Quantum states are the key mathematical objects in quantum theory. It is therefore surprising that physicists have been unable to agree on what a quantum state truly represents. One possibility is that a pure quantum state corresponds directly to reality. However, there is a long history of suggestions that a quantum state (even a pure state) represents only knowledge or information about some aspect of reality. Here we show that any model in which a quantum state represents mere information about an underlying physical state of the system, and in which systems that are prepared independently have independent physical states, must make predictions that contradict those of quantum theory.
Reported by Science Daily, 21 March 2012.
The alphabet of data processing could include more elements than the “0″ and “1″ in future. An international research team has achieved a new kind of bit with single electrons, called quantum bits, or qubits. With them, considerably more than two states can be defined. So far, quantum bits have only existed in relatively large vacuum chambers. The team has now generated them in semiconductors. They have put an effect in practice, which the RUB physicist Prof. Dr. Andreas Wieck had already theoretically predicted 22 years ago. This represents another step along the path to quantum computing.
Together with colleagues from Grenoble and Tokyo, Wieck from the Chair of Applied Solid State Physics reports on the results in the journal Nature Nanotechnology.
Physicists have found the strongest evidence yet of quantum effects fueling photosynthesis.
Multiple experiments in recent years have suggested as much, but it’s been hard to be sure. Quantum effects were clearly present in the light-harvesting antenna proteins of plant cells, but their precise role in processing incoming photons remained unclear.
In an experiment published Dec. 6 in Proceedings of the National Academy of Sciences, a connection between coherence — far-flung molecules interacting as one, separated by space but not time — and energy flow is established.
“There was a smoking gun before,” said study co-author Greg Engel of the University of Chicago. “Here we can watch the relationship between coherence and energy transfer. This is the first paper showing that coherence affects the probability of transport. It really does change the chemical dynamics.” Continue reading »
Northwestern University researchers have developed a new switching device that takes quantum communication to a new level. The device is a practical step toward creating a network that takes advantage of the mysterious and powerful world of quantum mechanics.
The researchers can route quantum bits, or entangled particles of light, at very high speeds along a shared network of fiber-optic cable without losing the entanglement information embedded in the quantum bits. The switch could be used toward achieving two goals of the information technology world: a quantum Internet, where encrypted information would be completely secure, and networking superfast quantum computers.
The device would enable a common transport mechanism, such as the ubiquitous fiber-optic infrastructure, to be shared among many users of quantum information. Such a system could route a quantum bit, such as a photon, to its final destination just like an e-mail is routed across the Internet today.
The research — a demonstration of the first all-optical switch suitable for single-photon quantum communications — is published by the journal Physical Review Letters.
“My goal is to make quantum communication devices very practical,” said Prem Kumar, AT&T Professor of Information Technology in the McCormick School of Engineering and Applied Science and senior author of the paper. “We work in fiber optics so that as quantum communication matures it can easily be integrated into the existing telecommunication infrastructure.”
The bits we all know through standard, or classical, communications only exist in one of two states, either “1” or “0.” All classical information is encoded using these ones and zeros. What makes a quantum bit, or qubit, so attractive is it can be both one and zero simultaneously as well as being one or zero. Additionally, two or more qubits at different locations can be entangled — a mysterious connection that is not possible with ordinary bits.
Researchers need to build an infrastructure that can transport this “superposition and entanglement” (being one and zero simultaneously) for quantum communications and computing to succeed.
The qubit Kumar works with is the photon, a particle of light. A photonic quantum network will require switches that don’t disturb the physical characteristics (superposition and entanglement properties) of the photons being transmitted, Kumar says. He and his team built an all-optical, fiber-based switch that does just that while operating at very high speeds.
To demonstrate their switch, the researchers first produced pairs of entangled photons using another device developed by Kumar, called an Entangled Photon Source. “Entangled” means that some physical characteristic (such as polarization as used in 3-D TV) of each pair of photons emitted by this device are inextricably linked. If one photon assumes one state, its mate assumes a corresponding state; this holds even if the two photons are hundreds of kilometers apart.
