String field theory could be the foundation of quantum mechanics: Connection could be huge boost to string theory

Artist's abstraction (stock illustration). Credit: © agsandrew / Fotolia

Two USC researchers have proposed a link between string field theory and quantum mechanics that could open the door to using string field theory -- or a broader version of it, called M-theory -- as the basis of all physics.

"This could solve the mystery of where quantum mechanics comes from," said Itzhak Bars, USC Dornsife College of Letters, Arts and Sciences professor and lead author of the paper.

Bars collaborated with Dmitry Rychkov, his Ph.D. student at USC. The paper was published online on Oct. 27 by the journal Physics Letters.

Rather than use quantum mechanics to validate string field theory, the researchers worked backwards and used string field theory to try to validate quantum mechanics.

In their paper, which reformulated string field theory in a clearer language, Bars and Rychov showed that a set of fundamental quantum mechanical principles known as "commutation rules'' may be derived from the geometry of strings joining and splitting.

"Our argument can be presented in bare bones in a hugely simplified mathematical structure," Bars said. "The essential ingredient is the assumption that all matter is made up of strings and that the only possible interaction is joining/splitting as specified in their version of string field theory."

Physicists have long sought to unite quantum mechanics and general relativity, and to explain why both work in their respective domains. First proposed in the 1970s, string theory resolved inconsistencies of quantum gravity and suggested that the fundamental unit of matter was a tiny string, not a point, and that the only possible interactions of matter are strings either joining or splitting.

Four decades later, physicists are still trying to hash out the rules of string theory, which seem to demand some interesting starting conditions to work (like extra dimensions, which may explain why quarks and leptons have electric charge, color and "flavor" that distinguish them from one another).

At present, no single set of rules can be used to explain all of the physical interactions that occur in the observable universe.

On large scales, scientists use classical, Newtonian mechanics to describe how gravity holds the moon in its orbit or why the force of a jet engine propels a jet forward. Newtonian mechanics is intuitive and can often be observed with the naked eye.

On incredibly tiny scales, such as 100 million times smaller than an atom, scientists use relativistic quantum field theory to describe the interactions of subatomic particles and the forces that hold quarks and leptons together inside protons, neutrons, nuclei and atoms.

Quantum mechanics is often counterintuitive, allowing for particles to be in two places at once, but has been repeatedly validated from the atom to the quarks. It has become an invaluable and accurate framework for understanding the interactions of matter and energy at small distances.

Quantum mechanics is extremely successful as a model for how things work on small scales, but it contains a big mystery: the unexplained foundational quantum commutation rules that predict uncertainty in the position and momentum of every point in the universe.

"The commutation rules don't have an explanation from a more fundamental perspective, but have been experimentally verified down to the smallest distances probed by the most powerful accelerators. Clearly the rules are correct, but they beg for an explanation of their origins in some physical phenomena that are even deeper," Bars said.

The difficulty lies in the fact that there's no experimental data on the topic -- testing things on such a small scale is currently beyond a scientist's technological ability.

Source: University of Southern California

Strange materials cropping up in condensed matter laboratories

A conformal field theory in condensed matter (labelled CFT) gets a hand from string theory – the string theory hand has an extra dimension and a black hole in it. Credit: Image courtesy of Perimeter Institute for Theoretical Physics

Subir Sachdev, William Witczak-Krempa, and Erik Sørensen are condensed matter physicists. They study exotic but tangible systems, such as superfluids. And their latest paper about one such system has a black hole in it.

How did a black hole get into a condensed matter paper? "Well, it's a long story," says Sachdev, who is a professor at Harvard and a Distinguished Visiting Research Chair at Perimeter Institute.

It's a long story, he might add, that in a way starts with him: he was one of the first condensed matter physicists to venture into the strange land of string theory, where the black holes live. But that is getting ahead of the tale.

"Let's start here," Sachdev says. "Condensed matter physicists study the behaviour of electrons in many materials -- semiconductors, metals, and exotic materials like superconductors."

Normally, these physicists can model the behaviour of a material as if electrons were moving freely around inside it. Even if that's not what's actually happening, because of complex interactions, it makes the model easy to understand and the calculations easier to do. Electrons (and occasionally other particles) used in this kind of short-hand model are called quasi-particles.

However, there are a handful of systems that cannot be described by considering electrons (or any other kind of quasi-particle) moving around.

"What we try to do is understand a quantum system where you have electricity without electrons," says Sachdev. "Of course, the system does have electrons in it, but the behaviour of the system doesn't look like electrons moving at all. What you see is not even particles, but some lumps of quantum excitations that are doing strange quantum things."

"Without quasi-particles, it's a mess," says William Witczak-Krempa. Witczak-Krempa, a Perimeter postdoctoral fellow, is also a condensed matter theorist who collaborated with Sachdev on the paper. "It's this quantum fuzzball of stuff."

Describing such a fuzzball system is a challenge -- but it's crucial to understanding many modern materials, including superfluids and high-temperature superconductors. The broad problem of how to model systems without quasi-particles has been stumping condensed matter theorists for decades.

"What we decided to do was look at a simple case of such an electricity-without-electrons system," says Witczak-Krempa. "That turns out to be a quantum phase transition between a superfluid and an insulator."

A fair amount of work had been done on such systems, such that the team was able to make progress modelling the system using the traditional mathematical tools of condensed matter. Sachdev and Witczak-Krempa worked with Erik Sørensen of McMaster University on this aspect of their paper. Sørensen used a computer simulation -- specifically, a quantum Monte Carlo simulation -- to predict how conductivity should change with temperature and frequency as a superfluid turns into an insulator.

