BTemplates.com

Powered by Blogger.

Pageviews past week

Quantum mechanics

Auto News

artificial intelligence

About Me

Recommend us on Google!

Information Technology

Popular Posts

Showing posts with label Joint Quantum Institute. Show all posts
Showing posts with label Joint Quantum Institute. Show all posts

Sunday, September 25, 2011

Roll over Einstein: Law of physics challenged


One of the very pillars of physics and Einstein's theory of relativity - that nothing can go faster than the speed of light - was rocked Thursday by new findings from one of the world's foremost laboratories.
This undated file photo shows famed physicist Albert Einstein. Scientists at the European Organization for Nuclear Research, or CERN, the world's largest physics lab, say they have clocked subatomic particles, called neutrinos, traveling faster than light, a feat that, if true, would break a fundamental pillar of science, the idea that nothing is supposed to move faster than light, at least according to Einstein's special theory of relativity: The famous E (equals) mc2 equation. That stands for energy equals mass times the speed of light squared. The readings have so astounded researchers that they are asking others to independently verify the measurements before claiming an actual discovery. (AP Photo)

European researchers said they clocked an oddball type of subatomic particle called a neutrino going faster than the 186,282 miles per second that has long been considered the cosmic speed limit.

The claim was met with skepticism, with one outside physicist calling it the equivalent of saying you have a flying carpet. In fact, the researchers themselves are not ready to proclaim a discovery and are asking other physicists to independently try to verify their findings.

"The feeling that most people have is this can't be right, this can't be real," said James Gillies, a spokesman for the European Organization for Nuclear Research, or CERN, which provided the particle accelerator that sent neutrinos on their breakneck 454-mile trip underground from Geneva to Italy.

Going faster than light is something that is just not supposed to happen according to Einstein's 1905 special theory of relativity - the one made famous by the equation E equals mc2. But no one is rushing out to rewrite the science books just yet.

It is "a revolutionary discovery if confirmed," said Indiana University theoretical physicist Alan Kostelecky, who has worked on this concept for a quarter of a century.

Stephen Parke, who is head theoretician at the Fermilab near Chicago and was not part of the research, said: "It's a shock. It's going to cause us problems, no doubt about that - if it's true."

Even if these results are confirmed, they won't change at all the way we live or the way the world works. After all, these particles have presumably been speed demons for billions of years. But the finding will fundamentally alter our understanding of how the universe operates, physicists said.

Einstein's special relativity theory, which says that energy equals mass times the speed of light squared, underlies "pretty much everything in modern physics," said John Ellis, a theoretical physicist at CERN who was not involved in the experiment. "It has worked perfectly up until now."

France's National Institute for Nuclear and Particle Physics Research collaborated with Italy's Gran Sasso National Laboratory on the experiment at CERN. CERN reported that a neutrino beam fired from a particle accelerator near Geneva to a lab 454 miles (730 kilometers) away in Italy traveled 60 nanoseconds faster than the speed of light. Scientists calculated the margin of error at just 10 nanoseconds. (A nanosecond is one-billionth of a second.)

Given the enormous implications of the find, the researchers spent months checking and rechecking their results to make sure there were no flaws in the experiment.

A team at Fermilab had similar faster-than-light results in 2007, but a large margin of error undercut its scientific significance.

If anything is going to throw a cosmic twist into Einstein's theories, it's not surprising that it's the strange particles known as neutrinos. These are odd slivers of an atom that have confounded physicists for about 80 years.

The neutrino has almost no mass, comes in three different "flavors," may have its own antiparticle and has been seen shifting from one flavor to another while shooting out from our sun, said physicist Phillip Schewe, communications director at the Joint Quantum Institute in Maryland.

Columbia University physicist Brian Greene, author of the book "Fabric of the Cosmos," said neutrinos theoretically can travel at different speeds depending on how much energy they have. And some mysterious particles whose existence is still only theorized could be similarly speedy, he said.



Fermilab team spokeswoman Jenny Thomas, a physics professor at the University College of London, said there must be a "more mundane explanation" for the European findings. She said Fermilab's experience showed how hard it is to measure accurately the distance, time and angles required for such a claim.

Nevertheless, Fermilab, which shoots neutrinos from Chicago to Minnesota, has already begun working to try to verify or knock down the new findings.

And that's exactly what the team in Geneva wants.

Gillies told The Associated Press that the readings have so astounded researchers that "they are inviting the broader physics community to look at what they've done and really scrutinize it in great detail, and ideally for someone elsewhere in the world to repeat the measurements."