The researchers used pairs of polarization-entangled photons emitted into standard telecom-grade fiber. One photon of the pair was transmitted through the all-optical switch. Using single-photon detectors, the researchers found that the quantum state of the pair of photons was not disturbed; the encoded entanglement information was intact.
“Quantum communication can achieve things that are not possible with classical communication,” said Kumar, director of Northwestern’s Center for Photonic Communication and Computing. “This switch opens new doors for many applications, including distributed quantum processing where nodes of small-scale quantum processors are connected via quantum communication links.”
The National Science Foundation through their Integrative Graduate Education and Research Traineeship (IGERT) program supported the research.
Journal Reference: Matthew Hall, Joseph Altepeter, Prem Kumar. Ultrafast Switching of Photonic Entanglement. Physical Review Letters, 2011; 106 (5) DOI: 10.1103/PhysRevLett.106.053901
Reported by Mark Anderson, in IEEE Spectrum, 14 February 2011.
Researchers upend understanding of olfactory organs with quantum tunneling experiment
Flash memory, scanning tunneling microscopes…and a fly’s sense of smell. According to new research, the same strange phenomenon—quantum tunneling—makes all three possible. If confirmed, the discovery could pave the way for a new generation of artificial scents, from perfumes to pheromones—and, perhaps someday, artificial noses.
The conventional theory of smell holds that the nose’s chemical receptors—some 400 different kinds in a human nose—sense the presence of odorant molecules by a lock-and-key process that reads the odorant’s physical shape. That theory has some problems, though. For instance, ethanol (which smells like vodka) and ethanethiol (which smells like rotten eggs) have essentially the same shape, differing from each other by only a single atom. (Ethanol is C2H6O, and ethanethiol is C2H6S.)
Evidence has emerged over the past decade suggesting that at least part of a molecule’s scent comes from chemical receptors in the nose that pump current through the odorant molecule and cause it to vibrate in an identifiable way. Lacking a direct electrical hookup to the odorant, the nose’s receptors would likely transmit electrons via quantum tunneling, a well-studied process that allows electrons to hop through nonconducting regions if they are small enough. Tunneling is what allows charge to be stored in flash memory cells. It also forms the image in scanning tunneling electron microscopes and is a source of wasted power in microchips.
A group of four scientists from MIT and the Alexander Fleming Biomedical Sciences Research Center, in Vari, Greece, says it has proved the tunneling theory in fruit flies (Drosophila), a favorite lab specimen of geneticists. If the quantum ”molecular vibrational” theory of smell is correct, then the flies should be able to smell the difference between molecules that have regular hydrogen in them versus the same molecules that contain its heavier isotope, deuterium. (The nucleus of regular hydrogen is a proton; the nucleus of deuterium is a proton plus a neutron.)
While regular and ”deuterated” molecules have exactly the same shape, when set in motion by the receptors’ tiny tunneling current, the two would vibrate in a very different pattern of molecular wobbles. A tunneling-based sense of smell should be able to detect the different vibrations. Deuterated molecules would, in other words, smell different to the flies.
According to the new research—published in this week’s Proceedings of the National Academy of Sciences—the flies appear to be able to smell the difference. The researchers had set up a tiny maze with a T-junction. To the right, the molecule acetophenone (which has a sweet and flowery scent to human noses) filled the air. To the left, the air contained an isotope of acetophenone in which eight of the molecule’s hydrogens were swapped out for deuterium. Nearly 30 percent more flies went toward the regular acetophenone. Meanwhile, flies that had been genetically engineered to lack a sense of smell showed no preference.