"This frequency dependence tells us how the quantum fluid behaves in time. This dynamic behaviour is notoriously hard to study using standard methods, including quantum Monte Carlo simulations," says Witczak-Krempa. "Erik's work was a huge computational achievement. It took months of processing time. And, in the end, the results still needed to be converted into a form that can be compared with experiments. This is where we tried something new."

To perform this conversion, Sachdev and Witczak-Krempa tackled the same system from a different angle: string theory. (Here, they build on Sachdev's previous work with Perimeter Faculty member Robert Myers and one of his graduate students, Ajay Singh.) One of the pillars of string theory is that certain quantum field theories (technically known as conformal field theories) can be translated into a theory of gravity with one extra dimension.

Sachdev explains where the extra dimension comes in. Wiggling his fingers above the tabletop, he conjures strings moving through the air.

"In certain configurations, the strings all end on a kind of membrane," he says, tapping his fingertips on the table's surface. "You might ask yourself: if you were living on the membrane [the table surface] -- and you didn't know about the extra dimensions where the strings were, what would you see?"

He answers himself: "Only the ends of the strings. They would look like particles. What's amazing is that string theorists found that the theories that you'd use to define the ends of the strings on the membrane are remarkably like the theory we want to use to describe our system."

The quantum field theory describing Sachdev and Witczak-Krempa's "fuzzball" system shares many fundamental properties with the conformal field theories associated with string theory -- so many that the researchers were able to map the two-dimensional field theory into a three-dimensional theory of gravity.

"We ended up studying the physics of this alternate reality," says Witczak-Krempa. "Using this technique, we were able to translate a very hard problem into a fairly easy one." Albeit a fairly easy problem involving a black hole.

"We wanted to look at the physics of the boundary -- the physics at the table top," says Witczak-Krempa. "But we wanted to heat it up a bit -- give it a finite temperature. It turns out that the natural way of doing this is to invoke a black hole." Really?

"There are various ways of developing an intuition about that," he says. "For instance, you can remember that the black hole will release Hawking radiation. The Hawking radiation escapes and eventually hits the boundary where the system lives, and heats it up."

Witczak-Krempa admits it's unorthodox: "Most condensed matter people would go: 'Why is there a black hole in this paper?' It's crazy. But what's even crazier is that this mathematical machinery works quite well. It gives you answers that make a lot of sense. You can compare them directly with Erik's Monte Carlo results, and they check out."

It's the first time results from a traditional large-scale condensed matter simulation have been compared to results from the new string theory approach.

Sachdev is cautiously thrilled: "There are a couple of issues we don't fully understand and one discrepancy we wish we understood better, but in general it's worked extremely well. It's progress on something I've been thinking about for more than 20 years. And now we finally have a theory that is perhaps not complete, but is encouragingly successful."

What's more, string theory has finally produced a set of physical predictions that experimentalists can go check. Sachdev and Witczak-Krempa are hoping that an experimental team will try soon.

"Let's see what happens," says Sachdev. "We're pushing string theory to a new regime. Whatever happens, we will learn more."

Source: Perimeter Institute for Theoretical Physics

Einstein's 'spooky' theory may lead to ultra-secure Internet

Could new research into Einstein's 'spooky action at a distance' pave the way for a new ultra-secure quantum Internet? Credit: © Serg Nvns / Fotolia

Einstein's skepticism about quantum mechanics may lead to an ultra-secure Internet, suggests a new paper by researchers from Swinburne University of Technology and Peking University.

Associate Professor Margaret Reid from Swinburne's Centre for Quantum and Optical Science said Einstein's reservations about quantum mechanics were highlighted in a phenomenon known as "'spooky' action at a distance."

In 1935, Einstein and researchers highlighted a 'spooky' theory in quantum mechanics, which is the strange way entangled particles stay connected even when separated by large distances.

"Until now the real application of this has been for messages being shared between two people securely without interception, regardless of the spatial separation between them," Professor Reid said.

"In this paper, we give theoretical proof that such messages can be shared between more than two people and may provide unprecedented security for a future quantum Internet."

In the 1990s, scientists realised you can securely transmit a message through encrypting and using a shared key generated by Einstein's strange entanglement to decode the message from the sender and receiver. Using the quantum key meant the message was completely secure from interception during transmission.

Sending Einstein's entanglement to a larger number of people means the key can be distributed among all the receiving parties, so they must collaborate to decipher the message, which Professor Reid said makes the message even more secure.

"We found that a secure message can be shared by up to three to four people, opening the possibility to the theory being applicable to secure messages being sent from many to many.

"The message will also remain secure if the devices receiving the message have been tampered with, like if an iPhone were hacked, because of the nature of Einstein's spooky entanglement.

"Discovering that it can be applied to a situation with more parties has the potential to create a more secure Internet -- with less messages being intercepted from external parties."

Source: Swinburne University of Technology

A new step towards using graphene in electronic applications

(A) This image is a diagram of molecular precursors, the resulting graphene nanoribbons and the heterostructured ones. (B) Tunnel microscopy images of the heterostructures synthesized on gold surfaces. Credit: UPV/EHU

Few materials have received as much attention from the scientific world or have raised so many hopes with a view to their potential deployment in new applications as graphene has. This is largely due to its superlative properties: it is the thinnest material in existence, almost transparent, the strongest, the stiffest and at the same time the most strechable, the best thermal conductor, the one with the highest intrinsic charge carrier mobility, plus many more fascinating features. Specifically, its electronic properties can vary enormously through its confinement inside nanostructured systems, for example. That is why ribbons or rows of graphene with nanometric widths are emerging as tremendously interesting electronic components. On the other hand, due to the great variability of electronic properties upon minimal changes in the structure of these nanoribbons, exact control on an atomic level is an indispensable requirement to make the most of all their potential.

