence of a complex dimension. There is no fortune at all in this since it is the only way to manifest the true face of its nature after a long struggle for a unified description of the causal universe up to now. General relativity and causal quantum physics can for the first time be on an equal footing in the foundation of the Hamiltonian outlook on causal physical processes. The biggest difference of Hamiltonian in the microscopic world is its additional term, namely quantum potential, as compared to the classical one. It states that there is a special field in the atom scale. It is on the decrease along with increasing mass, and finally vanishes in our daily scale. This field is so-called the quantum field, which can take responsibility for all marvelous quantum phenomena from the perspective of causality. Thus, a concrete object having specific physical quantities can be discussed after a complete description of carried energy has been expressed. In other words, it can be regarded as a classical particle for those who once considered it as the most bizarre particle in the quantum scale.

One of the most elusive parts of this causal quantum physics, or so-called quantum Hamilton mechanics, is the existence of complex dimension. It becomes an objectionable point for those who only can be convinced through measurements. In reality, there is an objective world whose nature and reality are independent of human observers. Hence, if the projection of causality on our living world, given by the complex dimension extension, can bring us a compatible result with what we can observe, then, it could be considered to bring complex dimension into existence. Unlike the quantum potential proposed in a hidden variable theory, the quantum potential we discuss in complex space can bring up compatible outcomes with a quantum probabilistic interpretation. The unpersuasive exposition of motion in an eigen state once becomes a fatal wound as a causal theory in Bohmian mechanics, however, becomes describable in quantum Hamilton mechanics. It is reasonable to explain the multi-path behavior by thinking of no specific initial position that has been made, since its imaginary initial position cannot be confirmed as the real one has been fixed in an experimental process. In addition, a particle is moving in a complex domain when a complex force associated with the quantum potential acts on it. As a result, a non-classical trajectory, which has deviations from the classical straight line, can be observed in the double-slit experiment issue. After a long period, the ensemble of trajectories presents us with the formation of a wave [5], which has been regarded as the matter wave, exhibited by the interference result of the dark and bright band on the screen. It is the best way to illustrate the on-again-off-again strange property of a quantum particle by extending dimension to a complex domain. In such a way, we can see everything through the transparent wall, and can explore all unexplainable and unpredictable quantum strangeness.

Therefore, the wave function decided by Schrodinger’s equation describes a particle’s motion statistically and cannot provide more detail about each trajectory. It is clearer to think of it as a water flow; since we cannot know a specific molecule’s motion by observing its whole flow, and can only understand the probability of this molecule passing by a specific area. This is the limitation of describing a quantum motion based on wave mechanics since it provides a macroscopic observation which cannot be overlooked. On the contrary, a fully informative view of a particle’s motion can be presented in terms of causal description from the same wave function. We can observe a specific particle’s motion with the help of quantum Hamilton mechanics since all its moving information can be traced in complex space. In other words, the incompleteness of a quantum mechanical description can be regarded as its macroscopic observation and becomes a functional tool that can only extract a portion of information from real conditions. Moreover, wave description itself has constraints on the local and accuracy standpoints of quantum motions. On the subject of the EPR problem, the non-local property can be explained by the information propagation through the quantum field between two particles. And the uncertainty principle can be regarded as the consequence of statistical observation. Now, we have a logically consistent and empirically adequate deterministic theory of quantum phenomena.

In summary, the revolutionary viewpoint of the complex dimension not only visualizes hidden variables but also provides them with a realistic physical meaning. As a causal theory, quantum Hamilton mechanics can be fully understood based on the classical standpoint, and invigorates complete description of the microscopic nature. However, this complex extension should be amenable to revision on empirical grounds as a scientific hypothesis. The prediction of small perturbation of the electron’s spin momentum⁶ can be one of the crucial testimonies, waiting for the comparison with experiments. On the other hand, the flying time of a photonic tunneling effect can be brought out by solving the equation of motion of the photon, which can be examined by the experimental data. The oscillation period of ammonia molecular can be estimated precisely by averaging the time that a trajectory cycle has made. Those issues have been executed in our laboratory recently, and could be worked out in the near future. We can conclude that quantum mechanics is a theory of phenomenology which gives average values for observed quantities; while quantum Hamilton mechanics is a theory of fatalism presenting specific particles and trajectories. Quantum Hamilton mechanics as a causal theory extends our understanding of the true nature of the microscopic world. It defends the causality, preserves physical laws while revealing the most mysterious feature of nature, indicating what is behind quantum mechanics.

E-mail: ngcmars@gmail.com

Reference

1 A. Einstein, B. Podolsky and N. Rosen, Can quantum-mechanical descriptions of

physical reality be considered complete?, Physical Review, 47, 777 (1935).

Reprinted in Quantum Theory and Measurement, p. 139, (1987).

2. Bohm, David. "A Suggested Interpretation of the Quantum Theory in Terms of

"Hidden Variables" I". Physical Review 85: 166-179. (1952).

3. R.P.Feynman: Space-Time Approach to Non-Relativistic Quantum Mechanics, Rev.

Mod. Phys. 367, (1948).

4. C. D. Yang, Quantum Hamilton mechanics: Hamilton equations of quantum motion, origin of quantum operators, and proof of quantization axiom, Ann. of Phys. 321, 2876-2926, (2006).

5. C. D. Yang, Wave particle duality in complex space, Ann. of Phys., 319, 444-470 (2005).

6. C. D. Yang, On Modeling and Visualizing Single-Electron Spin Motion, Chaos, Solitons, & Fractals, 30, 41 -50, (2006)

From: http://issuu.com/jiaps/docs/2-2008

Black Holes May Make Good Mommies

By Phil Berardelli

ScienceNOW Daily News
22 August 2008

Black holes might have a nurturing side. Scottish researchers have created a computer simulation that explains how supermassive black holes, such as the one at the center of the Milky Way, could promote the birth of nearby stars. The findings expand the possible scenarios for star formation and could help astronomers determine how stars emerged in the very young universe.

The popular conception of black holes is that they obliterate anything in their path, including time, space, and matter. Stars can escape annihilation if their orbits keep them far enough away. But some stars not only orbit perilously close to a supermassive black hole but also appear to have formed in its vicinity. Earlier in this decade, for example, astronomers spotted a population of very young--under 10 million years old--and very massive stars locked in elliptical orbits around the Milky Way's central black hole (ScienceNOW, 13 October 2005).

