Quantum Computing? Quantum Bar Magnets in a Transparent Salt

ScienceDaily (June 15, 2012) — Scientists have managed to switch on and off the magnetism of a new material using quantum mechanics, making the material a test bed for future quantum devices.

This image shows the antiferromagnetic arrangement of the spins (colored arrows) in the magnetic salt used by the Swiss-German-US-London team. (Credit: University College London)

The international team of researchers led from the Laboratory for Quantum Magnetism (LQM) in Switzerland and the London Centre for Nanotechnology (LCN), found that the material, a transparent salt, did not suffer from the usual complications of other real magnets, and exploited the fact that its quantum spins -- which are like tiny atomic magnets -- interact according to the rules of large bar magnets. The study is published in Science.

Anybody who has played with toy bar magnets at school will remember that opposite poles attract, lining up parallel to each other when they are placed end to end, and anti-parallel when placed adjacent to each other. As conventional bar magnets are simply too large to reveal any quantum mechanical nature, and most materials are too complex for the spins to interact like true bar magnets, the transparent salt is the perfect material to see what's going on at the quantum level for a dense collection of tiny bar magnets.

The team were able to image all the spins in the special salt, finding that the spins are parallel within pairs of layers, while for adjacent layer pairs, they are antiparallel, as large bar magnets placed adjacent to each other would be. The spin arrangement is called "antiferromagnetic." In contrast, for ferromagnets such as iron, all spins are parallel.

By warming the material to only 0.4 degrees Celsius above the absolute "zero" of temperature where all classical (non-quantum) motion ceases, the team found that the spins lose their order and point in random directions, as iron does when it loses its ferromagnetism when heated to 870 Celsius, much higher than room temperature because of the strong and complex interactions between electron spins in this very common solid.

The team also found that they could achieve the same loss of order by turning on quantum mechanics with an electromagnet containing the salt. Thus, physicists now have a new toy, a collection of tiny bar magnets, which naturally assume an antiferromagnetic configuration and for which they can dial in quantum mechanics at will.

"Understanding and manipulating magnetic properties of more traditional materials such as iron have of course long been key to many familiar technologies, from electric motors to hard drives in digital computers," said Professor Gabriel Aeppli, UCL Director of the LCN.

"While this may seem esoteric, there are deep connections between what has been achieved here and new types of computers, which also rely on the ability to tune quantum mechanics to solve hard problems, like pattern recognition in images."

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IBM takes giant step to faster, quantum computers

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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A quantum connection between light and motion

 February 6, 2012
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© 2012 EPFL

(PhysOrg.com) -- Physicists have demonstrated a system in which light is used to control the motion of an object that is large enough to be seen with the naked eye at the level where quantum mechanics governs its behavior.

The movement of objects is ultimately governed by the laws of quantum mechanics, which predict some intriguing phenomena: An object could simultaneously be in two places at the same time, and it should always be moving a little, even at a temperature of absolute zero - the oscillator is then said to be in its quantum 'ground state'. Until recently, these strange predictions of quantum mechanics have only been observed in the motion of tiny objects such as individual atoms. For large objects, the unavoidable coupling of the object to the surrounding environment quickly washes out the quantum properties, in a process known as decoherence. But researchers in EPFL’s Laboratory of Photonics and Quantum Measurements have now shown that it is possible to use light to control the vibrational motion of a large object, consisting of a hundred trillion atoms, at the quantum level. The results of their research have been published in the February 2nd edition of Nature magazine.



A ring of light

The object they used was circular in design - a 30-micrometer diameter glass donut mounted on a microchip. Under the direction of Tobias Kippenberg, the team injected a laser into a thin optical fiber, and brought the fiber close to the donut, allowing light to 'jump' to the object and circulate around the circumference of the donut up to a million times. Just as the pressure of a finger running along the rim of a wineglass will cause it to hum, the tiny force exerted by the photons traveling inside the glass ring can cause it to vibrate at a well-defined frequency. But the force can in fact also dampen the vibrations, and thus cool down the oscillatory motion.

Cold, colder...

Cooling is crucial to reaching the regime of quantum mechanical motion, as this is normally overshadowed by random thermal fluctuations. For this reason, the structure is placed in a cryostat that brings it to a temperature of less than one degree above absolute zero (−273.15°C). The light launched into the donut slows down the motion one hundred times, thus cooling it even more, very close to the quantum 'ground state'. And more importantly, the interaction between light and the movement of the oscillator can be made so strong that the two form an intimate connection: A small excitation in the form of a light pulse was fully transformed into a small vibration and back again. For the first time, this transformation between light and motion was made to occur within a time that is short enough so that the quantum properties of the original light pulse are not lost in the process through decoherence. By outpacing decoherence, these results demonstrate the possibility of controlling the quantum properties of an object’s motion. It also provides a way to see the peculiar predictions of quantum mechanics at play in man-made objects.

Looking forward

Mechanical vibrations can be coupled to quantum systems of completely different nature (such as electric currents), as well as to light. They could therefore be used to ‘translate’ quantum information between those systems and light signals. This is especially beneficial as it allows to transport quantum information - the basic ingredient of a future quantum computer - over large distances in optical fibers.

More information: Quantum-coherent coupling of a mechanical oscillator to an optical cavity mode, E. Verhagen, S. Deléglise, S. Weis, A. Schliesser, Tobias J. Kippenberg, Nature, January 2012. DOI: 10.1038/nature10787

Provided by Ecole Polytechnique Federale de Lausanne

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