The lithographic techniques used in conventional nanotechnology do not yet have such resolution and precision. In the year 2010, however, a way was found to synthesise nanoribbons with atomic precision by means of the so-called molecular self-assembly. 

Molecules designed for this purpose are deposited onto a surface in such a way that they react with each other and give rise to perfectly specified graphene nanoribbons by means of a highly reproducible process and without any other external mediation than heating to the required temperature. In 2013 a team of scientists from the University of Berkeley and the Centre for Materials Physics (CFM), a mixed CSIC (Spanish National Research Council) and UPV/EHU (University of the Basque Country) centre, extended this very concept to new molecules that were forming wider graphene nanoribbons and therefore with new electronic properties. This same group has now managed to go a step further by creating, through this self-assembly, heterostructures that blend segments of graphene nanoribbons of two different widths.

The forming of heterostructures with different materials has been a concept widely used in electronic engineering and has enabled huge advances to be made in conventional electronics. "We have now managed for the first time to form heterostructures of graphene nanoribbons modulating their width on a molecular level with atomic precision. What is more, their subsequent characterisation by means of scanning tunnelling microscopy and spectroscopy, complemented with first principles theoretical calculations, has shown that it gives rise to a system with very interesting electronic properties which include, for example, the creation of what are known as quantum wells," pointed out the scientist Dimas de Oteyza, who has participated in this project. This work, the results of which are being published this very week in the journal Nature Nanotechnology, therefore constitutes a significant success towards the desired deployment of graphene in commercial electronic applications.

Dr Dimas G. de Oteyza, who was previously at Berkeley and at the CFM, is currently working at the Donostia International Physics Center (DIPC) as a Fellow Gipuzkoa.

Source: University of the Basque Country

The simplest element: Turning hydrogen into 'graphene'

This image is a comparison of the carbon compound graphene with a similar hydrogen-based structure synthesized by Carnegie scientists. Credit: Carneige's Ivan Naumov and Russell Hemley

New work from Carnegie's Ivan Naumov and Russell Hemley delves into the chemistry underlying some surprising recent observations about hydrogen, and reveals remarkable parallels between hydrogen and graphene under extreme pressures. Their work is the cover story in the December issue of Accounts of Chemical Research.

Hydrogen is the most-abundant element in the cosmos. With only a single electron per atom, it is deceptively simple. As a result, hydrogen has been a testing ground for theories of the chemical bond since the birth of quantum mechanics a century ago. Understanding the nature of chemical bonding in extreme environments is crucial for expanding our understanding of matter over the broad range of conditions found in the universe.

Observing hydrogen's behavior under very high pressures has been a great challenge for researchers. But recently teams have been able to observe that at pressures of 2-to-3.5 million times normal atmospheric pressure it transforms into an unexpected structure consisting of layered sheets, rather than a close-packed metal as had been predicted many years ago.

These hydrogen sheets resemble the carbon compound graphene. Graphene's layers are each constructed of a honeycomb structure made of six-atom carbon rings. This conventional carbon graphene, first synthesized about a decade ago, is very light, but incredibly strong, and conducts heat and electricity very efficiently. These properties promise revolutionary technology, including advanced optical electronics for screens, high-functioning photovoltaic cells, and enhanced batteries and other energy storage devices.

The new work from Naumov and Hemley shows that the stability of the unusual hydrogen structure arises from the intrinsic stability of its hydrogen rings. These rings form because of so-called aromaticity, which is well understood in carbon-containing molecules such as benzene, as well as in graphene. Aromatic structures take on a ring-like shape that can be thought of as alternating single and double bonded carbons. But what actually happens is that the electrons that make up these theoretically alternating bonds become delocalized and float in a shared circle around the inside of the ring, increasing stability.

Naumov and Hemley's study also indicates that hydrogen initially becomes a dark poorly conducting metal like graphite instead of a conventional shiny metal and a good conductor, as was originally suggested in theoretical calculations going back to the 1930's using early quantum mechanical models for solids.

Though the discovery of this layered sheet character of dense hydrogen has come as a surprise to many, chemists 30 years ago--before the discovery of graphene--predicted the structure based on simple chemical considerations. Their work is validated and extended by the new findings.

"Overall, our results indicate that chemical bonding occurs over a much broader range of conditions than people had previously considered. However, the structural effects of that chemical bonding under extreme conditions can be very different than that observed under the ordinary conditions that are familiar to us," Hemley said.

Source: Carnegie Institution

'Topological insulators' promising for spintronics, quantum computers

Purdue University doctoral student Yang Xu, lead author of a new research paper on "topological insulators," an emerging class of materials that could make possible "spintronic" devices and practical quantum computers far more powerful than today's technologies, is shown here inspecting devices made from topological insulators under a microscope before electrical measurements.
Credit: Purdue University photo / Ting-fung Chung

Researches have uncovered "smoking-gun" evidence to confirm the workings of an emerging class of materials that could make possible "spintronic" devices and practical quantum computers far more powerful than today's technologies.

The materials are called "topological insulators." Unlike ordinary materials that are either insulators or conductors, topological insulators are in some sense both at the same time -- they are insulators inside but always conduct electricity via the surface. Specifically, the researchers have reported the clearest demonstration of such seemingly paradoxical conducting properties and observed the "half integer quantum Hall effect" on the surface of a topological insulator.

"This is unambiguous smoking-gun evidence to confirm theoretical predictions for the conduction of electrons in these materials," said Purdue University doctoral student Yang Xu, lead author of a paper appearing this week in the journal Nature Physics.

Yong P. Chen, a Purdue associate professor of physics and astronomy and electrical and computer engineering, led a team of researchers from Purdue, Princeton University and the 

University of Texas at Austin in studying the bismuth-based material.

