Quantum Teleportation Achieved over Record Distances

Quantum Teleportation Achieved over Record Distances-NOW TO STARS?

By John Matson | August 9, 2012 | 44

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The European Space Agency's Optical Ground Station on Tenerife in the Canary Islands was used as a receiver in recent quantum teleportation experiments. Credit: ESA

Two teams of researchers have extended the reach of quantum teleportation to unprecedented lengths, roughly equivalent to the distance between New York City and Philadelphia. But don’t expect teleportation stations to replace airports or train terminals—the teleportation scheme shifts only the quantum state of a single photon. And although part of the transfer happens instantaneously, the steps required to read out the teleported quantum state ensure that no information can be communicated faster than the speed of light.

Quantum teleportation relies on the phenomenon of entanglement, through which quantum particles share a fragile, invisible link across space. Two entangled photons, for instance, can have correlated, opposite polarization states—if one photon is vertically polarized, for instance, the other must be horizontally polarized. But, thanks to the intricacies of quantum mechanics, each photon’s specific polarization remains undecided until one of them is measured. At that instant the other photon’s polarization snaps into its opposing orientation, even if many kilometers have come between the entangled pair.

An entangled photon pair serves as the intermediary in the standard teleportation scheme. Say Alice wants to teleport the quantum state of a photon to Bob. First she takes one member of a pair of entangled photons, and Bob takes the other. Then Alice lets her entangled photon interfere with the photon to be teleported and performs a polarization measurement whose outcome depends on the quantum state of both of her particles.

Because of the link between Alice and Bob forged by entanglement, Bob’s photon instantly feels the effect of the measurement made by Alice. Bob’s photon assumes the quantum state of Alice’s original photon, but in a sort of garbled form. Bob cannot recover the quantum state Alice wanted to teleport until he reverses that garbling by tweaking his photon in a way that depends on the outcome of Alice’s measurement. So he must await word from Alice about how to complete the teleportation—and that word cannot travel faster than the speed of light. That restriction ensures that teleported information obeys the cosmic speed limit.

Even though teleportation does not allow superluminal communication, it does provide a detour around another physics blockade known as the no-cloning theorem. That theorem states that one cannot perfectly copy a quantum object to, for instance, send a facsimile to another person. But teleportation does not create a copy per se—it simply shifts the quantum information from one place to another, destroying the original in the process.

Teleportation can also securely transmit quantum information even when Alice does not know where Bob is. Bob can take his entangled particle wherever he pleases, and Alice can broadcast her instructions for how to ungarble the teleported state over whatever conventional channels—radio waves, the Internet—she pleases. That information would be useless to an eavesdropper without an entangled link to Alice.

Physicists note that quantum entanglement and teleportation could one day form the backbone of quantum channels linking hypothetical quantum processors or enabling secure communications between distant parties. But for now the phenomenon of teleportation is in the gee-whiz exploratory phase, with various groups of physicists devising new tests to push the limits of what is experimentally possible.

In the August 9 issue of Nature, a Chinese group reports achieving quantum teleportation across Qinghai Lake in China, a distance of 97 kilometers. (Scientific American is part of Nature Publishing Group.) That distance surpasses the previous record, set by a group that included several of the same researchers, of 16 kilometers.

But a more recent study seems to have pushed the bar even higher. In a paper posted May 17 to the physics preprint Web site arXiv.org, just eight days after the Chinese group announced their achievement on the same Web site, a European and Canadian group claims to have teleported information from one of the Canary Islands to another, 143 kilometers away. That paper has not been peer-reviewed but comes from a very reputable research group.

Both teams of physicists faced serious experimental challenges—sending a single photon 100 kilometers and then plucking it out of the air is no easy task. In practical terms, both groups’ Alices and Bobs needed laser-locked telescopes for sending and receiving their photons, as well as complex optics for modifying and measuring the photons’ quantum states.

But that’s nothing compared to what the physicists have in mind for future experiments. Both research groups note that their work is a step toward future space-based teleportation, in which quantum information would be beamed from the ground to an orbiting satellite.

About the Author: John Matson is an associate editor at Scientific American focusing on space, physics and mathematics. Follow on Twitter @jmtsn.

Coming Soon: Artificial Limbs Controlled by Thoughts

The idea that paralyzed people might one day control their limbs just by thinking is no longer a Hollywood-style fantasy

By Miguel A. L. Nicolelis

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Image: Kemp Remillard

In Brief

• Brain waves can now control the functioning of computer cursors, robotic arms and, soon, an entire suit: an exoskeleton that will allow a paraplegic to walk and maybe even move gracefully.
• Sending signals from the brain's outer rindlike cortex to initiate movement in the exoskeleton represents the state of the art for a number of bioelectrical technologies perfected in recent years.
• The 2014 World Cup in Brazil will serve as a proving ground for a brain-controlled exoskeleton if, as expected, a handicapped teenager delivers the ceremonial opening kick.

In 2014 billions of viewers worldwide may remember the opening game of the World Cup in Brazil for more than just the goals scored by the Brazilian national team and the red cards given to its adversary. On that day my laboratory at Duke University, which specializes in developing technologies that allow electrical signals from the brain to control robotic limbs, plans to mark a milestone in overcoming paralysis.

