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Spooky Action at a Distance.” Ground Stations Could 2 NEWS | IN DEPTH QUANTUM PHYSICS Spooky action achieved at record distance Entangled photons from Chinese satellite foreshadow space-based quantum network By Gabriel Popkin repeaters” that rebroadcast quantum infor- power on par with the United States and Eu- mation could extend a network’s reach, but rope (Science, 22 July 2016, p. 342). uantum entanglement—physics at they aren’t yet mature. Many physicists have In their first experiment, the team sent a its strangest—has moved out of this dreamed instead of using satellites to send laser beam into a light-altering crystal on the world and into space. In a study that quantum information through the near- satellite. The crystal emitted pairs of photons shows China’s growing mastery of vacuum of space. “Once you have satellites entangled so that their polarization states both the quantum world and space distributing your quantum signals through- would be opposite when one was measured. science, a team of physicists reports out the globe, you’ve done it,” says Verónica The pairs were split, with photons sent to Qthat it sent eerily intertwined quantum Fernández Mármol, a physicist at the Spanish separate receiving stations in Delingha and particles from a satellite to ground stations National Research Council in Madrid. “You’ve Lijiang, 1200 kilometers apart. Both stations separated by 1200 kilometers, smashing the leapfrogged all the problems you have with are in the mountains of Tibet, reducing the previous world record. The result is a step- losses in fibers.” amount of air the fragile photons had to tra- Downloaded from ping stone to ultrasecure communication Jian-Wei Pan, a physicist at the University verse. This week in Science, the team reports networks and, eventually, a space-based of Science and Technology of China in Shang- simultaneously measuring more than 1000 quantum internet. hai, got the chance to test the idea when photon pairs. They found the photons had “It’s a huge, major achievement,” says the Micius satellite, named after an ancient opposite polarizations far more often than Thomas Jennewein, a physicist at the Uni- Chinese philosopher, was launched in Au- would be expected by chance, thus confirm- versity of Waterloo in Canada. “They started gust 2016. The satellite is the foundation of ing spooky action over a record distance http://science.sciencemag.org/ with this bold idea and managed to do it.” the $100 million Quantum Experiments at (though the 2015 test over a shorter distance Entanglement involves putting objects Space Scale program, one of several missions was more stringent). in the peculiar limbo of quantum super- that China hopes will make it a space science The team had to overcome many hurdles, position, in which an object’s quantum properties occupy multiple states at once: like Schrödinger’s cat, dead and alive at the Quantum leaps same time. Then those quantum states are China’s Micius satellite, launched in August 2016, has now validated across a record 1200 kilometers the shared among multiple objects. Physicists “spooky action” that Albert Einstein abhorred (1). The team is planning other quantum tricks (2–4). have entangled particles such as electrons and photons, as well as larger objects such 1. Spooky action on September 5, 2017 as superconducting electric circuits. Light-altering crystal Micius Entangled photons were Theoretically, even if entangled objects are creates entangled (500-km altitude) sent to separate stations. separated, their precarious quantum states photon pairs. Measuring one photon’s quantum state instantly should remain linked until one of them is determines the other’s, no measured or disturbed. That measurement matter how far away. instantly determines the state of the other object, no matter how far away. The idea is so counterintuitive that Albert Einstein mocked 4. Global network 1 Pair Future satellites and it as “spooky action at a distance.” ground stations could 2. Quantum key distribution Starting in the 1970s, however, physicists enable a quantum 3 Micius will send strings began testing the effect over increasing dis- internet. of entangled photons tances. In 2015, the most sophisticated of to the stations, creating a key for eavesdrop-proof these tests, which involved measuring entan- 2 communications. gled electrons 1.3 kilometers apart, showed 4 once again that spooky action is real. Beyond the fundamental result, such ex- Pair string periments also point to the possibility of Ali Observatory 3. Quantum teleportation ((((( -( ( ((((( hack-proof communications. Long strings Micius will send one entangled A) of entangled photons, shared between dis- India (( photon to Earth while keeping (D ( tant locations, can be “quantum keys” that Delinghaa its mate on board. When a third secure communications. Anyone trying to photon with an unknown state ground SCIENCE eavesdrop on a quantum-encrypted mes- station is entangled with the one on Earth, and their states jointly