'Quantum criticality': Ultracold experiments heat up quantum research

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This false color image shows the average density of cesium atoms taken during multiple experimental cycles for studying quantum criticality in the ultracold laboratory of Cheng Chin, associate professor in physics at UChicago. The density is lowest in the white area on the outside, highest toward the center, where higher numbers of atoms are blocking the incoming infrared laser light. Xibo Zhang collected these data in connection with his recently completed doctoral research at UChicago. (Xibo Zhang and Cheng Chin)

(PhysOrg.com) -- University of Chicago physicists have experimentally demonstrated for the first time that atoms chilled to temperatures near absolute zero may behave like seemingly unrelated natural systems of vastly different scales, offering potential insights into links between the atomic realm and deep questions of cosmology.

This ultracold state, called “quantum criticality,” hints at similarities between such diverse phenomena as the gravitational dynamics of black holes or the exotic conditions that prevailed at the birth of the universe, said Cheng Chin, associate professor in physics at UChicago. The results could even point to ways of simulating cosmological phenomena of the early universe by studying systems of atoms in states of quantum criticality.

“Quantum criticality is the entry point for us to make connections between our observations and other systems in nature,” said Chin, whose team is the first to observe quantum criticality in ultracold atoms in optical lattices, a regular array of cells formed by multiple laser beams that capture and localize individual atoms.

UChicago graduate student Xibo Zhang and two co-authors published their observations online Feb. 16 in Science Express and in the March 2 issue of Science.

Quantum criticality emerges only in the vicinity of a quantum phase transition. In the physics of everyday life, rather mundane phase transitions occur when, for example, water freezes into ice in response to a drop in temperature. The far more elusive and exotic quantum phase transitions occur only at ultracold temperatures under the influence of magnetism, pressure or other factors.

“This is a very important step in having a complete test of the theory of quantum criticality in a system that you can characterize and measure extremely well,” said Harvard University physics professor Subir Sachdev about the UChicago study.

Physicists have extensively investigated quantum criticality in crystals, superconductors and magnetic materials, especially as it pertains to the motions of electrons. “Those efforts are impeded by the fact that we can’t go in and really look at what every electron is doing and all the various properties at will,” Sachdev said.

Sachdev’s theoretical work has revealed a deep mathematical connection between how subatomic particles behave near a quantum critical point and the gravitational dynamics of black holes. A few years hence, offshoots of the Chicago experiments could provide a testing ground for such ideas, he said.

There are two types of critical points, which separate one phase from another. The Chicago paper deals with the simpler of the two types, an important milestone to tackling the more complex version, Sachdev said. “I imagine that’s going to happen in the next year or two and that’s what we’re all looking forward to now,” he said.

Other teams at UChicago and elsewhere have observed quantum criticality under completely different experimental conditions. In 2010, for example, a team led by Thomas Rosenbaum, the John T. Wilson Distinguished Service Professor in Physics at UChicago, observed quantum criticality in a sample of pure chromium when it was subjected to ultrahigh pressures.

Zhang, who will receive his doctorate this month, invested nearly two and a half years of work in the latest findings from Chin’s laboratory. Co-authoring the study with Zhang and Chin were Chen-Lung Hung, PhD’11, now a postdoctoral scientist at the California Institute of Technology, and UChicago postdoctoral scientist Shih-Kuang Tung.

In their tabletop experiments, the Chicago scientists use sets of crossed laser beams to trap and cool up to 20,000 cesium atoms in a horizontal plane contained within an eight-inch cylindrical vacuum chamber. The process transforms the atoms from a hot gas to a superfluid, an exotic form of matter that exists only at temperatures hundreds of degrees below zero.

“The whole experiment takes six to seven seconds and we can repeat the experiment again and again,” Zhang said.
The experimental apparatus includes a CCD camera sensitive enough to image the distribution of atoms in a state of quantum criticality. The CCD camera records the intensity of laser light as it enters that vacuum chamber containing thousands of specially configured ultracold atoms.

“What we record on the camera is essentially a shadow cast by the atoms,” Chin explained.