The group repeated the experiment with a different molecule, octanol, and its deuterated cousin, and got the same result. In a third experiment, the researchers in Greece trained their flies to avoid a deuterated octanol. The vibrations between carbon and deuterium in octanol, says study coauthor Luca Turin, a visiting scientist in biomedical engineering at MIT, are very similar to those of carbon and nitrogen in chemicals called nitriles. So if the vibration theory holds, then some deuterated molecules might share similar odors to the nitriles, even though their shapes are worlds apart.
Indeed, when the trained flies were presented with a whiff of a nitrile, they avoided it. Conversely, flies conditioned to avoid nitriles also avoided the deuterated octanol. ”The only thing the deuterium and nitrile have in common is vibration [pattern],” Turin says.
Andrew Horsfield, senior lecturer in the department of materials at Imperial College London, has been working on early applications of the vibrational theory that Turin’s group tested. Horsfield and his colleagues have developed an indium-arsenide nanowire detector that might crudely ”smell” itself by tunneling electrons between special structures within the nanowire and reading off the vibrations produced. Horsfield’s group plans to extend this idea to smaller devices that can smell external molecules. The group has so far only been fine-tuning its nanowire setup, and Horsfield says its first ”sniff” test might be more than a year away.
Horsfield says that the research being done by Turin’s group justifies his group’s continued nanowire studies. ”It’s very clear to me that this is a very important paper,” he says. ”Time will prove that to be true.”
In the weird world of quantum physics, two linked particles can share a single fate, even when they’re miles apart.
Now, two physicists have mathematically described how this spooky effect, called entanglement, could also bind particles across time.
If their proposal can be tested, it could help process information in quantum computers and test physicists’ basic understanding of the universe.
“You can send your quantum state into the future without traversing the middle time,” said quantum physicist S. Jay Olson of Australia’s University of Queensland, lead author of the new study.
In ordinary entanglement, two particles (usually electrons or photons) are so intimately bound that they share one quantum state — spin, momentum and a host of other variables — between them. One particle always “knows” what the other is doing. Make a measurement on one member of an entangled pair, and the other changes immediately.
Physicists have figured out how to use entanglement to encrypt messages in uncrackable codes and build ultrafast computers. Entanglement can also help transmit encyclopedias’ worth of information from one place to another using only a few atoms, a protocol called quantum teleportation.
In a new paper posted on the physics preprint website arXiv.org, Olson and Queensland colleague Timothy Ralph perform the math to show how these same tricks can send quantum messages not only from place to place, but from the past to the future.
The equations involved defy simple mathematical explanation, but are intuitive: If it’s impossible to describe one particle without including the other, this logically extends to time as well as space.
“If you use our timelike entanglement, you find that [a quantum message] moves in time, while skipping over the intermediate points,” Olson said. “There really is no difference mathematically. Whatever you can do with ordinary entanglement, you should be able to do with timelike entanglement.”
Olson explained them with a Star Trek analogy. In one episode, “beam me up” teleportation expert Scotty is stranded on a distant planet with limited air supply. To survive, Scotty freezes himself in the transporter, awaiting rescue. When the Enterprise arrives decades later, Scotty steps out of the machine without having aged a day.
“It’s not time travel as you would ordinarily think of it, where it’s like, poof! You’re in the future,” Olson said. “But you get to skip the intervening time.”
According to quantum physicist Ivette Fuentes of the University of Nottingham, who saw Olson and Ralph present the work at a conference, it’s “one of the most interesting results” published in the last year.
“It stimulated our imaginations,” said Fuentes. “We know entanglement is a resource and we can do very interesting things with it, like quantum teleportation and quantum cryptography. We might be able to exploit this new entanglement to do interesting things.”
One such interesting thing could involve storing information in black holes, said physicist Jorma Louko, also of the University of Nottingham.
“They show that you can use the vacuum, that no-particle state, to store a lot of information in just a couple of atoms, and recover that info from other atoms later on,” Louko said. “The details of that have not been worked out, but I can foresee that the ideas that these authors use could be adapted to the black hole context.”