"This experimental system provides an excellent platform to pursue a plethora of exotic physics and novel device applications predicted for topological insulators," Chen said.

For example, by further combining topological insulators with a superconductor, which conducts electricity with no resistance, researchers may be able to build a practical quantum computer. Such a technology would perform calculations using the laws of quantum mechanics, making for computers much faster than conventional computers at certain tasks such as database searches and code-breaking.

"One of the main problems with prototype quantum computers developed so far is that they are prone to errors," Chen said. "But if topologically protected, there is a mechanism to fundamentally suppress those errors, leading to a robust way to do quantum computing."

The topological insulators were synthesized at Purdue and fabricated into electrical devices at the Birck Nanotechnology Center in the university's Discovery Park.

The researchers for the first time demonstrated a three-dimensional material with an electrical resistance not dependent on the thickness of the material, a departure from conventional behavior. Whereas electrons usually have a mass, in the case of topological insulators the conducting electrons on the surface have no mass and are automatically "spin polarized," leading to the unique half-integer quantum Hall effect observed and also making the material promising for various potential applications.

Topological insulators could bring future computing platforms based on "spintronics." Conventional computers use the presence and absence of electric charges to represent ones and zeroes in a binary code needed to carry out computations. Spintronics, however, uses the "spin state" of electrons to represent ones and zeros.

"Compounds based on bismuth, antimony, telluride and selenide are the cleanest and most intrinsic topological insulators demonstrated so far, with no measurable amount of undesirable conduction inside the bulk that often spoils the topological conduction properties in earlier topological insulator materials," Chen said.

The researchers also found evidence consistent with the conduction of electrons being "topologically protected," meaning its surface is guaranteed to be a robust conductor. Studying thin-slab-shaped samples cut from this material down to ever decreasing thickness while observing the conductance, the researchers found that the conductance -- which occurs always and only at the surface -- barely changes.

"For the thinnest samples, such topological conduction properties were even observed at room temperature, paving the way for practical applications," Xu said.

The paper was authored by Xu; Purdue research scientist Ireneusz Miotkowski, who created the high-quality materials; Princeton postdoctoral research associate Chang Liu; Purdue postdoctoral research associate Jifa Tian; UT Austin graduate student Hyoungdo Nam; Princeton graduate student Nasser Alidoust; Purdue graduate student Jiuning Hu; Chih-Kang Shih, Jane and Roland Blumberg Professor at UT Austin; M. Zahid Hasan, a Princeton professor of physics; and Chen.

In addition to the material growth and electrical measurements performed by the Purdue researchers, the Princeton and UT Austin groups contributed to this study by performing advanced characterizations that further confirmed important properties of the material as a topological insulator.

The research was funded by the Defense Advanced Research Projects Agency, which supports a Purdue-led program with participation from Princeton and other institutions aiming to develop energy efficient electronic devices based on topological insulators. The electrical measurements revealing the signature half-integer quantum Hall effect were performed at the National Science Foundation's National High Magnetic Field Laboratory. UT Austin's contribution to this study was supported the Welch Foundation and U.S. Army Research Office.

Source: Purdue University

New records set for silicon quantum computing

Artist impression of an electron wave function (blue), confined in a crystal of nuclear-spin-free 28-silicon atoms (black), controlled by a nanofabricated metal gate (silver).
Credit: Stephanie Simmons, UNSW

Two research teams working in the same laboratories at UNSW Australia have found distinct solutions to a critical challenge that has held back the realisation of super powerful quantum computers.

The teams created two types of quantum bits, or "qubits" -- the building blocks for quantum computers -- that each process quantum data with an accuracy above 99%. The two findings have been published simultaneously today in the journal Nature Nanotechnology.

"For quantum computing to become a reality we need to operate the bits with very low error rates," says Scientia Professor Andrew Dzurak, who is Director of the Australian National 

Fabrication Facility at UNSW, where the devices were made.

"We've now come up with two parallel pathways for building a quantum computer in silicon, each of which shows this super accuracy," adds Associate Professor Andrea Morello from UNSW's School of Electrical Engineering and Telecommunications.

The UNSW teams, which are also affiliated with the ARC Centre of Excellence for Quantum Computation & Communication Technology, were first in the world to demonstrate single-atom spin qubits in silicon, reported in Nature in 2012 and 2013.

Now the team led by Dzurak has discovered a way to create an "artificial atom" qubit with a device remarkably similar to the silicon transistors used in consumer electronics, known as MOSFETs. Post-doctoral researcher Menno Veldhorst, lead author on the paper reporting the artificial atom qubit, says, "It is really amazing that we can make such an accurate qubit using pretty much the same devices as we have in our laptops and phones."

Meanwhile, Morello's team has been pushing the "natural" phosphorus atom qubit to the extremes of performance. Dr Juha Muhonen, a post-doctoral researcher and lead author on the natural atom qubit paper, notes: "The phosphorus atom contains in fact two qubits: the electron, and the nucleus. With the nucleus in particular, we have achieved accuracy close to 99.99%. That means only one error for every 10,000 quantum operations."

Dzurak explains that, "even though methods to correct errors do exist, their effectiveness is only guaranteed if the errors occur less than 1% of the time. Our experiments are among the first in solid-state, and the first-ever in silicon, to fulfill this requirement."

The high-accuracy operations for both natural and artificial atom qubits is achieved by placing each inside a thin layer of specially purified silicon, containing only the silicon-28 isotope. This isotope is perfectly non-magnetic and, unlike those in naturally occurring silicon, does not disturb the quantum bit. The purified silicon was provided through collaboration with Professor Kohei Itoh from Keio University in Japan.

The next step for the researchers is to build pairs of highly accurate quantum bits. Large quantum computers are expected to consist of many thousands or millions of qubits and may integrate both natural and artificial atoms.

