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Three weeks ago, Chinese scientists launched into space a beautiful physics experiment that will, if successful, establish a new world record for the long-distance "nonlocal entanglement" of quantum particles.

Quantum pioneer Erwin Schrodinger considered entanglement the most characteristic of the many strange new quantum phenomena, but his friend and colleague Albert Einstein declared it "spooky action at a distance" that can't happen in the real world. When two quantum objects, such as two photons (particles of light), interact with each other, there's a chance they will come away "entangled" after the interaction is finished. Even though the entangled photons may separate widely after the interaction, they remain intimately "in touch" or "correlated" (coordinated) with each other, instantly adjusting their own behavior in response to changes -- which might be sudden and unpredictable -- in the other photon.

Picture a photon as a tiny arrow that can point in various directions. We say the photon is "polarized" in a particular direction -- a property that sunglasses manufacturers take advantage of to reduce glare because reflected sunlight is highly polarized in a direction parallel to the reflecting surface.

When two photons are entangled, they become a single unified "atom of light," even though they might be widely separated. As a way two such photons can exhibit their togetherness, each photon carries information about the other photon's polarization and instantly adjusts its own polarization to properly correlate with any changes in its entangled partner's polarization, even though those changes might be sudden and unpredictable. This behavior is called "nonlocal" because each photon seems to follow instructions issued by the other distant photon, even though the connection is instantaneous and thus cannot be caused by any normal "signaling" such as a radio wave from one photon to the other. This "spooky action" was one reason for Einstein's lifelong belief that quantum physics was inadequate and that something better, something more "complete," would eventually replace it. But scientists have searched in vain for such a non-spooky theory, while "spooky" quantum physics continues to meet every experimental challenge.

Entanglement experiments have been conducted since 1972. In each of many repeated experimental trials, two entangled photons are sent out in two directions from a central source, to separated locations traditionally called "Alice" and "Bob." They have devices that can quickly and unpredictably shift the polarization of their photon while it is speeding, at 186,000 miles per second, toward detectors in their laboratories. To the surprise of most physicists, the results confirmed the quantum predictions.

Many such experiments have been done by now. In the most impressive of these, by Anton Zeilinger's group at the University of Vienna in 2007, Alice was on one of the Canary Islands and Bob was 144 kilometers away on another island. When Alice altered her photon's polarization quickly and unpredictably, Bob's photon responded instantly by adjusting its own polarization. The entangled photon pair acted instantaneously as a single highly unified object throughout its 144-kilometer extension.

Chinese quantum physicist Pan Jianwei, who obtained his Ph.D. under Zeilinger and has numerous scientific publications and awards, heads the Chinese experiment. In 2004, Pan's team at the University of Science and Technology in Hefei, China, was the first to demonstrate five-photon entanglement and to show that it is truly nonlocal, just like the two-photon entanglement described above. Pan is chief scientist for the Quantum Space Satellite, or QUESS, that will be central to several experiments in quantum entanglement and quantum communication. These experiments will be conducted in cooperation with the University of Vienna and the Austrian Academy of Sciences.

The QUESS satellite orbits at an altitude of 600 km (375 miles) in a "sun-synchronous orbit" that maintains an unchanging local solar (or "sundial") time at all times, except for the day changes upon crossing the international date line. This orbit ensures a nearly unchanging space environment, which I assume is chosen by the scientists to be the darkest hour of night.

The satellite is a small remotely-controlled physics lab. For the entanglement experiment, the lab will create a series of entangled photon pairs and send one photon of each pair down to "Alice" at a ground lab in Delingha in western China and the other photon down to "Bob" in Lijiang, 1,200 kilometers (750 miles) to the southeast. If all goes well, Alice and Bob will verify that they are looking at opposite ends of a single instantaneously unified atom of light 1,200 kilometers long: Spooky action indeed, at a greater distance than ever.

Commentary on 09/06/2016

Print Headline: Spooky action at greater distances

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