Recently in Quantum physics Category

Science News: Studying with the radio on may not be the best way to remember what you've read. But scientists have now built a data storage device whose memory gets a boost from noise.

The device can store one bit of information, such as a 0 or a 1, only when surrounded by electronic noise, which is normally a problem in computer circuits.

"If you remove the noise, it doesn't store the bit at all," says Diego Grosz of the Instituto Tecnológico de Buenos Aires, a coauthor of the study.

Related Link
One-bit stochastic resonance storage device

ScienceNOW: The way light moves, with its fixed speed and its ability to act like either a wave or a particle, often leads to some of the most curious paradoxes of physics. A new one has just been found: Make holes in a film of gold so thin that it's already semitransparent, and less light gets through.

Nature: Quantum systems habitually leak information, limiting their usefulness for practical applications. By optimally reversing the leak, this information loss has been reduced to a trickle in the solid state.

Related Link
Preserving electron spin coherence in solids by optimal dynamical decoupling

Science: Since the work of James Clerk Maxwell and Heinrich Hertz, we have known that light is an electromagnetic wave. An intricate mechanism generates magnetic fields around the electric fields, and vice versa. In the optical-wavelength range, experimental studies have been limited to probing only the electric-field components.

In Science, Matteo Burresi and colleagues report direct measurements of the magnetic-field components of light obtained with a nanostructured metallic probe at the tip of a sharp glass fiber.

Related Link
Probing the magnetic field of light at optical frequencies

Various: In a talk entitled Higgs, dark matter and supersymmetry, what the Large Hadron Collider will tell us, given to science writers attending the Council for the Advancement of Science Writing’s annual symposium, Nobel laureate Steven Weinberg of the University of Texas at Austin gave his opinion of what the LHC will discover.

The LHC will eventually attain sufficient energy to produce the Higgs boson, he says, but evidence of supersymmetry is a much more speculative possibility.

"If the Congress had not had the imbecility to cancel the Superconducting Super Collider [in 1993], it would have been discovered long ago here in Texas," says Weinberg in comments reported by Tom Siegfried of Science News.

"Many of us are terrified that the LHC will discover a Higgs particle and nothing more," Weinberg said. That would just confirm the standard model, which everybody believes already. It would not point the way to further progress in solving a deeper problem that physics faces—how to add gravity to the unified theory of the other forces.

Peter Woit of "Not Even Wrong" says that what he found interesting about Weinberg’s talk was that, "whatever Weinberg’s views on more speculative theories in physics such as extra dimensions or string theory landscape, he decided not to mention these at all in his talk."

"As a result, both questioners wanted to ask Weinberg about string theory, which he hadn’t talked about, not about the solid science he did talk about," says Woit.

String theory or superstring theory, is one of the candidates for unifying all the forces in the universe into one theory.

If the LHC creates new particles generated by supersymmetry, then clues to what makes up the bulk of dark matter in the universe would be found, which may give some tangible evidence to whether string theory is correct.

But string theory to this point has not produced a cohesive and clear guide to testing its fit with all the observable features of physical existence. Weinberg said:

"It’s developed mathematically, but not to the point where there is any one theory, or to the point that even if we had one theory we would know how to do calculations to predict things like the mass of the electron, or the masses of the quarks. So, I would say, although there has been theoretical progress... I find it disappointing. One of the hopes would be that the LHC would provide a clue to something we’re missing in superstring theory and I think that supersymmetry is the most likely place to look."

"One of the troubles with superstring theory is that although in a sense the theorists think there is only one theory, there are an infinite number of approximate solutions of it and we don’t know which one corresponds to our world. But at least in a large variety of the solutions of superstring theory there is supersymmetry visible at low energies, and if we see supersymmetry at low energies, superstring theorists may be able to derive from it some kind of clue as to how to solve these theories. But I haven’t talked about it in this lecture because I don’t see how that would work... I mean I couldn’t say that it was likely with any degree of sincerity, and certainly the LHC and any other accelerator that we can imagine being built will not get up to energies which are high enough so that we can directly see the structures that are described by superstring theory, the strings or the D-branes or whatever it is. Those will not be accessible at the LHC, so any clue we get will be very indirect."