Morello's research team also established a world-record "coherence time" for a single quantum bit held in solid state. "Coherence time is a measure of how long you can preserve quantum information before it's lost," Morello says. The longer the coherence time, the easier it becomes to perform long sequences of operations, and therefore more complex calculations.

The team was able to store quantum information in a phosphorus nucleus for more than 30 seconds. "Half a minute is an eternity in the quantum world. Preserving a 'quantum superposition' for such a long time, and inside what is basically a modified version of a normal transistor, is something that almost nobody believed possible until today," Morello says.

"For our two groups to simultaneously obtain these dramatic results with two quite different systems is very special, in particular because we are really great mates," adds Dzurak.

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Source: University of New South Wales

Toward quantum computing, spintronic memory, better displays: Nuclear spins control current in plastic LED

An organic light-emitting diode, or OLED, glows orange when electrical current flows through it. University of Utah physicists used this kind of OLED – basically a plastic LED instead of a conventional silicon semiconductor LED – to show that they could read the subatomic “spins” in the center or nuclei of hydrogen isotopes and use those spins to control current to the OLED. It is a step toward “spintronic” devices such as faster computers, better data storage and more efficient OLEDS for TV, computer and cell phone displays.  Credit: Andy Brimhall, University of Utah Marketing and Communications

University of Utah physicists read the subatomic "spins" in the centers or nuclei of hydrogen isotopes, and used the data to control current that powered light in a cheap, plastic LED -- at room temperature and without strong magnetic fields.

The study -- published in Friday's issue of the journal Science -- brings physics a step closer to practical machines that work "spintronically" as well as electronically: superfast quantum computers, more compact data storage devices and plastic or organic light-emitting diodes, or OLEDs, more efficient than those used today in display screens for cell phones, computers and televisions.

"We have shown we can use room-temperature, plastic electronic devices that allow us to see the orientation of the tiniest magnets in nature -- the spins in the smallest atomic nuclei," says physics professor Christoph Boehme, one of the study's principal authors. "This is a step that may lead to new ways to store information, produce better displays and make faster computers."

The experiment is a much more practical version of a study Boehme and colleagues published in Science in 2010, when they were able to read nuclear spins from phosphorus atoms in a conventional silicon semiconductor. But they could only do so when the apparatus was chilled to minus 453.9 degrees Fahrenheit (nearly absolute zero), was bombarded with intense microwaves and exposed to superstrong magnetic fields.

In the new experiments, the physicists were able to read the nuclear spins of two isotopes of hydrogen: a single proton and deuterium, which is a proton, neutron and electron. The isotopes were embedded in an inexpensive plastic polymer or organic semiconductor named MEH-PPV, an OLED that glows orange when current flows.

The researchers flipped the spins of the hydrogen nuclei to control electrical current flowing though the OLED, making the current stronger or weaker. They did it at room temperature and without powerful light bombardment or magnetic fields -- in other words, at normal operating conditions for most electronic devices, Boehme says.

"This experiment is remarkable because the magnetic forces created by the nuclei are millions of times smaller than the electrostatic forces that usually drive currents," yet they were able to control currents, he says.

Harnessing nuclear spins can increase the efficiency "of electronic materials out of which so much technology is made," Boehme adds. "It also raises the question whether this effect can be used for technological applications such as computer chips that use nuclear spins as memory and our method as a way to read the spins."

The U.S. Department of Energy funded the new study, and the physicists used facilities of the University of Utah's Materials Research Science and Engineering Center, funded by the National Science Foundation.

Boehme conducted the study with fellow University of Utah physicists: first author and postdoctoral fellow Hans Malissa; research professor and co-senior author John Lupton, who also is on the faculty of the University of Regensburg, Germany; distinguished professor Z. Valy Vardeny; professor Brian Saam; graduate students Marzieh Kavand and David Waters; and postdoctoral fellow Kipp van Schooten. Another co-author was Paul Burn of Australia's University of Queensland.

Spintronics: Storing Data in Atomic Nuclei

Electronic devices use electrical current or electrons, which are negatively charged particles orbiting the nuclei or centers of atoms. Modern computers store data electronically: data are stored as binary "bits" in which zero is represented by "off," or no electrical charge, and one is represented by "on" or the presence of electrical charge.

In spintronics, data are stored by the spins of either electrons or, preferably, atomic nuclei. Spin often is compared with a tiny bar magnet like a compass needle, either pointing up or down -- representing one or zero -- in an electron or an atom's nucleus. Nuclear spin orientations live longer, so are better for storing data.

The 2010 study by Boehme and colleagues showed that nuclear spins of phosphorus in a silicon semiconductor could control electrical current, but at impractically low temperatures and strong magnetic fields. They had to use the magnetic fields to align spins of phosphorus electrons in the same direction, and then use intense light to transfer the same alignment to the spins of phosphorus nuclei. Then they bombarded the semiconductor with radio waves to reverse the nuclear spins and control the current.

Boehme says scientists previously have claimed that current in plastic semiconductors -- known formally as pi-conjugated polymers -- can be controlled by the nuclear spins in hydrogen. Until the new study, "nobody has ever shown it directly" at room temperature by turning nuclear spins to change an electrical current, he adds.

The New Study

In the new experiments, the physicists used magnetic resonance to reverse the nuclear spins in hydrogen isotopes embedded in the OLED, and then were able to detect how the reversed 

spins caused a change in the electrical current through the OLED.

In the first two experiments, Boehme says, the physicists made nuclear spins in a proton and deuterium wiggle in characteristic ways, and were able to read corresponding wiggles in the resulting electrical current. In a third experiment, they flipped the spins back and forth at a rate they wanted instead of at the characteristic frequencies.