Only two labs elsewhere in the world can try to replicate the work: Fermilab and a Japanese installation that has been slowed by the tsunami and earthquake. And Fermilab's measuring systems aren't nearly as precise as the Europeans' and won't be upgraded for a while, said Fermilab scientist Rob Plunkett.

Drew Baden, chairman of the physics department at the University of Maryland, said it is far more likely that the CERN findings are the result of measurement errors or some kind of fluke. Tracking neutrinos is very difficult, he said.

"This is ridiculous what they're putting out," Baden said. "Until this is verified by another group, it's flying carpets. It's cool, but ..."

So if the neutrinos are pulling this fast one on Einstein, how can it happen?

Parke said there could be a cosmic shortcut through another dimension - physics theory is full of unseen dimensions - that allows the neutrinos to beat the speed of light.

Indiana's Kostelecky theorizes that there are situations when the background is different in the universe, not perfectly symmetrical as Einstein says. Those changes in background may alter both the speed of light and the speed of neutrinos.

But that doesn't mean Einstein's theory is ready for the trash heap, he said.

"I don't think you're going to ever kill Einstein's theory. You can't. It works," Kostelecky said. There are just times when an additional explanation is needed, he said.

If the European findings are correct, "this would change the idea of how the universe is put together," Columbia's Greene said. But he added: "I would bet just about everything I hold dear that this won't hold up to scrutiny."


More information: The results are pre-published on ArXiv: http://arxiv.org/abs/1109.4897

Tuesday, August 16, 2011

Searching for Spin Liquids: Much-Sought Exotic Quantum State of Matter Can Exist


The world economy is becoming ever more reliant on high tech electronics such as computers featuring fingernail-sized microprocessors crammed with billions of transistors. For progress to continue, for Moore's Law -- according to which the number of computer components crammed onto microchips doubles every two years, even as the size and cost of components halves -- to continue, new materials and new phenomena need to be discovered.
Diagram depicting anti-ferromagnetic order (upper) compared to a spin liquid phase (lower). In an anti-ferromagnet, the spins are anti-aligned. A spin liquid has no order and the spins can be viewed as bobbing about like water molecules in liquid water. (Credit: E. Edwards)

Furthermore, as the sizes of electronic components shrink, soon down to the size of single atoms or molecules, quantum interactions become ever more important. Consequently, enhanced knowledge and exploitation of quantum effects is essential. Researchers at the Joint Quantum Institute (JQI) in College Park, Maryland, operated by the University of Maryland and the National Institute of Standards and Technology (NIST), and at Georgetown University have uncovered evidence for a long-sought-after quantum state of matter, a spin liquid.

The research was performed by JQI postdoctoral scientists Christopher Varney and Kai Sun, JQI Fellow Victor Galitski, and Marcos Rigol of Georgetown University. The results appear in an editor-recommended article in the 12 August issue of the journal Physical Review Letters.

You can't pour a spin liquid into a glass. It's not a material at all, at least not a material you can touch. It is more like a kind of magnetic disorder within an ordered array of atoms. Nevertheless, it has many physicists excited.

To understand this exotic state of matter, first consider the concept of spin, which is at the heart of all magnetic phenomena. For instance, a refrigerator magnet, at the microscopic level, consists of trillions of trillions of iron atoms all lined up. Each of these atoms can be thought of loosely as a tiny spinning ball. The orientation of that spin is what makes the atom into a tiny magnet. The refrigerator magnet is an example of a ferromagnet, the ferro part coming from the Latin word for iron. In a ferromagnet, all the atomic spins are lined up in the same way, producing a large cooperative magnetic effect.

Important though they may be, ferromagnets aren't the only kind of material where magnetic interactions between spins are critical. In anti-ferromagnets, for instance, the neighboring spins are driven to be anti-aligned. That is, the orientations of the spins alternate up and down (see top picture in figure). The accumulative magnetic effect of all these up and down spins is that the material has no net magnetism. The high-temperature superconducting materials discovered in the 1980s are an important example of an anti-ferromagnetic structure.

More complicated and potentially interesting magnetic arrangements are possible, which may lead to a quantum spin liquid. Imagine an equilateral triangle, with an atom (spin) at each corner. Anti-ferromagnetism in such a geometry would meet with difficulties. Suppose that one spin points up while a second spin points down. So far, so good. But what spin orientation can the third atom take? It can't simultaneously anti-align with both of the other atoms in the triangle. Physicists employ the word "frustration" to describe this baffling condition where all demands cannot be satisfied.

In everyday life frustration is, well, frustrating, and actually this condition is found throughout nature, from magnetism to neural networks. Furthermore, understanding the different manifestations of a collection of magnetically interacting spins might help in designing new types of electronic circuitry.