If we succeed in meeting still formidable challenges, the first ceremonial kick of the World Cup game may be made by a paralyzed teenager, who, flanked by the two contending soccer teams, will saunter onto the pitch clad in a robotic body suit. This suit—or exoskeleton, as we call it—will envelop the teenager's legs. His or her first steps onto the field will be controlled by motor signals originating in the kicker's brain and transmitted wirelessly to a computer unit the size of a laptop in a backpack carried by our patient. This computer will be responsible for translating electrical brain signals into digital motor commands so that the exoskeleton can first stabilize the kicker's body weight and then induce the robotic legs to begin the back-and-forth coordinated movements of a walk over the manicured grass. Then, on approaching the ball, the kicker will visualize placing a foot in contact with it. Three hundred milliseconds later brain signals will instruct the exoskeleton's robotic foot to hook under the leather sphere, Brazilian style, and boot it aloft.

This scientific demonstration of a radically new technology, undertaken with collaborators in Europe and Brazil, will convey to a global audience of billions that brain control of machines has moved from lab demos and futuristic speculation to a new era in which tools capable of bringing mobility to patients incapacitated by injury or disease may become a reality. We are on our way, perhaps by the next decade, to technology that links the brain with mechanical, electronic or virtual machines. This development will restore mobility, not only to accident and war victims but also to patients with ALS (also known as Lou Gehrig's disease), Parkinson's and other disorders that disrupt motor behaviors that impede arm reaching, hand grasping, locomotion and speech production. Neuroprosthetic devices—or brain-machine interfaces—will also allow scientists to do much more than help the disabled. They will make it possible to explore the world in revolutionary ways by providing healthy human beings with the ability to augment their sensory and motor skills.

In this futuristic scenario, voluntary electrical brain waves, the biological alphabet that underlies human thinking, will maneuver large and small robots remotely, control airships from afar, and perhaps even allow the sharing of thoughts and sensations of one individual with another over what will become a collective brain-based network.

Thought Machines

The lightweight body suit intended for the kicker, who has not yet been selected, is still under development. A prototype, though, is now under construction at the lab of my great friend and collaborator Gordon Cheng of the Technical University of Munich—one of the founding members of the Walk Again Project, a nonprofit, international collaboration among the Duke University Center for Neuroengineering, the Technical University of Munich, the Swiss Federal Institute of Technology in Lausanne, and the Edmond and Lily Safra International Institute of Neuroscience of Natal in Brazil. A few new members, including major research institutes and universities all over the world, will join this international team in the next few months.

The project builds on nearly two decades of pioneering work on brain-machine interfaces at Duke—research that itself grew out of studies dating back to the 1960s, when scientists first attempted to tap into animal brains to see if a neural signal could be fed into a computer and thereby prompt a command to initiate motion in a mechanical device. Back in 1990 and throughout the first decade of this century, my Duke colleagues and I pioneered a method through which the brains of both rats and monkeys could be implanted with hundreds of hair-thin and pliable sensors, known as microwires. Over the past two decades we have shown that, once implanted, the flexible electrical prongs can detect minute electrical signals, or action potentials, generated by hundreds of individual neurons distributed throughout the animals' frontal and parietal cortices—the regions that define a vast brain circuit responsible for the generation of voluntary movements.

Biomarker Predicts Recovery from a Type of Depression

A new study signifies the beginning of the end of psychiatrists' guess-work in figuring out which antidepressants work best for individual patients

By Amy Maxmen

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Image: Jiri Hodan

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By Amy Maxmen of Nature magazine

People who live with clinical depression must also suffer the ‘trial and error’ approach that psychiatrists take when prescribing antidepressants. Now, a study published this week signifies the beginning of the end of guesswork. In it, a blood test predicts who will respond well to a novel treatment for depression, and who might even fare worse.“We haven’t had a test like this in psychiatry before,” says Andy Miller, a professor of psychiatry at Emory University and an author on the study in Archives of General Psychiatry. “There is no brain scan, no physiological measure that tells you whether a patient will respond to one drug more than another.”

The test identifies an inflammatory protein in blood, C-reactive protein or CRP, that indicates internal inflammation. Whereas 62% of depressed participants with high CRP levels responded well to the new treatment, only 33% of participants with low CRP levels did.

The correlation was not entirely unexpected, because the drug suppresses inflammation, and Miller thinks that inflammation underlies depression in some people. To test whether a potent anti-inflammatory could soothe the malady, his team recruited 60 people who had lived with major depression for more than a decade and had received no relief from antidepressants.

Half of the participants received monthly treatments of the rheumatoid arthritis drug, Janssen’s Infliximab, and half received a placebo. Overall, Infliximab did not seem to work. However, when Miller’s team analyzed how the subset of participants with high CRP faired, it turns out they responded well to the drug, with a relief from sadness, suicidal thoughts, anxiety and other symptoms.

Since the late 1980s, researchers have sporadically hypothesized that inflammation can lead to depression. The theory is that depressed behavior might be beneficial in the short term because it reserves an injured animal’s energy for healing rather than romping around in the sunlight. Although the hypothesis has never received widespread support, researchers have found that some depressed patients indeed bear elevated levels of inflammatory proteins.

On the basis of the results from this relatively small study, a biologic drug such as Infliximab might be a better option in the anti-inflammatory realm than Cox-2 inhibitors such as aspirin, which come with unwanted side effects, says Miller. Although he knows of no Infliximab-like drug in development for depression, he says that companies might be encouraged by his team’s results. What’s more, with a biomarker to predict a response, companies will have a better chance of success.

Robert Dantzer, a neuroimmunologist at MD Anderson Cancer Center in Houston, Texas, notes that some of the participants in the low-CRP group fared worse on Infliximab than on placebo. Thus, the CRP test could be as important a tool for excluding depressed patients from taking anti-inflammatory therapies as for predicting responders.

This article is reproduced with permission from the magazine Nature. The article was first published on September 5, 2012.