Chinese Physicists Smash Distance Record For Teleportation

The ability to teleport photons through 100 kilometres of free space opens the way for satellite-based quantum communications, say researchers
 

Teleportation is the extraordinary ability to transfer objects from one location to another without travelling through the intervening space.

The idea is not that the physical object is teleported but the information that describes it. This can then be applied to a similar object in a new location which effectively takes on the new identity.

And it is by no means science fiction. Physicists have been teleporting photons since 1997 and the technique is now standard in optics laboratories all over the world.

The phenomenon that makes this possible is known as quantum entanglement,  the deep and mysterious link that occurs when two quantum objects share the same existence and yet are separated in space.

Teleportation turns out to be extremely useful. Because teleported information does not travel through the intervening space, it cannot be secretly accessed by an eavesdropper.

For that reason, teleportation is the enabling technology behind quantum cryptography, a way of sending information with close-to-perfect secrecy.

Unfortunately, entangled photons are fragile objects. They cannot travel further than a kilometre or so down optical fibres because the photons end up interacting with the glass breaking the entanglement. That severely limits quantum cryptography's usefulness.

However, physicists have had more success teleporting photons through the atmosphere. In 2010, a Chinese team announced that it had teleported single photons over a distance of 16 kilometres. Handy but not exactly Earth-shattering.

Now the same team says it has smashed this record. Juan Yin at the University of Science and Technology of China in Shanghai, and a bunch of mates say they have teleported entangled photons over a distance of 97 kilometres across a lake in China.

That's an impressive feat for several reasons. The trick these guys have perfected is to find a way to use a 1.3 Watt laser and some fancy optics to beam the light and receive it.

Inevitably photons get lost and entanglement is destroyed in such a process. Imperfections in the optics and air turbulence account for some of these losses but the biggest problem is beam widening (they did the experiment at an altitude of about 4000 metres). Since the beam spreads out as it travels, many of the photons simply miss the target altogether.

So the most important advance these guys have made is to develop a steering mechanism using a guide laser that keeps the beam precisely on target. As a result, they were able to teleport more than 1100 photons in 4 hours over a distance of 97 kilometres.

That's interesting because it's the same channel attenuation that you'd have to cope with when beaming photons to a satellite with, say, 20 centimetre optics orbiting at about 500 kilometres. "The successful quantum teleportation over such channel losses in combination with our high-frequency and high-accuracy [aiming] technique show the feasibility of satellite-based ultra-long-distance quantum teleportation," say Juan and co.

So these guys clearly have their eye on the possibility of satellite-based quantum cryptography which would provide ultra secure communications around the world. That's in stark contrast to the few kilometres that are possible with commercial quantum cryptography gear.

Of course, data rates are likely to be slow and the rapidly emerging technology of quantum repeaters will extend the reach of ground-based quantum cryptography so that it could reach around the world, in principle at least.

But a perfect, satellite-based security system might be a useful piece of kit to have on the roof of an embassy or distributed among the armed forces.

Something for western security experts to think about.

Ref: arxiv.org/abs/1205.2024: Teleporting Independent Qubits Through A 97 Km Free-Space Channel

Newscribe : get free news in real time 

Related posts:
Quantum Rainbow Photon Gun Unveiled
Quantum strategy offers game-winning advantages ...
IBM Scalable Quantum Computing
IBM takes giant step to faster, quantum computers
Can The Human Brain See Quantum Images?
A quantum connection between light and motion
Quantum criticality': Ultracold experiments heat up ...
Quantum Computing Thrives on Chaos

Quantum strategy offers game-winning advantages, even without entanglement

By Lisa Zyga PhysOrg.com

Enlarge

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

Related posts:

IBM Scalable Quantum Computing
Can The Human Brain See Quantum Images?
A quantum connection between light and motion
Quantum Computing Thrives on Chaos
The Quantum Physics Behind The Death Of Osama Bin ... 

 Newscribe : get free news in real time 

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?

TRSF: Read the Best New Science Fiction inspired by today’s emerging technologies.

 Newscribe : get free news in real time