Entanglement in time could also be used to investigate as-yet-untested fundamentals of particle physics. In the 1970s, physicist Bill Unruh predicted that, if a spaceship accelerates through the empty space of a vacuum, particles should appear to pop out of the void. Particles carry energy, so they would be, in effect, a warm bath. Wave a thermometer outside, and it would record a positive temperature.
Called the Unruh effect, this is a solid prediction of quantum field theory. It’s never been observed, however, as a spaceship would have to accelerate at as-yet-unrealistic speeds to generate an effect large enough to be testable. But because timelike entanglement also involves particles emerging from vacuums, it could be used to conduct more convenient searches, relying on time rather than space.
Finding the Unruh effect would provide support for quantum field theory. But it might be even more exciting not to see the effect, Olson said.
“It would be more of a shocking result,” Olson said. “If you didn’t see it, something would be very wrong with our understanding.”
Reported by PhysOrg in December 17, 2010.
Understanding the transport of electrons in nanostructures and biological molecules is crucial to understanding properties such as electrical conductivity or the biochemical behavior of molecules. However, determining whether the electrons are behaving according to the classical laws of motion or the quantum mechanical regime at the nanoscale is challenging because many nanostructures fall in a grey area between both regimes. Japanese researchers from the RIKEN Advanced Science Institute in Wako, with colleagues from Germany and Taiwan, have now devised a set of mathematical equations that can distinguish classical from quantum mechanical behavior of electrons in nanostructures.
On a macroscopic scale, objects follow the classical laws of motion. Golf or billiard balls, for example, will follow exact, predictable paths. On a microscopic scale, objects such as electrons move according to the laws of quantum mechanics, where processes occur in a probabilistic manner. Measuring the properties of quantum mechanical systems, however, is challenging.
“In microscopic systems, it is very difficult to perform ideal measurements without disturbing the system,” explains Neill Lambert from the research team. As a consequence, measurements on quantum mechanical systems are difficult to distinguish from invasive measurements on classical systems, says Franco Nori from RIKEN and the University of Michigan, who led the research team. “It is important to be confident that experimental results are not originating from a classical effect, giving a false impression of quantum behavior.”
As a model system, the researchers chose the transport of electrons through vanishingly small pieces of matter known as quantum dots. “Even measuring the current passing through a quantum dot represents an invasive measurement of the system,” Lambert notes. To identify quantum effects, he and his colleagues developed a set of criteria expressed as a mathematical inequality relationship for experimental data from these quantum dots. Any excess over a critical threshold in the formula by a parameter represents a clear sign of quantum behavior. In their simulations the researchers found several regimes at low temperatures where quantum effects in the dynamics of electrons in the quantum dots should occur.
The inequality relation derived by the researchers is based on fundamental principles and therefore applies not only to the transport of electrons through quantum dots, but also to many open, microscopic electron transport systems, says Nori. He believes that it will soon be easier to determine whether electrons in nanostructures follow the rules of quantum mechanics or take the classical route of their billiard-ball counterparts.
More information: Lambert, N., et al. Distinguishing quantum and classical transport through nanostructures. Physical Review Letters 105, 176801 (2010) . See the article here: http://prl.aps.org … /i17/e176801 .
The more one probes the universe at smaller and smaller scales, the weirder matter and energy seem to behave.
But this strangeness may limit its own extent in quantum mechanics, the theory describing the behavior of matter at an infinitesimal level, according to a new study by an ex-hacker and a physicist.
“We’re interested in this question of why quantum theory is as weird as it is, but not weirder,” said physicist Jonathan Oppenheim of the University of Cambridge. “It was an unnatural question for people to have asked even 20 years ago. The reason we’re able to get these results is that we’re thinking of things in the way a hacker might think of things.”