"I myself, well I was working on superstring theory in the 80s and gave it up because... I moved into cosmology, which in the last couple of decades has had the excitement that elementary particle physics had in the 60s and 70s, a wonderful coming together of theory and observation. Cosmology now reminds me of the excitement that I felt when I was younger and doing particle physics... and it’s a pity that superstring hasn’t developed better. I still think it’s the best hope we have, I don’t know of anything else. My own work very recently has been trying to develop an alternative to superstring theory as a way of making sense out of quantum gravity at very high energies. But even though I’m working on this I still find superstring theory more attractive, but not attractive enough…"

ISNS: With the speed of computers so regularly seeing dramatic increases in their processing speed, it seems that it shouldn't be too long before the machines become infinitely fast—except they can't.

A pair of physicists has shown that computers have a speed limit as unbreakable as the speed of light. If processors continue to accelerate as they have in the past, we'll hit the wall of faster processing in less than a century.

Intel co-founder Gordon Moore predicted 40 years ago that manufacturers could double computing speed every two years or so by cramming ever-tinier transistors on a chip. His prediction became known as Moore's Law, and it has held true throughout the evolution of computers—the fastest processor today beats out a ten-year-old competitor by a factor of about 30.

If components are to continue shrinking, physicists must eventually code bits of information onto ever smaller particles. Smaller means faster in the microelectronic world, but physicists Lev Levitin and Tommaso Toffoli at Boston University in Massachusetts, have slapped a speed limit on computing, no matter how small the components get.

"If we believe in Moore's law ... then it would take about 75 to 80 years to achieve this quantum limit," Levitin said.

"No system can overcome that limit. It doesn't depend on the physical nature of the system or how it's implemented, what algorithm you use for computation ... any choice of hardware and software," Levitin said. "This bound poses an absolute law of nature, just like the speed of light."

Scott Aaronson, an assistant professor of electrical engineering and computer science at the Massachusetts Institute of Technology in Cambridge, thought Levitin's estimate of 75 years extremely optimistic.

Moore's Law, he said, probably won't hold for more than 20 years.

In the early 1980s, Levitin singled out a quantum elementary operation, the most basic task a quantum computer could carry out. In a paper published in Physical Review Letters, Levitin and Toffoli present an equation for the minimum sliver of time it takes for this elementary operation to occur. This establishes the speed limit for all possible computers.

Using their equation, Levitin and Toffoli calculated that, for every unit of energy, a perfect quantum computer spits out ten quadrillion more operations each second than today's fastest processors. 

"It's very important to try to establish a fundamental limit—how far we can go using these resources," Levitin explained.

The physicists pointed out that technological barriers might slow down Moore's law as we approach this limit. Quantum computers, unlike electrical ones, can't handle "noise"—a kink in a wire or a change in temperature can cause havoc. Overcoming this weakness to make quantum computing a reality will take time and more research.

As computer components are packed tighter and tighter together, companies are finding that the newer processors are getting hotter sooner than they are getting faster. Hence the recent trend in duo and quad-core processing; rather than build faster processors, manufacturers place them in tandem to keep the heat levels tolerable while computing speeds shoot up. Scientists who need to churn through vast numbers of calculations might one day turn to superconducting computers cooled to drastically frigid temperatures. But even with these clever tactics, Levitin and Toffoli said, there's no getting past the fundamental speed limit.

Aaronson called it beautiful that such a limit exists.

"From a theorist's perspective, it's good to know that fundamental limits are there, sort of an absolute ceiling," he said. "You may say it's disappointing that we can't build infinitely fast computers, but as a picture of the world, if you have a theory of physics allows for infinitely fast computation, there could be a problem with that theory."

Lauren Schenkman
First published at Inside Science News

NYTimes.com: Holger Bech Nielsen, of the Niels Bohr Institute in Copenhagen, and Masao Ninomiya of the Yukawa Institute for Theoretical Physics in Kyoto, Japan, have suggested that the hypothesized Higgs boson, which physicists hope to produce with CERN's Large Hadron Collider, might be so abhorrent to nature that its creation would ripple backward through time and stop the collider before it could make one, like a time traveler who goes back in time to kill his grandfather.