The UChicago scientists first looked for signs of quantum criticality in experiments performed at ultracold temperatures from 30 to 12 nano-Kelvin, but failed to see convincing evidence. Last year they were able to push the temperatures down to 5.8 nano-Kelvin, just billionths of a degree above absolute zero (minus 459 degrees Fahrenehit). “It turns out that you need to go below 10 nano-Kelvin in order to see this phenomenon in our system,” Chin said.

Chin’s team has been especially interested in the possibility of using ultracold atoms to simulate the evolution of the early universe. This ambition stems from the quantum simulation concept that Nobel laureate Richard Feynman proposed in 1981. Feynman maintained that if scientists understand one quantum system well enough, they might be able to use it to simulate the operations of another system that can be difficult to study directly.

For some, like Harvard’s Sachdev, quantum criticality in ultracold atoms is worthy of study as a physical system in its own right. “I want to understand it for its own beautiful quantum properties rather than viewing it as a simulation of something else,” he said.

More information: “Observation of Quantum Criticality with Ultracold Atoms in Optical Lattices,” by Xibo Zhang, Chen-Lung Hung, Shih-Kuang Tung, and Chen Chin, Science, March 2, 2012, Vol. 335, No. 6072, pp. 1070-1072, and online Feb. in Science Express Feb. 16.

Provided by University of Chicago (news : web)

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Quantum strategy offers game-winning advantages, even without entanglement

By Lisa Zyga PhysOrg.com

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Experimental and theoretical results both show that quantum gain - measured as the difference between the winning chances for classical and quantum players - is highest under maximum entanglement. Quantum gain remains even when entanglement disappears, and approaches zero along with the discord. Image credit: Zu, et al. ©2012 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

(PhysOrg.com) -- Quantum correlations have well-known advantages in areas such as communication, computing, and cryptography, and recently physicists have discovered that they may help players competing in zero-sum games, as well. In a new study, researchers have found that a game player who uses an appropriate quantum strategy can greatly increase their chances of winning compared with using a classical strategy.

The researchers, Chong Zu from Tsingua University in Beijing, China, and coauthors, have published their study on how quantum mechanics can help game players in a recent issue of the New Journal of Physics.

In their study, the researchers focused on a two-player game called matching pennies. In the classical version of this game, each player puts down one penny as either heads or tails. If both pennies match, then Player 1 wins and takes both pennies. If one penny shows heads and the other shows tails, then Player 2 wins and takes both pennies. Since one player’s gain is always the other player’s loss, the game is a zero-sum game.

In the classical version of the game, neither player has any incentive to choose one side of the coin over the other, so players choose heads or tails with equal probability. The random nature of the players’ strategies results in a “mixed strategy Nash equilibrium,” a situation in which each player has only a 50% chance of winning, no matter what strategy they use.

But here, Zu and coauthors have found that a player who has the option of using a quantum strategy can increase his or her chances of winning from 50% to 94%. This quantum version of the game uses entangled photons as qubits instead of pennies. And instead of choosing between heads and tails, players use a polarizer and single-photon detector to implement their strategies. While the classical player can still choose only one of two states, the quantum player has more choices due to her ability to rotate a polarizer 360° before the single-photon detector. The researchers calculated that the quantum player can maximize his or her chances of winning by rotating the polarizer at a 45° angle.

“Each player can apply any operation to their qubit (or coin), and then measure it in computational basis,” Zu explained to PhysOrg.com. “For a classical player, the operation he can do is to flip the bit or just leave it unchanged. However, if a player has quantum power, he can apply arbitrary single-bit operations to his qubit. But the measurement part is the same for the quantum and classical players.”

The researchers found that the quantum advantage depends heavily on how correlated the original photons are, with a maximally entangled state providing the largest gain. The researchers were surprised to find that the quantum advantage doesn’t decrease to zero when entanglement disappears completely, since a different kind of quantum correlation – quantum discord – also provides an advantage. This finding may even be the most interesting part of the study.