Could the stars have migrated there? Not likely. They're too young, and there are no nearby star hatcheries that could have produced them. The other possibility is that the stars formed in place. But astronomers also considered that idea unlikely, because the supermassive black hole would have shredded any cloud of gas--from which all stars condense--pulled into its influence.

Now the homegrown scenario seems more realistic, thanks to a computer model developed by astrophysicists Ian Bonnell of the University of St. Andrews in Fife, U.K., and Kenneth Rice of the University of Edinburgh, U.K. The simulation, which required more than a year of supercomputer time, tracked two hypothetical clouds of molecular hydrogen--the basic stellar building material--moving within a light-year or so of a supermassive black hole, much like the one anchoring the Milky Way. The researchers report today in Science that as the clouds fell toward the black hole, its gravity disrupted but did not destroy their clumpy structure. Eventually, the clouds flattened and merged into a disk that followed an elliptical orbit. During the flattening, nearly 200 new stars ignited, within a few hundred thousand years. Nearly all the resulting stars were very massive, meaning that they will live short and violent lives ending in supernovae.

The findings raise the question of where the star-forming clouds in the Milky Way would have come from. Bonnell and Rice speculate that they drifted freely within the galaxy until interaction with some other object or objects, such as larger clouds or other black holes, sent them hurtling toward the supermassive central black hole. But the answer remains unclear.

The simulation is a "breakthrough," says astronomer Mark Voit of Michigan State University in East Lansing, because it helps explain why those massive young stars around the Milky Way's center follow such elongated orbits. It "addresses one of the big open questions in astrophysics," adds Volker Bromm, an astrophysicist at the University of Texas, Austin. Thanks to this work, he says, "one wonders what the next-generation telescopes will find in the far-away universe just a few years from now."

From: http://sciencenow.sciencemag.org/cgi/content/full/2008/822/2?etoc

Why Sleep?

Phys. Rev. E 77, 011922 (issue of January 2008) Title and Authors
8 January 2008
Why Sleep?

Getty Images
Concentrate on napping. Researchers aren't sure why animals need to sleep, but a new study suggests that any system is more efficient when it focuses on one task at a time, rather than trying to multitask. With a sleep-wake cycle, the brain collects information during the day and processes it at night.
Why we sleep remains a mystery. Competing theories claim various "house-cleaning" brain activities occur during sleep, but they can't say why we need to power down to accomplish them. A study in the January Physical Review E suggests that a sleep-wake cycle, allowing the brain to focus on one task at a time, may be the most efficient way to operate. The researcher shows mathematically that processing a continuously changing resource--sensory input, in the brain's case--is best done "offline," when there's no input. This sort of analysis may lead to a more precise biological explanation for why sleep and other biological cycles evolved.
Humans spend a third of their lives asleep, and sleep is essential to our health. But scientists do not yet agree on its purpose. One theory is that the brain requires sleep to consolidate information collected during the day, while another theory says that the brain needs to sweep out harmful free radicals that build up during waking hours. But turning off the senses seems impractical, if not outright dangerous. It would seem better for an organism to perform sleep-related tasks in parallel with being awake.
To address this question, Emmanuel Tannenbaum of Ben Gurion University in Beer Sheva, Israel, proposes the concept of temporal differentiation, in which a system focuses on one task at a time, rather than trying to multitask. The advantages of a time-varying strategy have been studied in traffic control, computer programming, and operations research. But Tannenbaum believes he is the first to consider the brain as a "factory" for information processing, for which certain routines are more efficient than others.
In his paper, Tannenbaum analyzes two models. The first involves a tank with two pipes--one for filling and one for emptying--which can be opened one at a time. Assuming the incoming resource flow continuously switches between "on" and "off," Tannenbaum proves mathematically that one way to maximize the flow through the tank is to fill whenever the resource is available and empty when it isn't. The resource is analogous to sensory information that fills the brain and needs to be processed (emptied). Tannenbaum reasons that many animals can only receive visual information when there is light, so an efficient strategy, according to the tank model, is to be alert during daylight hours and devote all one's time in darkness to processing. As a comparison, Tannenbaum calculates the productivity of alternating rapidly between filling and emptying (equivalent to being half-asleep and half-awake simultaneously) and finds this approach less efficient.
Certain sleep behaviors, like episodic REM sleep and nocturnal habits, do not fit this picture, so Tannenbaum formulated a more generic model, in which a resource supplied at a fixed rate is processed in three separate steps, such that the initial, intermediate, and final products are all present in varying concentrations. The model bears some resemblance to the cyclic reactions of circadian rhythm proteins, which keep many organisms on 24-hour clocks even in complete darkness. Tannenbaum finds that a temporally differentiated case, where the steps are performed separately, is 33 percent more efficient at producing the final product than an undifferentiated case, where all three steps run simultaneously. This result depends on the details of the model, but he believes that optimization through temporal differentiation might explain why certain cyclic behaviors evolved.
James Krueger, a sleep expert at Washington State University, says that this is definitely a new approach, but he thinks Tannenbaum ignores a host of sleep phenomena, such as the localization of sleep to specific areas of the brain and the fact that some sensory input continues during sleep. Still, he welcomes the effort and admits that "any new idea cannot address everything at once." --Michael Schirber Michael Schirber is a freelance science writer in Montpellier, France.
Temporal Differentiation and the Optimization of System Output Emmanual Tannenbaum Phys. Rev. E 77, 011922 (issue of January 2008)

From: http://focus.aps.org/story/v21/st1

Squeezed into Darkness

Phys. Rev. Lett. 100, 203601 (issue of 23 May 2008) Title and Authors
8 May 2008
Squeezed into Darkness