sage would disrupt the shared key, alerting measured, the properties of everyone to a compromised channel. the last photon are instantly But entangled photons degrade rapidly as Lijiangian teleported up to Micius. ground station ( ( ( ( ( (( )(( (( ((( ( ( ( ( they pass through the air or optical fibers. So ( far, the farthest anyone has sent a quantum Indian China ocean key is a few hundred kilometers. “Quantum ( ( ( ( ((( ( 1110 16 JUNE 2017 • VOL 356 ISSUE 6343 sciencemag.org SCIENCE including keeping the beams of photons fo- BIOMEDICINE cused on the ground stations as the satellite hurtled through space at nearly 8 kilometers per second. “Showing and demonstrating it In a major shift, cancer drugs is quite a challenging task,” says Alexander Ling, a physicist at the National University of Singapore. “It’s very encouraging.” However, go ‘tissue-agnostic’ Ling notes that Pan’s team recovered only about one photon out of every 6 million sent Encouraged by an FDA approval, companies design cancer from the satellite—far better than ground- drugs to work in any solid tumor based experiments but still far too few for practical quantum communication. Pan expects China’s National Space Sci- By Ken Garber, in Chicago, Illinois are making their way through clinical trials ence Center to launch additional satellites (see table, p. 1112). “It’s a sign of the times,” with stronger and cleaner beams that could arely 2 years after enrolling its first says Alice Shaw, director of thoracic onco- be detected even when the sun is shining. patient, a small biotech company has logy at Massachusetts General Hospital in (Micius operates only at night.) “In the next shown the worth of the first drug Boston. “The molecular biology of the tu- 5 years we plan to launch some really practi- deliberately targeted to any solid tu- mor is really leading the way over the tissue Downloaded from cal quantum satellites,” he says. In the mean- mor carrying a specific mutation. The of origin.” time, he plans to use Micius to distribute mutation is found in less than 1% of The May FDA approval “is a very clear quantum keys to Chinese ground stations, Bcommon cancers, and the firm, Loxo Onco- signal to the research community that they which will require longer strings of pho- logy in Stamford, Connecticut, scoured cities [the FDA] view this favorably—the idea of tons and additional steps. Then he wants to on three continents to find using cancer genomes” to demonstrate intercontinental quantum key 55 adults and children with select drugs regardless of http://science.sciencemag.org/ distribution between stations in China and 17 kinds of advanced cancer, tumor type, says cancer bio- Austria, which will require holding one half from bone to breast, all har- logist Bert Vogelstein of of an entangled photon pair on board until boring the mutation. Johns Hopkins University the Austrian ground station appears within The effort paid off. School of Medicine in Bal- view of the satellite. He also plans to teleport Of that diverse patient timore, Maryland. But such a quantum state—a technique for transfer- group, 43 saw their tumors drugs will remain the ex- ring quantum-encoded information without shrink by 30% or more and ception, he and others note, moving an actual object—from a third Ti- seven had their tumors until researchers extend the betan observatory to the satellite. disappear completely, the principle beyond the rela- Other countries are inching toward quan- company reported at an tively rare gene alterations tum space experiments of their own. Ling American Society of Clinical now targeted. on September 5, 2017 is teaming up with physicists in Australia Oncology (ASCO) meeting Merck’s Keytruda was to send quantum information between two here earlier this month. “I’ve first approved in 2014, for a satellites, and the Canadian Space Agency done a lot of studies, but I’ve specific cancer, melanoma. recently announced funding for a small never seen a drug this active, A monoclonal antibody, quantum satellite. European and U.S. teams with as little side effects,” the drug does not attack are also proposing putting quantum instru- says oncologist David Hong tumors directly. Instead, it ments on the International Space Station. of the University of Texas unleashes antitumor im- One goal is to test whether entanglement is MD Anderson Cancer Center munity by shutting down a affected by a changing gravitational field, by in Houston, one of the aca- negative signal, or “check- comparing a photon that stays in the weaker demic investigators for the point,” in the immune war- gravitational environment of orbit with an trial sponsored by Loxo. “It’s A tumor in a child’s leg had grown riors known as T cells. entangled partner sent to Earth, says Anton really been kind of a mira- so large (top) that an amputation Almost a decade ago, Zeilinger, a physicist at the Austrian Acad- cle drug.” It also validates a was planned, but a drug targeting Vogelstein and immuno- emy of Sciences in Vienna.
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