"It worked," Boehme says. "This shows you can turn a nuclear spin when you want, and only then the current turns around. We can control a current by controlling nuclear spins."

The researchers measured the current change directly, but not resulting changes in the OLED's light output -- changes so small they aren't detectable with the naked eye.

In both the 2010 and the new studies, the physicists did not read the spins of individual nuclei, but the collective spins of more than 1 million nuclei at a time. The ultimate goal is to be able to read the spins of nuclei individually.

"If you want to store information, the highest storage density would be to store information in single nuclear spins," Boehme says. Since the 2010 study, other physicists have achieved that in phosphorus nuclei, he adds.

Benefits of Spintronics

By storing information using both spins and electrical charge, spintronic devices should have greater storage capacity and process data more quickly -- although researchers still have years to go to figure out how to connect and process spintronically stored information in futuristic computers, conventional and quantum.

"We don't know if its five years, 50 years or never," Boehme says.

Yet he says spintronics already resulted in today's terabyte-sized computer hard drives, which use spintronic "read heads" so small that data can be stored more densely.

In 2012, Boehme and colleagues showed the same spintronic OLED in the new study works as a "dirt cheap" magnetic field sensor at room temperature without being compromised by degradation. Such sensors may enable more accurate spacecraft navigation systems, he says.

Because nuclear spin-controlled electrical current regulates output of light by the OLED, it provides a way to study how to make OLEDs more efficient. OLEDs convert far more electricity into light than incandescent light bulbs, which turn most incoming electricity into heat. But there is much more room for improved efficiency.

"Hopefully, OLEDs will become better -- use less electricity and produce more light -- because we learned here how nuclear spins' orientation influences how well the OLED works," Boehme says. "Any sort of efficiency limitation can only be overcome if the mechanism that imposes this limitation is understood."

Source: University of Utah

A new, tunable device for spintronics

Tunable spin Hall angle device based on GaAs through field induced intervalley repopulation.
Credit: Copyright Jairo Sinova

An international team of scientists including physicist Jairo Sinova from the University of Mainz has developed a tunable spin-charge converter made of GaAs.

Spin-charge converters are important devices in spintronics, an electronic which is not only based on the charge of electrons but also on their spin and the spin-related magnetism. Spin-charge converters enable the transformation of electric into magnetic signals and vice versa. Recently, the research group of Professor Jairo Sinova from the Institute of Physics at Johannes Gutenberg University Mainz in collaboration with researchers from the UK, Prague, and Japan, has for the first time realised a new, efficient spin-charge converter based on the common semiconductor material GaAs. Comparable efficiencies had so far only been observed in platinum, a heavy metal. In addition, the physicists demonstrated that the creation or detection efficiency of spin currents is electrically tunable in a certain regime. This is important when it comes to real devices. The underlying mechanism, that was revealed by theoretical works of the Sinova group, opens up a new approach in searching and engineering spintronic materials. These results have recently been published in the journal Nature Materials.

Spintronics does not only make use of the electron's charge to transmit and store information but it takes also advantage of the electron's spin. The spin can be regarded as a rotation of the electron around its own axis, and generates a magnetic field like a small magnet. In some materials, electron spins spontaneously align their direction, leading to the phenomenon of ferromagnetism which is well known e.g. in iron. Additionally, "spin-up" or "spin-down" directions can be used to represent two easily distinguishable states -- 0 and 1 -- used in information technology. This is already used for memory applications such as computer hard discs.

Making use of electron spin for information transmission and storage, enables the development of electronic devices with new functionalities and higher efficiency. To make real use of the electron spin, it has to be manipulated precisely: it has to be aligned, transmitted and detected. The work of Sinova and his colleagues shows, that it is possible to do so using electric fields rather than magnetic ones. Thus, the very efficient, simple and precise mechanisms of charge manipulation well established in semiconductor electronics can be transferred to the world of spintronic and thereby combine semiconductor physics with magnetism.

Spin-charge converters are essential tools for that. They can transform charge currents into spin currents, and vice versa. The main principle behind these converters is the so called spin-Hall effect. Jairo Sinova had already been involved in the prediction and discovery of this relativistic phenomenon in 2004.

The spin-Hall effect appears when an electric field drives electrons through a (semi-) conductor plate. Taking a look at the classical Hall effect that is known from undergraduate physics, the interaction of moving electrons and an external magnetic field forces the electrons to move to one side of the plate, perpendicular to their original direction. This leads to the so called Hall voltage between both sides of the plate. For the spin-Hall effect electron-spins are generated by irradiating the sample with circularly polarised light. The electron spins are then parallel or antiparallel, and their direction is perpendicular to the plate and the direction of movement. The moving electron spins are now forced to one or the other side of the plate, depending on the spin orientation. The driving force behind this is the so called spin-orbit coupling, a relativistic electromagnetic effect which influences moving electron spins. This leads to the separation of both spin orientations.

To make practical use of this effect, it is essential to get a highly efficient spin separation. Up to now, platinum has been the most efficient spin-charge converter material, as it is a heavy metal, and the spin-orbit coupling of heavy metals is known to be especially strong due to the large amount of protons (positive charge) in their core.

Now, Sinova and his colleagues have shown that gallium-arsenide (GaAs), a very common and widely used semiconductor material, can be an as efficient spin-charge converter as platinum, even at room temperature, which is important for practical applications. Moreover, the physicists have demonstrated for the first time that the efficiency can be tuned continuously by varying the electric field that drives the electrons.

The reason for this -- as theoretical calculations of the Sinova group have shown -- lies in the existence of certain valleys in the conduction band of the semiconductor material. One can think of the conduction band and its valleys as of a motor highway with different lanes, each one requiring a certain minimum velocity. Applying a higher electric field enables a transition from one lane to the other.