One compromise that a frustrated spin system makes is to simultaneously exist in many spin orientations. In a quantum system, this simultaneous existence, or superposition, is allowed.

Here's where the JQI researchers have tried something new. They have studied what happens when frustration occurs in materials with a hexagonal (six sided) unit cell lattice.

What these atoms do is interact via their respective spins. The strength of the interaction between nearest neighbor (NN) atoms is denoted by the parameter J1. Similarly, the force between next nearest neighbors (NNN) -- that is, pairs of atoms that have at least one intervening atom between them -- is denoted by J2. Letting this batch of atoms interact among themselves, even on a pretend lattice as small as this, entails an immense calculation. Varney and his colleagues have calculated what happens in an array of hexagons consisting of 30 sites where the spins are free to swing about in a two-dimensional plane (this kind of approach is called an XY model).



Christopher Varney, who has appointments at Maryland and Georgetown, said that the interactions of atoms can be represented by a matrix (essentially a two-dimensional spreadsheet) with 155 million entries on each side. This huge number corresponds to the different spin configurations that can occur on this honeycomb-structured material.

What the researchers found were a "kaleidoscope" of phases, which represent the lowest-energy states that are allowed given the magnetic interactions. Just as water can exist in different phases -- steam, liquid, and ice -- as the temperature is changed, so here a change in the strengths of the interactions among the spins (the J1 and J2 parameters) results in different phases. For example, one simple solution is an antiferromagnet (upper picture in figure).

But one phase turns out to be a true quantum spin liquid having no order at all. When J2 is between about 21% and 36% of the value of J1, frustration coaxes the spins into disorder; the entire sample co-exists in millions of quantum states simultaneously.

It's difficult for the human mind to picture a tiny two-dimensional material in so many states at the same time. JQI fellow, Victor Galitski, suggests that one shouldn't think of the spins as residing at the original atomic sites but rather as free ranging particle-like entities dubbed "spinons." These spinons bob about, just as water molecules bob about in liquid water (see lower picture in figure). Hence the name quantum spin liquid.

Another reason for using the word liquid, Galitski says, is this 'bobbing about' is analogous to what happens inside a metal. There, the outer electrons of most atoms tend to leave their home atoms and drift through the metal sample as if they constituted a fluid, called a "Fermi liquid."

Electrons in a metal are able to drift since it takes only an infinitesimal amount of energy to put them into motion. The same is true for the fluctuating spins in the hexagonal model studied by the JQI scientists. Indeed, their spin model assumes a temperature of absolute zero, where quantum effects abound.

Writing in an essay that accompanied the article in Physical Review Letters, Tameem Albash and Stephan Haas, scientists at the University of Southern California, say that the JQI/Georgetown team "present a convincing example" of the new spin liquid state.

How can this new frustration calculation be tested? The experimental verification of the spin liquid state in a 2-dimenstional hexagonal lattice, Albash and Haas suggest, "will probably be tested using cold atoms trapped in optical lattices. In the past few years, this technology has become a reliable tool to emulate quantum many body lattice systems with tunable interactions." Indeed the authors propose such an experiment.

What would such a spin liquid material be good for? It's too early to tell. But some speculations include the idea that these materials could support some exotic kind of superconductivity or would organize particle-like entities that possessed fractional electric charge.

"Kaleidoscope of Exotic Quantum Phases in a Frustrated XY Model" by Christopher N. Varney, Kai Sun, Victor Galitski, and Marcos Rigol, Physical Review Letters, 107, 077201, (12 August 2011).

Saturday, March 5, 2011

New Kinds of Superconductivity? Physicists Demonstrate Coveted 'Spin-Orbit Coupling' in Atomic Gases


Physicists at the Joint Quantum Institute (JQI), a collaboration of the National Institute of Standards and Technology (NIST) and the University of Maryland-College Park, have for the first time caused a gas of atoms to exhibit an important quantum phenomenon known as spin-orbit coupling. Their technique opens new possibilities for studying and better understanding fundamental physics and has potential applications to quantum computing, next-generation "spintronics" devices and even "atomtronic" devices built from ultracold atoms.
In an ultracold gas of nearly 200,000 rubidium-87 atoms (shown as the large humps) the atoms can occupy one of two energy levels (represented as red and blue); lasers then link together these levels as a function of the atoms' motion. At first atoms in the red and blue energy states occupy the same region (Phase Mixed), then at higher laser strengths, they separate into different regions (Phase Separated). (Credit: Ian Spielman, JQI/NIST)


 

In the researchers' demonstration of spin-orbit coupling, two lasers allow an atom's motion to flip it between a pair of energy states. The new work, published in Nature, demonstrates this effect for the first time in bosons, which make up one of the two major classes of particles. The same technique could be applied to fermions, the other major class of particles, according to the researchers. The special properties of fermions would make them ideal for studying new kinds of interactions between two particles -- for example those leading to novel "p-wave" superconductivity, which may enable a long-sought form of quantum computing known as topological quantum computation.