Nielsen and Ninomiya put this idea forward in a series of papers: "Test of Effect From Future in Large Hadron Collider: a Proposal" and "Search for Future Influence From LHC," posted on the physics Web site arXiv.org.

According to the so-called Standard Model that rules almost all physics, the Higgs is responsible for imbuing other elementary particles with mass.

"It is our prediction that all Higgs producing machines shall have bad luck," Nielsen told the New York Times in an email.

Science News: By linking the electrical currents of two superconductors large enough to be seen with the naked eye, researchers have extended the domain of observable quantum effects. Billions of flowing electrons in the superconductors can collectively exhibit a weird quantum property called entanglement, usually confined to the realm of tiny particles, say scientists in Nature.

"It's an exciting piece of work," comments physicist Steven Girvin of Yale University. "People are interested in pushing the boundaries of quantum mechanics."

Nature News: Suspending a cat between life and death is one of the best-known thought experiments in quantum mechanics.

Now researchers from Germany and Spain are proposing a real experiment to probe whether a virus can exist in a superposition of two quantum states.

Such superpositions are typically the domain of smaller, inanimate objects such as atoms. But the team believes that their technique, using finely tuned lasers, will soon allow for the superposition of something much closer to a living organism. They outline the experiment in a paper posted to the arXiv pre-print server.

Related Link
Towards quantum superposition of living organisms

Science: The Moon isn't made of green cheese and almost certainly doesn't harbor hypothetical particles called "strangelets," an analysis of lunar soil has shown. The result undermines a possible strangelet sighting a decade ago and strengthens the case that the bizarre particles, which protesters once feared might emerge from an atom smasher and consume Earth, don't exist.

"I'm not surprised," says Frank Wilczek, a theorist at the Massachusetts Institute of Technology (MIT) in Cambridge. "It would be a great discovery to find strangelets, but the theoretical case for them is pretty shaky." Still, he says, "it's not crazy" to look for them.

Forbes.com: Outsider Peter Woit is challenging the debate about physics "theory of everything."

NYTimes.com: IBM researcher Frances Ross is growing a crop of mushroom-shaped silicon nanowires that may one day become a basic building block for a new kind of electronics.

Nanowires are just one example, although one of the most promising, of a transformation now taking place in the material sciences as researchers push to create the next generation of switching devices smaller, faster, and more powerful than today's transistors.

The reason that many computer scientists are pursuing this goal is that the shrinking of the transistor has approached fundamental physical limits.

guardian.co.uk: For all we know we may live in a world in which windows unbreak and cold cups of coffee spontaneously heat up, we just don't remember. The explanation is quantum entanglement, says Lorenzo Maccone at the Massachusetts Institute of Technology, but other physicists, such as Huw Price, head of the Centre for Time at the University of Sydney, remain skeptical.

Related Link
Quantum Solution to the Arrow-of-Time Dilemma Phys. Rev. Lett.

Various: NOνA is a collaboration of 180 scientists and engineers from 28 institutions which plans to study neutrino oscillations using the existing NuMI neutrino beam at Fermilab. The NOνA experiment is designed to search for oscillations of muon neutrinos to electron neutrinos by comparing the electron neutrino event rate measured at the Fermilab site with the electron neutrino event rate measured at a location just south of International Falls, Minnesota, 810 kilometers distant from Fermilab. If oscillations occur, the far site will see the appearance of electrons in the muon neutrino beam produced at Fermilab.

As the Washington Post describes it in this story:

Scientists are playing an exotic game of pitch and catch between Illinois and Minnesota. Their catcher's mitt is solid iron, weighs 5,500 tons, and is parked in northern Minnesota in an abandoned iron mine. With millions of dollars from the federal stimulus package, construction crews are now building a second mitt near the Canadian border. It's even heavier, some 15,000 tons, and is made of 385,000 liquid-filled cells of PVC plastic.

Five hundred miles to the south is the pitcher: Fermilab, a sprawling U.S. government laboratory west of Chicago where physicists do violent things with tiny particles.

Science News: In a new study, researchers found telltale signs of quantum weirdness lurking in an optical trick called ghost imaging. Discovered over a decade ago, ghost imaging allows researchers to create an image of something using light that never bounced off the actual object. The new work adds to the debate over whether ghost imaging is quantum in nature, or if normal, everyday physics can explain the phenomenon.

