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

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

Atoms queue up for quantum computer networks

The experiment is carried out in a glass cell with very low pressure. In here is an ultra-thin glass fiber and a gas of cesium atoms. Using lasers and a magnetic field, the atoms are cooled down to almost absolute zero (minus 273 degrees Celsius) and the atoms gather as a cloud around the glass fiber. Then two laser beams with very different frequencies are transmitted into the fiber, thereby capturing atoms above the fiber surface. By measuring the difference in the speed of light for two other light beams on each side of the atoms' absorption line, you can measure the number of atoms.
Credit: Ola Jakup Joensen, Niels Bohr Institute

In order to develop future quantum computer networks, it is necessary to hold a known number of atoms and read them without them disappearing. To do this, researchers from the Niels Bohr Institute have developed a method with a trap that captures the atoms along an ultra thin glass fiber, where the atoms can be controlled. The results are published in the scientific journal, Physical Review Letters.

The research is carried out in the quantum optics laboratory in the basement of the Niels Bohr Institute in Copenhagen. The underground laboratory is set back from the road so there are no vibrations from traffic. Here, the researchers have designed experiments in which they can perform ultrasensitive trials with quantum optics.

"We have an ultra-thin glass fiber with a diameter of half a micrometer (a hundred times smaller than a strand of hair). Along this glass fiber we capture cesium atoms. They are cooled down to 100 micro Kelvin using a laser -- this is almost absolute zero, which is equivalent to minus 273 degrees Celsius. This system acts like a trap that holds the atoms on the side of the glass fiber," explains Jürgen Appel, Associate Professor in the research group Quantop at the Niels Bohr Institute, University of Copenhagen.

Atoms and light linked together

When light is transmitted through the glass fiber thread, the light will also move along the surface because the fiber is thinner than wavelength of the light. This creates strong interaction between the light and the atoms sitting securely above the surface of the fiber.

"We have developed a method where we can measure the number of atoms. We send two laser beams with different frequencies through the glass fiber. If there were no atoms on the fiber, the speed of light would be the same for both light beams. However, the atoms affect the two frequencies differently and by measuring the difference in the speed of light for the two light beams on each side of the atoms' absorption lines, you can measure the number of atoms along the fiber. We have shown that we can hold 2,500 atoms with an uncertainty of just eight atoms," says Jürgen Appel.

These are fantastic results. Without this method, you would have to use resonant light (light that the atoms absorb) and then you would scatter photons, which would kick the atoms out of the trap, says Jürgen Appel and explains that with this new method they can measure and control the atoms so that only 14 percent are kicked out of the trap and are lost.

"Our resolution is only limited by the natural quantum noise (the laser light's own minimal fluctuations) so our method could be used for so-called entangled states of atoms along the fiber. Such an entangled system with strongly interacting atoms and light is of great interest for future quantum computer networks," notes Jürgen Appel.

Source: University of Copenhagen - Niels Bohr Institute

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