In an unexpected development, the team also discovered that the lasers modified how the atoms interacted with each other and caused atoms in one energy state to separate in space from atoms in the other energy state.

One of the most important phenomena in quantum physics, spin-orbit coupling describes the interplay that can occur between a particle's internal properties and its external properties. In atoms, it usually describes interactions that only occur within an atom: how an electron's orbit around an atom's core (nucleus) affects the orientation of the electron's internal bar-magnet-like "spin." In semiconductor materials such as gallium arsenide, spin-orbit coupling is an interaction between an electron's spin and its linear motion in a material.

"Spin-orbit coupling is often a bad thing," said JQI's Ian Spielman, senior author of the paper. "Researchers make 'spintronic' devices out of gallium arsenide, and if you've prepared a spin in some desired orientation, the last thing you'd want it to do is to flip to some other spin when it's moving."

"But from the point of view of fundamental physics, spin-orbit coupling is really interesting," he said. "It's what drives these new kinds of materials called 'topological insulators.'"

One of the hottest topics in physics right now, topological insulators are special materials in which location is everything: the ability of electrons to flow depends on where they are located within the material. Most regions of such a material are insulating, and electric current does not flow freely. But in a flat, two-dimensional topological insulator, current can flow freely along the edge in one direction for one type of spin, and the opposite direction for the opposite kind of spin. In 3-D topological insulators, electrons would flow freely on the surface but be inhibited inside the material. While researchers have been making higher and higher quality versions of this special class of material in solids, spin-orbit coupling in trapped ultracold gases of atoms could help realize topological insulators in their purest, most pristine form, as gases are free of impurity atoms and the other complexities of solid materials.

Usually, atoms do not exhibit the same kind of spin-orbit coupling as electrons exhibit in gallium-arsenide crystals. While each individual atom has its own spin-orbit coupling going on between its internal components (electrons and nucleus), the atom's overall motion generally is not affected by its internal energy state.

But the researchers were able to change that. In their experiment, researchers trapped and cooled a gas of about 200,000 rubidium-87 atoms down to 100 nanokelvins, 3 billion times colder than room temperature. The researchers selected a pair of energy states, analogous to the "spin-up" and "spin-down" states in an electron, from the available atomic energy levels. An atom could occupy either of these "pseudospin" states. Then researchers shined a pair of lasers on the atoms so as to change the relationship between the atom's energy and its momentum (its mass times velocity), and therefore its motion. This created spin-orbit coupling in the atom: the moving atom flipped between its two "spin" states at a rate that depended upon its velocity.

"This demonstrates that the idea of using laser light to create spin-orbit coupling in atoms works. This is all we expected to see," Spielman said. "But something else really neat happened."

They turned up the intensity of their lasers, and atoms of one spin state began to repel the atoms in the other spin state, causing them to separate.

"We changed fundamentally how these atoms interacted with one another," Spielman said. "We hadn't anticipated that and got lucky."

The rubidium atoms in the researchers' experiment were bosons, sociable particles that can all crowd into the same space even if they possess identical values in their properties including spin. But Spielman's calculations show that they could also create this same effect in ultracold gases of fermions. Fermions, the more antisocial type of atoms, cannot occupy the same space when they are in an identical state. And compared to other methods for creating new interactions between fermions, the spin states would be easier to control and longer lived.

A spin-orbit-coupled Fermi gas could interact with itself because the lasers effectively split each atom into two distinct components, each with its own spin state, and two such atoms with different velocities could then interact and pair up with one other. This kind of pairing opens up possibilities, Spielman said, for studying novel forms of superconductivity, particularly "p-wave" superconductivity, in which two paired atoms have a quantum-mechanical phase that depends on their relative orientation. Such p-wave superconductors may enable a form of quantum computing known as topological quantum computation.

Friday, December 4, 2009

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


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

A pair of laser beams (red arrows) impinges upon an ultracold gas 
cloud of rubidum atoms (green oval) to create synthetic magnetic fields 
(labeled Beff).(Inset) The beams, combined with an external magnetic 
field (not shown) cause the atoms to "feel" a rotational force; the 
swirling atoms create vortices in the gas. (Credit: JQI)

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