Phys. Rev. Lett. 100, 203601 (2008)
Symmetry breaking. An optical cavity containing a special crystal can emit a beam with this intensity pattern (beam coming toward you). Theorists calculate that a related mode can lead to a beam with unwavering intensity, without some of the usual technical requirements.
According to quantum mechanics, empty space teems with random electromagnetic oscillations that limit the precision of optical measurements. Schemes to "squeeze" light and dodge this quantum limit require a carefully-tuned light intensity and other conditions. In the 23 May Physical Review Letters, Spanish researchers propose an alternative squeezing strategy that should be less finicky. If it proves experimentally feasible, the technique could permit improved measurements of gravitational waves or more practical ways to transmit quantum information.
Physicists often describe quantum-mechanical precision limits using the Heisenberg uncertainty principle, which places strict limits on how well quantities can be measured, even with perfect equipment. But this rule limits the combined uncertainty of pairs of related quantities, like the position and momentum of a particle. Researchers are free to measure the particle's position exactly, as long as they abandon any knowledge of its momentum, or vice versa.
A similar tradeoff applies to light waves, which have an intrinsic variability reflecting their quantum nature. Beginning in the 1980s, researchers learned how to experimentally "squeeze" light, for example, to precisely determine the light wave's amplitude at the expense of its phase, the number that measures the wave's progress through its oscillating cycle. But Germán de Valcárcel, of the University of Valencia in Spain, says that squeezing is significant only when the light intensity is chosen carefully. In his team's new technique the intensity of the input light "need not be tuned in order to obtain, ideally, perfectly squeezed light," he says.
To generate squeezed light, physicists typically shine laser light into an optical cavity, where it bounces back and forth between two partially transparent mirrors. The cavity contains a "nonlinear" crystal that converts the light into squeezed light of a new color with twice the wavelength. To optimize the effect, the input light must be carefully tuned to have intensity at or near a specific value called the threshold.
De Valcárcel and his colleagues propose using an input intensity well above the threshold and adjusting the mirror spacing so that the new light emerges with an intensity pattern in the shape of a dumbbell: with the beam coming toward you, you might see bright regions above and below, for example, with a dark lane horizontally across the middle. The critical ingredient, says de Valcárcel, is "symmetry breaking": the pattern is free to emerge with any orientation angle. "This angle is arbitrary," he says, so over time the pattern will rotate randomly.
Although the light "chooses" a particular orientation for the dumbbell pattern, or mode, the cavity also allows a second mode, which is identical but rotated by 90 degrees around the beam axis with respect to the first one. The researchers calculate that the completely unknown orientation of the first mode results in perfect precision for one aspect of the second, "dark" mode. Specifically, a component of this mode, the light signal that is exactly a quarter-cycle delayed from the bright mode, should be precisely zero--lacking even the usual quantum-mechanical fluctuations of empty space. Researchers could mix this "super-dark" mode with another laser to make a low-noise beam for precise measurements, such as detecting the tiny motions caused by gravitational waves.
Julio Gea-Banacloche, of the University of Arkansas in Fayetteville, is intrigued by the theoretical intuition that leads to the surprising new result. But he cautions that experimentalists usually avoid working above threshold because any noise reduction in one mode is hard to measure in the presence of very bright and noisy light in the other mode. --Don Monroe Don Monroe is a freelance science writer in Murray Hill, New Jersey.
Related Information:
Demonstration of a tenfold reduction in noise power using traditional squeezing techniques:H. Vahlbruch et al., "Observation of Squeezed Light with 10-dB Quantum-Noise Reduction," Phys. Rev. Lett. 100, 033602 (2008).
Noncritically Squeezed Light Via Spontaneous Rotational Symmetry Breaking Carlos Navarrete-Benlloch, Eugenio Roldán, and Germán J. de Valcárcel Phys. Rev. Lett. 100, 203601 (issue of 23 May 2008)

From: http://focus.aps.org/story/v21/st16

Laser Cooling of Atoms

Phys. Rev. Lett. 61, 169 (issue of 11 July 1988) Phys. Rev. Lett. 48, 596 (issue of 1 March 1982) Phys. Rev. Lett. 40, 1639 (issue of 19 June 1978) Titles and Authors
2 April 2008
Landmarks: Laser Cooling of Atoms