Since the spin-orbit coupling is different in each lane, a transition also affects the strength of the spin-hall effect. By varying the electric field, the scientists can distribute the electron spins on the different lanes, thus varying the efficiency of their spin-charge converter.

By taking into account the valleys in the conduction band, Sinova and his colleagues open up new ways to find and engineer highly efficient materials for spintronics. Especially, since current semiconductor growth technologies are capable of engineering the energy levels of the valleys and the strength of spin-orbit coupling, e.g. by substituting Ga or As with other materials like Aluminum.

Source: Universität Mainz

Study of brokers' potential conflict of interest in routing limit orders leaked to Wall Street

A new academic paper about potential conflict of interest in large retail brokers' routing of limit orders has stirred controversy on Wall Street and caught regulators' attention -- even before the paper has been submitted to a journal.

While some in the industry have compared the study's possible impact to an earlier one that reformed Nasdaq trading, the authors caution that the paper is not yet final and the findings should be taken in proper context.

The authors, professors at Indiana University's Kelley School of Business and the University of Notre Dame Mendoza College of Business, found that some large retail brokers regularly route clients' limit orders to the exchange that pays them the highest rebates. Under certain circumstances, this can lead to some clients' trades not being executed at the best possible times -- or not being executed at all.

"Certain brokers, led by Ameritrade and including E*Trade, Scott Trade and Fidelity, were bifurcating the order flow -- sending market and limit orders to different exchanges -- but seemed to send all their limit orders to one place," said Robert Jennings, the Gregg T. and Judith A. Summerville Professor of Finance at Kelley.

"Brokers were paid for almost every order received; the conflict of interest occurs because some exchanges will pay brokers more to route limit order flows there, even if the chance that the limit order gets executed is lower on that exchange than somewhere else."

U.S. equity exchanges typically charge traders taking liquidity -- such as market orders -- and pay traders making liquidity -- such as limit orders. The payments, or rebates, are funded by take fees, so exchanges with the highest liquidity rebates also have the highest take fees. Brokers can generate revenue from customers' order flow.

According to the paper, "Can Brokers Have It All? On the Relation Between Make Take Fees & Limit Order Execution Quality," study results also indicate that, under some market conditions and for certain stocks:

-- Fill rates for displayed limit orders are lower on exchanges with higher fees. -- Limit orders executed on venues with high fees take longer to execute than those with low fees. -- On average, limit orders executed on venues with low/negative take fees are more likely to fill at the most opportune time for the limit order customer.

"Our results suggest that order routing decisions have an important impact of at least some measures of limit order execution quality and routing decisions based primarily on rebates/fees appear to be inconsistent with best execution," Jennings said. "Even if fees/rebates are passed directly through to the investor, the decision to use a single venue that offers the highest liquidity rebates does not appear to be consistent with the objective of obtaining best execution."

Paper leaked to financial community; FINRA asks brokers for data

The authors presented the paper to relevant industry representatives, including several brokerages, the Securities and Exchange Commission and the National Association of Securities Dealers. This common practice is generally accepted by all parties to be a confidential forum to test and refine study hypotheses and findings.

However in this case, the paper was leaked to the broader financial community without the authors' knowledge or permission. This led some to suggest the paper's impact could equal that of a 1994 study by Bill Christie of Vanderbilt University and Paul Schultz of Notre Dame showing implicit collusion among Nasdaq market makers; it led to sweeping reform of Nasdaq market (and a billion-dollar legal settlement).

The paper's leak -- and the Financial Industry Regulatory Authority's subsequent request for routing data from the 50 largest brokers -- has the authors concerned that the findings about brokers' maximizing liquidity rebates might be oversimplified.

Routing limit orders to maximize make rebates reduces fill rates, produces less profitable limit order executions -- and might be inconsistent with a broker's fiduciary responsibility to obtain best execution, the authors concluded.

"This is a classic case of adverse selection. If there's really bad news about the stock, everybody gets filled. If there's good news about the stock, then only the places where the order gets filled first get filled," Jennings said. "We are not alleging that the use of such rebates is illegal or that it violates securities laws, but there is a need for further transparency for consumers."

The authors expect to publish the revised and final version of the paper in the near future, after incorporating feedback they received.

"Given the competitive nature of the retail brokerage business, if brokers can get exchanges to pay for their orders, they could charge lower commissions," Jennings said. "Thus, customers may be slightly better off; if the payment was eliminated, commissions might have to be higher."

Commissions may be based on the total revenue that brokers receive, "but lower commissions do not compensate those investors who miss out on profitable limit order executions," Jennings and his colleagues concluded. "Brokers cannot have it all."

Jennings co-authored "Can Brokers Have It All?" with Robert Battalio, a professor of finance; and Shane Corwin, an associate professor of finance, both at Notre Dame. Battalio earned his doctorate at Kelley.

Source: Indiana University

Instant-start computers possible with new breakthrough

Power button (stock image)

To encode data, today's computer memory technology uses electric currents -- a major limiting factor for reliability and shrinkability, and the source of significant power consumption. If data could instead be encoded without current -- for example, by an electric field applied across an insulator -- it would require much less energy, and make things like low-power, instant-on computing a ubiquitous reality.

A team at Cornell University led by postdoctoral associate John Heron, who works jointly with Darrell Schlom, professor of Industrial Chemistry in the Department of Materials Science and Engineering, and Dan Ralph, professor of Physics in the College of Arts and Sciences, has made a breakthrough in that direction with a room-temperature magnetoelectric memory device. Equivalent to one computer bit, it exhibits the holy grail of next-generation nonvolatile memory: magnetic switchability, in two steps, with nothing but an electric field. Their results were published online Dec. 17 in Nature, along with an associated "News and Views" article.