“There is no wonder that quantum mechanics will lead to advantages in game theory, but the interesting part of our work is that we find out the quantum gain does not decrease to zero when entanglement disappears,” Zu said. “Instead, it links with another kind of quantum correlation described by discord for the qubit case, and the connection is demonstrated both theoretically and experimentally.”

He added that this finding could potentially be useful for making real-world strategies.

“Our work may help people to understand how quantum mechanics works in game theory (in some cases, entanglement is not necessary for a quantum player to achieve a positive gain),” he said. “It may also give a good example of people making strategies in a future quantum network.”

More information: C. Zu, et al. “Experimental demonstration of quantum gain in a zero-sum game.” New Journal of Physics, 14 (2012) 033002. DOI: 10.1088/1367-2630/14/3/033002

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Can The Human Brain See Quantum Images?

Nobody knows whether humans can access exotic images based on quantum entanglement. Now one physicist has designed an experiment to find out

The strange rules of the quantum world lead to many weird phenomena. One of these is the puzzling process of quantum imaging, which allows images to form in hitherto unimagined ways.

Researchers begin by creating entangled pairs by sending a single laser  beam into a non-linear crystal, which converts single photons into entangled pairs of lower frequency photons, a process known as parametric down conversion. A continuous beam generates a series of pairs of entangled photons.

Next, they send the entangled photons towards a pair of detectors. Each member of an entangled pair by itself fluctuates in random ways that make its time and position of arrival uncertain.

Use one of the detectors to receive just one half of the entangled photons and the result is a blur, smeared by the process of randomness.

But use two detectors to receive both sets of photons and the uncertainties disappear, or at least are dramatically reduced. In this case, the 'image' is pinsharp. The uncertainty disappears because of the quantum correlation between the entangled pairs.

Researchers have extended this technique by superimposing a pattern on the wavefront of the initial laser beam, creating shapes such as a donut. They've shown that a single detector alone cannot 'see' a such a donut image even though it appears clean and sharp when two detectors pick up both sets of the entangled pairs.

These strange pictures are called quantum images or higher order images and quantum physicists think they can use them to carry out exotic processes such as sending information secretly and performing quantum lithography.

Today, Geraldo Barbosa at Northwestern University in Evanston, Illinois, raises another interesting possibility. He asks whether it is possible for humans to see higher order images and suggests that a relatively simple experiment could settle the question.

This experiment consists of a laser beam shaped into an image, such as the letter A. This laser then hits a non-linear crystal, generating entangled pairs of photons that retain this image shape. The set up is such that these photons are then detected, not by conventional detectors, but by human eyeballs.

The question is whether the human retina/brain combination can access the correlation that exists between the entangled pairs. If so, the human would see the letter A. If not, he or she would see only a blur.

Of course, there are some significant experimental challenges. One is to design the experiment in a way  that ensures the subject can only receive the image through this quantum process and not through some other channel, such as talking to the experimenter. However, that should be straightforward for any psychologist to design.

Another problem, however, is that the retina can only detect photons in groups of 7 or more and these have to arrive within a specific time window. Only then can a human subject 'see' the result. Generating the required intensity of entangled photons is one challenge.

The key question is whether the entanglement survives this group process. If the brain can access the quantum correlations, the image will be visible. If not, the result will be a blur.

That's a fascinating experiment not least because a positive result would be astounding. It would show that we humans can essentially 'see' entanglement.

Barbosa points out that new forms of imaging are not unknown in the animal world. Various animals and insects see in the infrared and ultraviolet, giving them an entirely different perspective on the world.

There is also some evidence that birds can 'see' the earth's magnetic field thanks to the quantum interaction between the field and light sensitive molecules in their retinas.

So the possibility that new ways of seeing the world can emerge is not unprecedented. However, the idea that humans can access higher order images thanks to quantum entanglement is clearly an idea of a different ilk.

Perhaps the most exciting aspect of Barbosa's idea is that it appears feasible now. There's no reason why this experiment couldn't be done in any quantum optics lab in the near future.

We'll look forward to seeing the results.

Ref: arxiv.org/abs/1202.5434: Can humans see beyond intensity images?

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