H. M. Helfer/NIST
Frozen. A cloud of cold sodium atoms (bright spot at center) floats in a trap. Researchers began cooling atoms with lasers in 1978, reaching below 40 Kelvin. They achieved temperatures a million times colder just ten years later, eventually leading to better atomic clocks and the observation of a new ultracold state of matter.
APS has put the entire Physical Review archive online, back to 1893. Focus Landmarks feature important papers from the archive.
In the 1970s and 80s, physicists learned how to use lasers to cool atoms to temperatures just barely above absolute zero. Three papers from that era, all published in Physical Review Letters, highlight some of the essential steps in the development of the technology. In 1978, researchers cooled ions somewhat below 40 Kelvin; ten years later, neutral atoms had gotten a million times colder, to 43 microkelvin. But the basic principle remained the same: use the force of laser light applied to atoms to slow them down. The work led to the creation of a new quantum form of matter called a Bose-Einstein condensate and to modern atomic clocks, as well as at least two Nobel prizes.
The original reason to cool atoms--that is, reduce the speed of their motion--was to allow more precise measurements of atomic spectra, and later, to improve atomic clocks. In 1978 Dave Wineland and his colleagues at what is now the National Institute of Standards and Technology (NIST) in Boulder, Colorado, followed theoretical proposals [1] and managed to laser cool magnesium ions.
As the team described in PRL, they confined the ions in an electromagnetic trap and hit them with a laser tuned to a frequency a bit below a "resonance" frequency for the ions. At rest, the ions absorb photons at the resonance frequency, but if they're moving toward the beam, its lower frequency appears Doppler shifted to the correct frequency, allowing them to absorb photons coming toward them. These photons slow down the ions until the cooling effect is balanced by the small heating that is always present when the laser is on. In later years, this heating--which comes from atoms recoiling every time they randomly emit or absorb a photon in any direction--would ultimately limit the cooling possible with this so-called Doppler cooling technique.
In Boston, William Phillips read Wineland's experimental article and a theoretical paper [2] with great interest. He was just finishing a postdoctoral fellowship at the Massachusetts Institute of Technology and heading to the NIST lab in Gaithersburg, Maryland. "The idea of cooling ions made me think that it might be possible to do the same thing with neutral atoms," says Phillips.
In 1982, Phillips and Harold Metcalf of Stony Brook University in New York published the first paper on laser cooling of neutral atoms. They sent a beam of sodium atoms through a magnetic field that was large at the entrance to the apparatus but became gradually smaller over a distance of 60 centimeters. While moving through the field, the atoms headed directly into an off-resonance laser that used Doppler cooling to reduce the range of atomic velocities among atoms in the beam. The laser also slowed the beam as a whole. During deceleration, the changing magnetic field changed the atoms' resonant frequency, so that the slowing and cooling continued over a long distance, allowing them to reach 40 percent of their initial velocity. Now called a Zeeman slower, this device has become a standard way of decelerating an atomic beam.
Laser cooling techniques improved, and by the late 1980s, researchers had achieved what they thought were the lowest possible temperatures, according to Doppler cooling theory--240 microkelvin for sodium atoms. Then in 1988, a group led by Phillips accidentally discovered that a technique developed three years earlier at another lab [3] could shatter the Doppler limit. They used three mutually perpendicular pairs of lasers to cool sodium atoms, with laser frequencies somewhat different from other labs. They discovered, using several new temperature measurement techniques, that their atoms were at about 43 microkelvin. Theorists quickly explained the unexpected cooling mechanisms by including more atomic states and the effects of laser polarization; previous cooling models were overly simplistic.
Guided by the new theory, experimentalists reached much colder temperatures and developed additional cooling techniques. Phillips' "sub-Doppler" cooling was an early step in the 1995 creation of a Bose-Einstein condensate, a new state of matter where gaseous atoms all drop to the lowest possible energy state.
Atomic clocks benefited as well. The latest generation uses techniques derived directly from what Phillips and others did in the 1980s. Phillips and others won the Nobel Prize in 1997 for developing laser cooling; another prize in 2001 was awarded for the creation of Bose-Einstein condensates.--Jason Socrates Bardi Jason Socrates Bardi is a senior science writer at the American Institute of Physics.
References:[1] D. J. Wineland and H. Dehmelt, Bull. Am. Phys. Soc. 20, 637 (1975); T. W. Hänsch and A. L. Schawlow, "Cooling of Gases by Laser Radiation," Opt. Commun. 13, 68 (1975).[2] A. Ashkin, "Trapping of Atoms by Resonance Radiation Pressure," Phys. Rev. Lett. 40, 729 (1978).[3] S. Chu et al., "Three-Dimensional Viscous Confinement and Cooling of Atoms by Resonance Radiation Pressure," Phys. Rev. Lett. 55, 48 (1985).
Related Information:
1997 Nobel Prize in physics
Observation of Atoms Laser Cooled below the Doppler Limit Paul D. Lett, Richard N. Watts, Christoph I. Westbrook, William D. Phillips, Phillip L. Gould, and Harold J. Metcalf Phys. Rev. Lett. 61, 169 (issue of 11 July 1988)
Laser Deceleration of an Atomic Beam William D. Phillips and Harold Metcalf Phys. Rev. Lett. 48, 596 (issue of 1 March 1982)
Radiation-Pressure Cooling of Bound Resonant Absorbers D. J. Wineland, R. E. Drullinger, and F. L. Walls Phys. Rev. Lett. 40, 1639 (issue of 19 June 1978)

From : http://focus.aps.org/story/v21/st11

Dark Physics Beats Light Limit

Dark Physics Beats Light Limit

Intel Corporation
Stamped out. Each of these chips contains over 400 million transistors. Calculations suggest that a new system using multiple lasers might be able to shrink the circuit elements even further.
Current laser-based techniques to make computer chips cannot fashion components much smaller than the light's wavelength, but researchers are devising tricks to beat this so-called diffraction limit. A new idea, detailed in the 22 February Physical Review Letters, is to use a dark state--which requires multiple laser beams--to write patterns in the absorbing material. Calculations show that the technique could create structures far smaller than the beams' wavelengths without using the dangerously high intensities needed with other proposed techniques.


In optical lithography, a "picture" of a microchip circuit is shone onto a semiconductor coated with a light-sensitive material called photoresist. Light-exposed areas of commonly-used photoresists become susceptible to the chemicals that etch out the integrated circuit pattern. According to classical physics, these exposed areas cannot be smaller than half a wavelength of the laser light. Engineers have ways to fudge this limit, such as immersing the semiconductors in liquids that help bend the light further. But to fundamentally break the limit, theorists have proposed systems where the photoresist molecule is activated by two or more photons of light, rather than one. Increasing the excitation energy reduces the effective wavelength, compared with a single photon. But multiphoton absorption requires all of the photons to be in the same place at the same time, which means high laser intensities that could damage materials or equipment.
Now Suhail Zubairy of Texas A&M University's campus in Qatar and his colleagues from the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, have proposed a new way to get below the diffraction limit without high-intensity lasers. The photoresist molecules would be activated by coherent population trapping (CPT), a process used in slow light and other atomic experiments. In the simplest case, two lasers drive two transitions from different lower energy states to a common excited state, but due to a quantum interference effect, the molecules are never excited. Instead they evolve into a so-called dark state--a stable combination of the lower states that is unaffected by light. With additional upper and lower states, there may exist more complex dark states that combine several low-energy states and that could be populated using additional lasers tuned to the different transitions. CPT does not require multiphoton absorption, so it can work at relatively low intensities.
Zubairy's team showed that one can use the lasers to create sub-wavelength-sized regions on the surface where the molecules are all in a special state--not simply the dark state, but the dark state "favoring" one of its component low-energy states over the others. In the simplest case, each of the two beams would be split in half and reflected back onto itself to form a pattern of light and dark stripes on the photoresist surface. By arranging how these patterns overlapped, some places would be exposed to more of one laser than the other. The researchers calculated how this varying illumination would affect the molecules in the dark state and found that they formed a pattern of their own--stripes alternating between molecules favoring one of the two component states and those favoring the other.
Assuming one state was more susceptible to etching, the process could lead to chip features as narrow as a half wavelength, according to their calculations. To beat the diffraction limit, the team included a third component in the dark state and an additional pair of lasers in their theory, which reduced the etch-sensitive stripes to quarter-wavelength thickness. More complex dark states led to even narrower stripes. By combining stripes of different widths, engineers could make the complicated patterns for a microchip, say the authors.
Jonathan Dowling of Louisiana State University in Baton Rouge thinks this new idea in "quantum trickery" could work, but the required energy level structure may be hard to reproduce in a commercial photoresist. "A lot of chemistry will be needed to translate these ideas into a practical technology," he says.--Michael Schirber Michael Schirber is a freelance science writer in Montpellier, France.