"The advantage here is low energy consumption," Heron said. "It requires a low voltage, without current, to switch it. Devices that use currents consume more energy and dissipate a significant amount of that energy in the form of heat. That is what's heating up your computer and draining your batteries."

The researchers made their device out of a compound called bismuth ferrite, a favorite among materials mavens for a spectacularly rare trait: It's both magnetic -- like a fridge magnet, it has its own, permanent local magnetic field -- and also ferroelectric, meaning it's always electrically polarized, and that polarization can be switched by applying an electric field. Such so-called ferroic materials are typically one or the other, rarely both, as the mechanisms that drive the two phenomena usually fight each other.

This combination makes it a "multiferroic" material, a class of compounds that has enjoyed a buzz over the last decade or so. Paper co-author Ramamoorthy Ramesh, Heron's Ph.D. adviser at University of California, Berkeley, first showed in 2003 that bismuth ferrite can be grown as extremely thin films and can exhibit enhanced properties compared to bulk counterparts, igniting its relevance for next-generation electronics.

Because it's multiferroic, bismuth ferrite can be used for nonvolatile memory devices with relatively simple geometries. The best part is it works at room temperature; other scientists, including Schlom's group, have demonstrated similar results with competing materials, but at unimaginably cold temperatures, like 4 Kelvin (-452 Fahrenheit) -- not exactly primed for industry. "The physics has been exciting, but the practicality has been absent," Schlom said.

A key breakthrough by this team was theorizing, and experimentally realizing, the kinetics of the switching in the bismuth ferrite device. They found that the switching happens in two distinct steps. One-step switching wouldn't have worked, and for that reason theorists had previously thought what they have achieved was impossible, Schlom said. But since the switching occurs in two steps, bismuth ferrite is technologically relevant.

The multiferroic device also seems to require an order of magnitude lower energy than its chief competitor, a phenomenon called spin transfer torque, which Ralph also studies, and that harnesses different physics for magnetic switching. Spin transfer torque is already used commercially but in only limited applications. They have some work to do; for one thing they made just a single device, and computer memory involves billions of arrays of such devices. They need to ramp up its durability, too. But for now, proving the concept is a major leap in the right direction.

"Ever since multiferroics came back to life around 2000, achieving electrical control of magnetism at room temperature has been the goal," Schlom said.

Source: Cornell University

Quantum physics just got less complicated: Wave-particle duality and quantum uncertainty are same thing

Quantum physics says that particles can behave like waves, and vice versa. Researchers have now shown that this 'wave-particle duality' is simply the quantum uncertainty principle in disguise. Credit: Timothy Yeo / CQT, National University of Singapore; CC-BY

Here's a nice surprise: quantum physics is less complicated than we thought. An international team of researchers has proved that two peculiar features of the quantum world previously considered distinct are different manifestations of the same thing. The result is published 19 December in Nature Communications.

Patrick Coles, Jedrzej Kaniewski, and Stephanie Wehner made the breakthrough while at the Centre for Quantum Technologies at the National University of Singapore. They found that 'wave-particle duality' is simply the quantum 'uncertainty principle' in disguise, reducing two mysteries to one.

"The connection between uncertainty and wave-particle duality comes out very naturally when you consider them as questions about what information you can gain about a system. Our result highlights the power of thinking about physics from the perspective of information," says Wehner, who is now an Associate Professor at QuTech at the Delft University of Technology in the Netherlands.

The discovery deepens our understanding of quantum physics and could prompt ideas for new applications of wave-particle duality.

Wave-particle duality is the idea that a quantum object can behave like a wave, but that the wave behaviour disappears if you try to locate the object. It's most simply seen in a double slit experiment, where single particles, electrons, say, are fired one by one at a screen containing two narrow slits. The particles pile up behind the slits not in two heaps as classical objects would, but in a stripy pattern like you'd expect for waves interfering. At least this is what happens until you sneak a look at which slit a particle goes through -- do that and the interference pattern vanishes.

The quantum uncertainty principle is the idea that it's impossible to know certain pairs of things about a quantum particle at once. For example, the more precisely you know the position of an atom, the less precisely you can know the speed with which it's moving. It's a limit on the fundamental knowability of nature, not a statement on measurement skill. The new work shows that how much you can learn about the wave versus the particle behaviour of a system is constrained in exactly the same way.

Wave-particle duality and uncertainty have been fundamental concepts in quantum physics since the early 1900s. "We were guided by a gut feeling, and only a gut feeling, that there should be a connection," says Coles, who is now a Postdoctoral Fellow at the Institute for Quantum Computing in Waterloo, Canada.

It's possible to write equations that capture how much can be learned about pairs of properties that are affected by the uncertainty principle. Coles, Kaniewski and Wehner are experts in a form of such equations known as 'entropic uncertainty relations', and they discovered that all the maths previously used to describe wave-particle duality could be reformulated in terms of these relations.

"It was like we had discovered the 'Rosetta Stone' that connected two different languages," says Coles. "The literature on wave-particle duality was like hieroglyphics that we could now translate into our native tongue. We had several eureka moments when we finally understood what people had done," he says.

Because the entropic uncertainty relations used in their translation have also been used in proving the security of quantum cryptography -- schemes for secure communication using quantum particles -- the researchers suggest the work could help inspire new cryptography protocols.

In earlier papers, Wehner and collaborators found connections between the uncertainty principle and other physics, namely quantum 'non-locality' and the second law of thermodynamics. The tantalising next goal for the researchers is to think about how these pieces fit together and what bigger picture that paints of how nature is constructed.

Source: Centre for Quantum Technologies at the National University of Singapore