A lot of eerie things happen in the quantum world. According to the Heisenberg uncertainty principle, for instance, it’s impossible to know everything about a quantum particle. The more precisely you know an electron’s position, the less precisely you know its momentum. Stranger still, the electron doesn’t even have properties like position and momentum until an observer measures them. It’s as if the particle exists in a plurality of worlds, and only by making a measurement can we force it to choose one.
In another weirdness, two particles can be bound together such that observing one causes changes in the other, even when they’re physically far apart. This quantum embrace, called entanglement (or more generally, nonlocality), made Einstein nervous. He famously called the phenomenon “spooky action at a distance.”
But there’s a limit to how useful nonlocality can be. Two separated people can’t send messages faster than the speed of light.
“It’s surprising that that happens,” said Stephanie Wehner, an ex-hacker and quantum-information theorist at the National University of Singapore. “Quantum mechanics is so much more powerful than the classical world, it should surely go up to the limits. But no, it turns out that there is some other limitation.”
As strange as quantum mechanics is, it could be stranger.
“The question is, can quantum mechanics be spookier?” Oppenheim said. “Researchers started asking why quantum theory doesn’t have more nonlocality, and if there’s another theory that could.”
It turns out that the amount of nonlocality you can have — that is, how much you can rely on two entangled particles to coordinate their changes — is limited by the uncertainty principle. Oppenheim and Wehner describe how they came to this conclusion in the Nov. 19 issue of the journal Science.
To see the link between uncertainty and nonlocality, Wehner suggests thinking of a game played by two people, Alice and Bob, who are far apart and not allowed to talk to each other.
On her desk, Alice has two boxes and two coffee cups. A referee flips a coin and tells her to put either an even or odd number of cups into the boxes. She has four choices: one cup in the left box, one in the right box, one cup in each box, or no cups at all. This is equivalent to Alice encoding two bits of information, Wehner says. If a cup in a box represents a 1 and no cup represents a 0, Alice can write 00, 01, 10 or 11.
Then the referee asks Bob to guess if there is a cup in either the left or the right box. If he guesses correctly, Alice and Bob both win. This is the same as Bob trying to retrieve one of the bits that Alice encoded.
In the normal, non-quantum world, the best strategy for this (admittedly really boring) game lets the duo win just 75 percent of the time. If they each have one of a pair of entangled particles, they can do better. Alice can influence the state of Bob’s particle by observing her own. Bob can then look at his particle and have some idea of what Alice’s looks like, and use that information to make a more educated guess about which box has a cup.
But this strategy only improves the pair’s odds of winning to 85 percent. Bob can’t always guess perfectly because the uncertainty principle says he can’t know both bits of information at the same time, Oppenheim and Wehner explained. The stronger the uncertainty principle is, the harder it will be for Bob to retrieve the bit.
“The reason we can’t win this game better than 85 percent is because quantum mechanics respects the uncertainty principle,” Oppenheim said.
Given the history of these two concepts, linking uncertainty to nonlocality is a little ironic, he noted. In 1935, Albert Einstein tried to tear down the uncertainty principle using entanglement, and wrote in a famous paper with Boris Podolsky and Nathan Rosen that “no reasonable definition of reality could be expected to permit this.”
“When people first discovered nonlocality, they hated it,” Oppenheim said. “It was just too weird. People tried to eradicate it and undermine it.”
As the century wore on, however, physicists realized that creating a near-psychic link between two particles could be useful in cryptography and enable ultra-fast quantum computers.
“Now we’ve gotten used to it, and we even like it,” Oppenheim said. “Then you start wishing there could be more of it.”
Although there aren’t any immediate practical applications of this link, the finding does reveal some mysteries about the fundamental nature of physics. The discovery could also inform future theories that go beyond quantum mechanics, such as a unified theory of everything.
“We know that our present theories are not consistent, and that there’s some underlying theory,” Oppenheim said. Physicists don’t know what the uncertainty principle or nonlocality will look like in this new theory, “but we at least know that these two things will be locked together.”