From: http://focus.aps.org/story/v21/st6

The shape of time

We all have the same question about what is time, could it be reversed? When I was a child, I love to see the movie "Back to the future", it says they go back to the past and try to come back to the future. It is always in my mind about "time travel". Could we find a way to travel forward or backward along time axis? Before think about that kind of question, we need to understand what is time firstly.
It is a qestionable issue and still an open discussion. If our physics describe the universe is somehow symmetric, then the physical symmetry should lead to time symmetry. Why there is only time axis propagates to one direction only without considering any possibility of "backward"? It is because there is no backward causation in humman intuition. But it is too anthropocentric to explain our nature phenomena. Maybe it's time to release our mind to widen the vision of the way we think about our universe. It is a great thing and a very important thing in my life, to pursue such an anwser for the most mystery in nature. This vedio is nice but not clear enough to explain the physics behind the time issue. I think it was made by some university students, if so, he will become a good science film producer one day.
Share it to all my friends, who love physics, animals and Earth.

http://tw.youtube.com/watch?v=y53hh-LAbLk&feature=related

World Science Fest: What's behind quantum mechanics?

World Science Fest: What's behind quantum mechanics?
By John Timmer Published: June 02, 2008 - 09:45AM CT
Friday night's session of the World Science Festival included a program on quantum mechanics entitled "The Invisible Universe," which included a panel discussion moderated by Alan Alda. Festival founder Brian Greene (who's actually a string theorist) provided the introduction to the quantum world, noting that, "100 years ago, one generation of physicists changed our understanding of reality." He said that society has adopted a lot of the lingo of quantum mechanics without really coming to terms of what it actually involved.
Greene tried to get the audience up to speed by talking about the now-famous double-slit experiment, using an example in which waves of water passing through a pool create an interference pattern. He then brought that into the world of electrons, which also form interference patterns when sent through a double slit, even when only one is sent through at a time. What is going through the slits, instead of an actual particle, appears to be a set of probability curves, which can interfere with each other on their way to determining where the electron lands.
The second example of quantum strangeness, which came during the ensuing discussion, was entanglement. Using up and down spin as an example, Greene described how two entangled particles could be separated, potentially by the entire length of the universe, and yet have their behavior remain linked. The consequence of the entanglement is that a measurement of the properties of one particle would instantly define the state of the other, no matter how large the separation between the two.
Quantum probabilities and a concrete world
These things profoundly violate what most people tend to think of as our orderly, causal universe. How do we make sense out of what we think of as a physical particle vanishing into a set of probabilities and then popping out, a particle again, at the far side? The panel included people who argued number of perspectives on this question.
Max Tegmark from MIT suggested that we register multiple things happening because all of them actually do happen in a series of related universes. According to Tegmark, a mathematical construct called Hilbert space can let us describe quantum behavior in a linear, causal, and 'real' manner but, to work, it requires that all quantum possibilities actually happen. Tegmark thinks they can, in nearby universes that split off and recombine to create the strange effects we measure.
Nobel Laureate Bill Phillips, who's a quantum experimentalist, took what he termed a "shut up and calculate" approach to the question. We're arguing over these different perspectives yet, "when we go into the lab, we get the same results." Phillips pointed out that there's no inherent reason to think that there's anything behind the quantum behavior we observe—it's just the nature of the universe. He noted that the graduate students he gets no longer have any issues with quantum behavior, and used that to suggest that it's really just a habit of mind (a mind that evolved to deal with a very concrete reality) that keeps many from being satisfied with quantum behavior.
The panel included a philosopher of physics, David Albert, that didn't really have a specific response he favored, although he clearly favored some response. Referring to the "shut up and calculate" approach, Albert pointed out that "nobody is born wanting to know the result of specific experiments," so to put the underlying principles off limits violates the nature of science. In Albert's view, "the problem isn't that Max's ideas are wacky—it's pretty clear that the world is wacky." The fear is that the wackiness may exceed our mental capacities; "is it stranger than we know, or stranger than we can know?" Albert asked.
New ideas vs. newfound comfort
Not surprisingly, the conversation frequently returned to Einstein, who was profoundly uncomfortable with quantum mechanics. Everyone agreed that this discomfort was often mischaracterized, though. It's frequently presented as an unease with the random nature of events, but nobody thinks Einstein expected that nature should make him (or anyone else) feel comfortable. Instead, Einstein seemed to have been unable to map quantum behavior onto anything he understood; as David Albert put it, it was like being told, "this bottle of water is Elvis Presley—it's not 'I don't believe that,' it's 'I don't know what that means.'"
Tegmark argued that any correct theory should seem weird, though, because the universe at the small scale is weird, and humans evolved to comprehend the large-scale world. He suggested that progress in the theoretical realm can be seen when people stop saying, "it's strange, and I hate it," and start saying, "I hate it." Tegmark was happy that there are a number of ways being tried to find some logic underlying the quantum world, saying, "it's better to bark up many trees than all going up the same one."
On that, everyone seemed to agree. Brian Greene argued that, as we get more information and ideas, quantum mechanics was likely to make more sense, and Alberts suggested three different proposals all had the potential to provide those ideas. Bill Phillips, who argued the shut up and calculate perspective, even agreed that these competing proposals were great. Calling experimentalists such as himself the real quantum mechanics, he said, "experimentalists love proving theorists wrong; I love having competing theories because somebody's gotta be wrong."