Related Link
Holographic Ghost Imaging and the Violation of a Bell Inequality. Physical Review Letters, in press

Technology Review: Researchers at the National Institute of Standards and Technology (NIST) in Boulder, CO, have demonstrated multiple computing operations on quantum bits—a crucial step toward building a practical quantum computer.

Nature News: Until recently, string theory—long heralded as a 'theory of everything'—hadn't been particularly good at explaining anything.

But at a workshop this month at the Kavli Institute for Theoretical Physics in Santa Barbara, California, scientists have been using the theory to make progress in tackling one of the biggest puzzles in condensed-matter physics: the origin of high-temperature superconductivity.

Science News: A team of theoretical physicists and astronomers has calculated that any hidden extra dimension beyond our familiar three-dimensional space, a world known in physics parlance as a 3-brane, must be less than 3 micrometers. The researchers base their findings on the recent discovery of one of the smallest and oldest black holes ever found.

The new limit is less than half that of previous limits on the length of an extra dimension, Oleg Gnedin of the University of Michigan in Ann Arbor and his colleagues report in a study posted online 30 June (http://arxiv.org/abs/0906.5351).

Nature: LaHaye and colleagues have taken an important step towards the observation of quantum phenomena in nearly macroscopic moving objects.

They report experimental evidence of an intriguing interplay between a superconducting artificial atom and a micrometre-size mechanical resonator. Remarkably, their findings can be described using the 'language' of radiation–matter interactions, which has also been successful in explaining the coupling of a superconducting artificial atom to microwave photon.

Related Link
Nanomechanical measurements of a superconducting qubit

ScienceNOW: The first electric motor whirred to life nearly 2 centuries ago, and in recent decades scientists and engineers have worked to build ever-smaller ones.

Now, a team of theoretical physicists has proposed a fully quantum-mechanical version of the classic spinning electric motor that consists of just two atoms trapped in a ring of light.

Experimenters might be able to construct the thing now, the researchers say, even though they themselves don't have an intuitive explanation of exactly how it works.

Nature: For almost a century, physicists have had in hand "the" theoretical framework of the known world—quantum mechanics. But whereas the world clearly comprises large complex systems, quantum mechanics is usually associated with the microworld of atoms and elementary particles, and is hardly ever considered as an underlying feature in our daily life.

This is even more pronounced for some of the seemingly weird predictions of quantum mechanics, such as entanglement, which asserts that the quantum state of physically separated objects is mutually and inextricably connected.

J. D. Jost and colleagues in Nature demonstrate quantum entanglement of two spatially separated mechanical oscillators. Although the quantum nature of mechanical oscillators has been known and observed for a long time, the entanglement of their oscillating motions has not, and its demonstration adds a valuable tool to the toolbox of quantum-state engineering.

Related Links
Entangled mechanical oscillators

Nature: The ability to produce arbitrarily superposed quantum states is a prerequisite for creating a workable quantum computer. Such highly complex states can now be generated on demand in superconducting electronic circuitry.

Related Link
Synthesizing arbitrary quantum states in a superconducting resonator

AFP: A team of French physicists led by Jean-Yves Bigot of the Institute of Materials Physics and Chemistry in Strasbourg say they have used a "femtosecond" laser, using ultra-fast bursts of laser light, to alter electron spin and thus speed up retrieval and storage.

The technique could increase the speed at which data is written and read from a hard drive up to 100,000 times, they say in this week's Nature Physics.

The work builds upon Albert Fert and Peter Gruenberg's discovery that tiny changes in magnetic fields can yield a large electric output. Their research led to the creation of a new electronics field called "spintronics" that relies on electron spin to store data; however, sensors for reading that data until now were too slow to be effective.

"Our method is called the photonics of spin, because it is photons [particles of light] that modify the state of the electrons' magnetisation" on the storage surface, Bigot told AFP.

Related Physics Today articles

Discoverers of Giant Magnetoresistance Win this Year's Physics Nobel (December 2007)
Quantum Spin Hall Effect Shows up in a Quantum Well Insulator, Just as Predicted (January 2008)
Magnetic Semiconductors Enable Efficient Electrical Spin Injection (April 2000)

Related Link

Coherent ultrafast magnetism induced by femtosecond laser pulses

In last week's Science Kukushkin and colleagues describe a variant of this technique for measuring the energy and momentum dependence of the excitations of a two-dimensional (2D) electron system.