From:http://arstechnica.com/journals/science.ars/2008/06/02/world-science-fest-whats-behind-quantum-mechanics

CONDENSED-MATTER PHYSICS:Single-Electron Spin Measurement Heralds Deeper Look at Atom

CONDENSED-MATTER PHYSICS:Single-Electron Spin Measurement Heralds Deeper Look at Atoms

Erik Stokstad

After 8 years of effort, physicists in California have pulled off a technical tour de force: detecting the spin of a single electron inside a glassy chip of silica. The feat marks a significant step toward seeing individual atoms inside a material--a prerequisite for building a microscope that could map the three-dimensional structure of molecules--and may prove critical for so-called spintronic devices, including some kinds of quantum computers. The new success at imaging a single spin "is a physics breakthrough," says John Sidles of the University of Washington, Seattle.
Researchers can already visualize individual atoms with scanning tunneling and atomic force microscopes, but only on the surface of a sample. They can peer inside materials with magnetic resonance imaging--aligning the spin (a quantum-mechanical property that's the essence of magnetism) of protons in a sample's hydrogen atoms, zapping them with radio waves, and using an induction coil to track the changing magnetic fields as the energized protons return to their equilibrium spin states. The resulting images are useful for doctors, but they are a far cry from atomic resolution: At least a quadrillion protons must respond for a pixel to light up.

Spin detector. Wiggling cantilever, tracked by laser, can spot coil-driven changes in an electron's magnetic orientation.
CREDIT: D. RUGAR ET AL., NATURE 430, 329 (2004)Researchers could sharpen the picture by detecting a proton's spin directly, via magnetic forces, rather than through voltages induced in a coil. That's a tough challenge. These forces, measured in attonewtons, are extremely weak. To make the job easier, a group led by physicist Daniel Rugar of the IBM Almaden Research Center in San Jose, California, set out in 1996 to detect the spin of a single electron, which has a magnetic moment 600 times stronger than a proton's.
The team made a flexible cantilever, just 85 micrometers long and 100 nanometers thin, and added a tiny but powerful magnet at the tip. Applying a high-frequency magnetic field, they manipulated the spin of the electron so that it would resonate with the magnetic field around the cantilever tip. Then they set the cantilever wiggling. If the tip was hovering above an electron with a detectable spin, the resonance repeatedly flipped the spin of the electron, giving the cantilever a slight boost each time. The regular nudging revealed the spin amid the noise created by much stronger electrostatic and van der Waals forces, the researchers report this week in Nature.
It's a slow process right now: Scanning a 170-nanometer stretch of the sample took several weeks. The cantilever's resolution--about 25 nanometers--isn't atomic-scale yet, but the technique can spot an electron as deep as 100 nanometers, or about 400 atomic layers below the surface.
"It's quite an impressive achievement," says physicist Chris Hammel of Ohio State University, Columbus. Unlike other methods of detecting single spins, he says, the new technique has the advantage of working on many kinds of materials. Rugar's team is improving the resolution by cooling the system and outfitting the cantilever with a stronger magnet. That should increase resolution and speed enough to enable 2D and 3D scans, Rugar says.

From: http://www.sciencemag.org/cgi/content/full/305/5682/322a

Quantum spin probe able to measure spin states at individual atoms; could have application in quantum computing

Quantum spin probe able to measure spin states at individual atoms; could have application in quantum computing 20 June 2001
By Robert Sanders, Media Relations

Image of the electron cloud around a nickle impurity in a high-temperature superconductor, obtained with a scanning tunneling microscope. The central spot is the nickle atom, surrounded by a cloverleaf pattern whose fourfold symmetry is indicative of the underlying d-wave nature of the high-temperature superconducting state.PHOTO CREDIT: Seamus Davis/UC BerkeleyBerkeley - University of California, Berkeley, scientists probing the quantum weirdness of high-temperature superconductors have built a scanning tunneling microscope that can measure for the first time the quantum spin of an electronic state of a single atom, in this case an impurity atom embedded in the superconductor.
Until now, scientists have had to trap isolated atoms and zap them with a laser to measure their spin state.
The technique already has improved understanding of high temperature superconductors, which, 15 years after their discovery, are still an enigma. It also can help probe the spin states of atoms in metals and semiconductors, as well as in new materials such as carbon nanotubes or strontium ruthenate superconductors.
The researchers, however, are most excited about its potential application in quantum computers.
The results are reported in the June 21 issue of Nature by J. C. S嶧mus Davis, professor of physics at UC Berkeley and a researcher in the Materials Sciences Division of Lawrence Berkeley National Laboratory, and his collaborators at Boston University, Tokyo University and the National Institute of Standards and Technology (NIST).
In atoms or nuclei, spin is analogous to counterclockwise rotation, like a top, and the energy of a state depends on whether the spin axis points up or down. Spin interactions can have important consequences - the pairing of two electrons of opposite spin, a Cooper pair, in a low-temperature superconductor is the source of its zero electrical resistance and other unique properties.
A two-state system - spin up or spin down - is also what scientists look for in the "qubit" of a quantum computer. The two spin states are analogous to the ones and zeros of the binary alphabet used by today's computers, from palm pilots to Cray supercomputers.
Scientists predict that quantum computers taking advantage of two-level quantum states like this will be able to perform calculations far faster than conventional transistor-based computers, and in the process shrink the size of computers immensely.
"One of the holy grails of solid state physics is to write and store information in just one atom," Davis said. "We now have the ability to separate the spin-up state from the spin-down state at a single atom. This could lead to new tools for using spins for quantum computing."
"This is a fantastic new atomic-scale quantum probe of atoms in the solid state," said theoretical chemist K. Birgitta Whaley, professor of chemistry at UC Berkeley. Whaley is leading an effort to create a new center at UC Berkeley to study quantum computing, and Davis is part of the team.
His co-authors on the Nature paper are former UC Berkeley postdoctoral associate Eric W. Hudson, PhD, now a National Research Council Fellow at NIST in Gaithersburg, Md.; Shuheng Pan, a professor at Boston University; Shin-ichi Uchida, professor, and Hiroshi Eisaki, PhD, of the University of Tokyo's Department of Superconductivity; and graduate student Kristine M. Lang and postdoctoral associate Vidya Madhavan of UC Berkeley. Eisaki currently is on sabbatical at Stanford University.
In a quantum computer, each one or zero is called a qubit or quantum bit, and various systems have been proposed to represent these qubits. They range from the up and down spins of atomic nuclei manipulated with a nuclear magnetic resonance machine, to ions trapped in a cryogenic vacuum chamber, their states flipped with pulses of radio waves.
The disadvantage of these systems is that the devices used to read or write the qubits are large and cumbersome and not scalable to large numbers of qubits, Davis said.
Scanning tunneling microscopes (STM), however, were designed to probe materials on an atomic scale by way of a microscopic tip that hovers over the surface and maps the intensity of the electron clouds. Qubits stored as closely packed impurity atoms in a solid crystal could be read out by an STM much like a recording head reads and writes magnetic bits on a CD. Davis notes that the scanning tunneling microscope should work just as well at measuring spin states around impurities in semiconductors and metals.
"Bruce Kane of the University of Maryland has proposed using phosphorus atom impurities in a silicon semiconductor as qubits of a quantum computer," Whaley said. "The problem has been, how do you manipulate the spins of single atoms? This is a great way to do that."
Davis's group discovered the power of their STM while studying the effect of single-atom impurities in pure samples of a copper oxide high-temperature superconductor (Bi2Sr2CaCu2O8+d) - dubbed BSCCO (BIS-ko) because it is composed of bismuth, strontium, calcium, copper and oxygen. The researchers' goal is to find out why these composite materials are superconductors at relatively high temperatures - 85 degrees above absolute zero (-307蚌) instead of 4 degrees above zero (-452蚌) typical of low-temperature superconductors - and perhaps find a recipe for making better ones.
Superconductors carry electricity without energy loss, and thus are attractive components of electrical transmission lines, motors or high-powered magnets.
Last year the UC Berkeley group used its STM to make the first images of electron clouds around a zinc atom impurity in a BSCCO superconductor. The researchers next placed a magnetic atom, nickel, into the regular crystal structure of the superconductor to see what more it would reveal about the unknown quantum processes at work in high-temperature superconductors. Because the superconductivity of high-temperature superconductors is thought to involve interactions among the magnetic moments of the individual atoms, theoreticians have predicted that nickel should disrupt the superconductivity less than a non-magnetic atom such as zinc.
In mapping the electron clouds with the STM, they found, in fact, that nickel left the superconductivity undamaged, in contrast to the nearby disruption caused by zinc. Because of this, the scientists were able to observe quantum phenomena around the nickel impurity that they did not see with zinc.
In high-temperature superconductors, many electrons fly about as unpaired quasiparticles - basically broken Cooper pairs, which are the form in which electrons travel in conventional low-temperature superconductors. These quasiparticles scatter off an impurity and form a small electron cloud that hovers around the impurity like a cloud around a mountain peak.
Around a zinc impurity, Davis and his team saw a simple quasiparticle cloud. Around nickel, however, they saw structure in the cloud created by the magnetic moment of the nickel atom. It was as if two different kinds of clouds could hug the impurity, one - a spin-up electron cloud - at a higher energy than the other, spin-down electron cloud. They could tell the difference by its effect on the electrical current flowing through the STM tip.
They observed another quantum phenomenon, too. Because of the ambiguities of quantum mechanics, quasiparticles in a superconductor are actually a 50/50 mixture of particle and non-particle, which is called a hole. The scientists verified this with the STM, detecting both particles and holes around the nickel impurity.
The presence of holes as well as particles proves that nickel does not impair the superconductivity, and it strongly supports the theory that high-temperature superconductors work through magnetic interactions among the atoms.
Theoretician Alexander V. Balatsky of Los Alamos National Laboratory, who predicted the spin splitting by a magnetic atom impurity, was amazed at how closely the experiment fit his predictions, which initially were met with skepticism.
"Our prediction was made long before the experimental capability to tunnel into a very clean system with very few impurities existed," he said. "It shows that some aspects of our theory work and gives us confidence that we have predictive power for these enigmatic materials."