In this technique, momentum is imparted with sound and, separately, energy is imparted with light. This approach allows the spectrum of "magnetorotons"—characteristic excitations of the states associated with the fractional quantum Hall effect—to be measured directly.
The observed spectral features were predicted many years ago but have eluded direct measurement until now.

Related Link
Dispersion of the Excitations of Fractional Quantum Hall States

Science: Graphene holds enormous promise for transistors and other electronic devices. But it is already making an impact in the arcane world of high-energy physics.

That's because electrons in graphene don't behave like electrons in a standard metal. In the lattice of a typical metal, electrons feel the push and pull of surrounding charges as they move. As a result, moving electrons behave as if they have a different mass from their less mobile partners. When electrons move through graphene, however, they act as if their mass is zero--behavior that makes them look more like neutrinos streaking through space near the speed of light.

At such "relativistic" speeds, particles don't follow the usual rules of quantum mechanics. Instead, physicists must invoke the mathematical language of quantum electrodynamics, which combines quantum mechanics with Albert Einstein's relativity theory. Even though electrons course through graphene at only 1/300 the speed of neutrinos, physicists realized several years ago that the novel material might provide a test bed for studying relativistic physics in the lab.

"This is pretty cool," says Alan Migdall of the National Institute of Standards and Technology in Gaithersburg, Maryland. "I haven't seen an approach like this before." Still, he and others say it's too soon to tell whether the new method, described on page 483 in Science, will outshine techniques that generate pairs of photons entangled from "birth."

Related Link
An entanglement filter

Related Article
Ballistic spin resonance

The water level at Homestake is down 254 feet since the high-water mark was reached last August.

Homestake is 8,000 feet deep. Mining stopped in 2001, and the underground pumps were turned off just before Homestake was sealed shut in 2003. Water was slowly filling the mine until last year. Now, the South Dakota Science and Technology Authority is pumping water out to reopen Homestake as an underground laboratory, with experiments as deep as 4,850 feet underground

ScienceNOW: This might be a rare case about which Einstein was wrong. More than 60 years ago, the great physicist scoffed at the idea that anything could travel faster than light, even though quantum mechanics had suggested such a condition. Now four Swiss researchers have brought the possibility closer to reality. Testing a concept called "spooky action at a distance"--a phrase used by Einstein in criticizing the phenomenon--they have shown that two subatomic particles can communicate nearly instantaneously, even if they are separated by cosmic distances.

Nature: A vacuum may be devoid of matter, but its shape is still important. The strength of the Casimir force caused by quantum fluctuations in the space between surfaces is critically dependent on their nanometre-scale shape.

Science: Images are superb conveyors of information. Recent research has shown how subtle quantum mechanical aspects of light can profoundly influence the nature of image formation.In the July 25 issue of Science, two important advances in this emerging area of quantum imaging are presented. Wagner et al. report on the behavior of two beams of light that are quantum mechanically entangled in position and direction of propagation--that is, the outcome of measurements on one beam depends on what sort of measurements have been performed on the other beam. Boyer et al. show that two image-bearing light beams can be entangled such that strong quantum correlations exist both between the two beams and between individual image features within each beam. They find two sorts of quantum correlations: The intensities of the two beams fluctuate in unison, at a level not permitted by classical statistics, and the noise in one part of the light field can be reduced, or "squeezed," at the expense of another part.

Science News: The length of bonds connecting water molecules could demonstrate quantum effects and help explain some of water’s weirdness.

Nature: Traditionally, entanglement was considered to be a quirk of microscopic objects that defied a common-sense explanation. Now, however, entanglement is recognized to be ubiquitous and robust. With the realization that entanglement can occur in macroscopic systems — and with the development of experiments aimed at exploiting this fact — new tools are required to define and quantify entanglement beyond the original microscopic framework.

Nature: A joint exploration of early modern physics and the surreal art movement shows these twentieth-century revolutions had more in common than we thought, explains Nature's Philip Ball.