From: http://berkeley.edu/news/media/releases/2001/06/20_physc.html

How to Create a Spin Current

PHYSICS:How to Create a Spin CurrentPrashant Sharma*
We usually think of a current as a flow of particles, such as the flow of electrons in a charge current generated by a battery. However, besides its charge, the electron also carries a spin, whose projection along the spin axis can point up or down. Conventional electronic devices ignore this property of the electron, but new devices are now being built that rely on the spin (1, 2). Such devices should have faster switching times and lower power consumption than conventional devices, mainly because spins can be manipulated faster and at lower energy cost than charges can.
All currently available spin-based devices are memory devices that use the spin to store information. Spin-based electronic (spintronic) devices such as transistors (2) require spin currents, just as conventional electronic devices require charge currents. Unfortunately, it is very difficult to generate and transport a spin current.
To understand what is meant by a spin current, consider an electron current that flows through a channel and contains only up-spin polarized electrons. Add to this a similar current in which all electrons are down-spin polarized and flow in the opposite direction. The result is a current of spins only; there is no net particle transfer across any cross section of the channel.
A spin current differs from a charge current in two important ways. First, it is invariant under time reversal: If the clock ran backward, spin current would flow in the same direction. Second, spin current is associated with a flow of angular momentum, which is a vector quantity. This feature allows quantum information to be sent across semiconducting structures, just as quantum optics involves distribution of information across optical networks via polarization states of the photon.
Most methods currently under investigation use ferromagnets to inject a spin current into a nonmagnetic material. However, this process is inefficient. For practical applications, the generation and detection of spin currents should not require strong magnetic fields and interfaces between semiconductors and ferromagnets.
Creating a spin current through spin pumping I. A single channel of electrons is formed in a 2D electron system through electrostatic confinement. When out-of-phase ac voltages are applied to the two gates, the channel is perturbed, resulting in a dc electron current. If one of the gates is replaced by an oscillating magnetic field, a spin current is pumped. One method that meets these requirements is spin pumping, which involves the scattering of electrons off a small region (a quantum cavity). In the cavity, electrons with different spins take dissimilar paths and therefore scatter differently off the cavity walls. Through modulating the shape of the cavity periodically in time, a constant spin current can be generated.
The theory of pumping electrons without changing their spin state is based on the idea (3) that one can create a traveling wave potential for electrons to ride on. At any moment, the electrons sit in the minima of the wave and move along with it. The efficiency of this mechanism depends on the depth of the traveling wave: The deeper the potential, the greater the chance that the electrons remain trapped and are transported along with the wave.
One practical way to create such a traveling wave is to periodically modulate the transmission of the electron flow at two points in space (4). This can be done by applying two metal gates on a channel (see the first figure) or on a quantum cavity (second figure, top panel). Experimentally, these structures are created in a two-dimensional (2D) layer of electrons (an electron gas), which forms in semiconducting heterostructures typically made from GaAs and AlGaAs. Switkes et al. have shown experimentally that a quantum cavity can indeed be modulated in time (5).
These ideas have recently been generalized to create spin currents. The earliest proposal (6) was to create a traveling wave for up spin-polarized electrons that is opposite in direction to that for down spin-polarized electrons. Creation of such spin-selective traveling waves requires the use of some externally controllable mechanism to break spin-rotation symmetry--that is, to define a unique spin axis in space. This can be achieved by replacing one of the metal gates in the first figure with an external, time-dependent magnetic field. To obtain efficient spin transport, one must increase the depth of the minima in the traveling waves by restricting the electrons to a long narrow channel--a quantum wire (7)--such that they repel each other.
Creating a spin current through spin pumping II. (Top) A quantum cavity is perturbed through out-of-phase ac voltages applied to the two gates. Electrons entering the cavity scatter off the cavity walls several times before leaving. (Bottom) In a sufficiently large cavity, pumping leads to a spin current with a tunable direction of spin polarization. Both in-plane (green arrow) and out-of-plane (red arrow) polarizations are possible. However, the picture of a single traveling wave is inadequate for describing pumping in a finite cavity, because the electron follows a complicated path before exiting the cavity. The direction of the current is therefore determined by the details of the scattering in the cavity (8). This sensitivity to the electron's path in the cavity is an essential feature of mesoscopic semiconductor devices, which can be up to several tens of micrometers in size.
In a theoretical proposal (9) for spin pumping, this dependence of the current through the cavity on externally controllable parameters is used to generate a spin current. The proposal is to modulate the shape of a quantum cavity through the use of two magnetic fields. A strong magnetic field applied in the plane of the cavity (see the second figure, top panel) couples only to the spin of the electrons. For such a strong in-plane magnetic field, there are more up-spin electrons (whose spins are aligned with the magnetic field) than down-spin ones inside the cavity. As a result, the pumped current is spin-polarized along the direction of the strong magnetic field and in the cavity plane; that is, it is a mixed charge and spin current.
To achieve only a spin current, a second, weak magnetic field is added. The field is weak enough not to affect the spin of an electron, but the Lorentz force exerted by this field affects the spatial motion of the charged electrons. One would therefore expect (on average) the up-spin charge current to flow in one direction while the down-spin charge current flows in the opposite direction. Spin currents have recently been produced experimentally with such a device (10).
An alternative to using magnetic fields in the cavity has also been proposed (11). It is based on the fact that the spin state of an electron moving inside a semiconductor is not independent of its momentum state. Because the quantum cavity is formed in a semiconductor--typically a GaAs/AlGaAs heterostructure--the spin of the electron is coupled to its motion inside the cavity. Because of this spin-orbit coupling, the direction of the spin of an electron follows the electron's motion. As a result, the direction of the spin polarization coming out of the cavity depends on the details of the scattering in the cavity (12). By increasing the number of times an electron scatters off the cavity walls, the in-plane spin projection of the electron can be made small, and its spin can be made to point in a direction perpendicular to the plane of the electron gas (see the second figure).
This approach should allow spin currents to be pumped through a quantum cavity without an in-plane magnetic field. Because of spin-orbit coupling, the outgoing current will be spin-polarized. By either applying a weak perpendicular magnetic field [as in the experiments in (10)] or by inducing small changes in the density of electrons in the cavity, a pure spin current can be obtained. In this method of pumping, the direction of polarization of the spin current can be changed from in-plane to out-of-plane by altering the shape and size of the quantum cavity (second figure, bottom panel). The approach has not yet been realized experimentally.
Spin pumping in mesoscopic systems allows the spin-polarization direction of currents to be manipulated without the use of strong magnetic fields and ferromagnets. However, some experimental challenges remain before this method is ready for use in actual spintronic devices. One difficulty lies in efficiently detecting spin currents whose polarization direction is arbitrary.
Spin currents polarized in the plane of the 2D electron system have been detected electrically (10). Out-of-plane polarization in a 2D electron system may be detected (11) via the spin Hall effect. Because of this effect, an electron with its spin polarized perpendicular to its momentum is deflected in a direction orthogonal to both its momentum and its spin. Reversing the direction of either the momentum or the spin polarization reverses the direction of deflection. As a result, a spin current with an out-of-plane polarization generates a transverse electric field in a material that shows the spin Hall effect. Recently, the spin Hall effect has been observed for the first time in a semiconducting GaAs/InGaAs heterostructure (13).
We have yet to create a spin pump that can generate spin currents with any chosen direction of spin polarization. Nonetheless, recent experimental and theoretical advances give hope that devices relying on spin currents will soon be realized.
References

S. A. Wolf et al., Science 294, [1488] (2001).
I. Zutic, J. Fabian, S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004) [APS].
D. J. Thouless, Phys. Rev. B 27, 6083 (1983) [APS].
B. L. Altshuler, L. I. Glazman, Science 283, [1864] (1999).
M. Switkes et al., Science 283, [1905] (1999).
P. Sharma, C. Chamon, Phys. Rev. Lett. 87, 096401 (2001) [APS].
T. Giamarchi, Quantum Physics in One Dimension (Clarendon, Oxford, 2004) [publisher's information].
P. W. Brouwer, Phys. Rev. B 58, 10135 (1998) [APS].
E. R. Mucciolo, C. Chamon, C. Marcus, Phys. Rev. Lett. 89, 146802 (2002) [APS].
S. K. Watson, R. M. Potok, C. M. Marcus, V. Umansky, Phys. Rev. Lett. 91, 258301 (2003) [APS].
P. Sharma, P. W. Brouwer, Phys. Rev. Lett. 91, 166801 (2003) [APS].
I. L. Aleiner, V. I. Fa'lko, Phys. Rev. Lett. 87, 256801 (2001) [APS].
Y. K. Kato, R. C. Myers, A. C. Gossard, D. D. Awschalom, Science 306, [1910] (2004).
10.1126/science.1099388

From: http://www.sciencemag.org/cgi/content/full/307/5709/531