Quantum Internet Author(s): Peter Weiss Source: Science News, Vol. 155, No. 14 (Apr. 3, 1999), pp. 220-221 Published by: Society for Science & the Public Stable URL: http://www.jstor.org/stable/4011480 Accessed: 28/10/2010 11:46

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http://www.jstor.org formation via entanglement. Teleportation also requires a classical transfer of information-in Kimble's ex- periment, along wires. By correctly com- bining the information from the wires and the beams, the scientists recreate the message at the receiving end. The quirks of quantum Along the classical path, information flows no faster than the speed of light. mechanics may lead to better So, quantum teleportation still obeys that universal speed limit. computer networks A lthough light-based quantum-infor- By PETER WEISS mation processing seems promis- ing, the quantum-Internet may not ultimately use light as its medium. Feverish experimentationnow under equivalent of sending just one quantum Attempts to use photons as bits in rudi- way in labs around the world may one bit, or qubit, worth of information. Unlike mentary quantum-computing experiments day lead to quantum computers. a conventional bit, which must represent have run into some serious snags. In an These extraordinarilypowerful calculating either 0 or 1, a qubit represents a mixture alternative pursued by some researchers, machines would employ as their bits not of 0 and 1 called a superposition. Only the quantum Internet might slosh more electric circuits but particles that obey the when a measurement is actually made, than blink. strange rules of quantum mechanics. These which destroys the superposition, is the A number of scientific teams are ex- devices are years away at best-maybe qubit forced into a specific value. ploring quantum computing and commu- decades, maybe more. While wonderfully versatile, a qubit nication using atoms in liquids manipu- Scientists dare to dream, anyway- still packs only one nugget of informa- lated via nuclear magnetic resonance not only of quantum computers but al- tion. Recently Kimble and his colleagues (NMR). The technique is also used in so of linking them together into net- have shown experimentally that quan- medicine to make images of body parts. works. The connections between the tum teleportation using light beams, With strong magnetic fields and radio- machines would operate on quantum made up of many photons, may be able wave pulses, NMR manipulates the spins mechanical principles, too. to carry a great deal more information, of atomic nuclei. Fuelingthose dreams is a growing ability enough perhaps to support practical In the Nov. 5, 1998 NATURE,researchers to transfer delicate quantum information computing. from Los Alamos (N.M.) National Labora- from one place to another. New experi- Kimble's team and scientists from tory and the University of New Mexico in ments that stretch quantum mechanical ef- Aarhus University in Denmark and the Albuquerque reported a short-distance fects across distances of kilometers are University of Wales in Bangor shipped a example of teleportation. They transmit- providing encouragement. characteristic of a pulse of light across a ted a characteristic known as spin orien- Theorists are fanning the flames as lab bench to another pulse a meter away. tation from one atom to another within a well. Studies of how such hypothetical Although the distance is short, in princi- molecule. The scientists manipulated networks would compare with conven- ple the same technique could work over molecules of trichloroethylene dissolved tional ones hint that greater computing unlimited spans, Kimble says. The re- in chloroform. power waits to be unleashed. searchers described their experiment in "We'veused different nuclei to transfer H. Jeff Kimble, an experimenter and the Oct. 23, 1998 SCIENCE. the information, not photons or electro- theorist at the California Institute of "With our quantum scheme, we could magnetic fields. We're the first ones to do Technology in Pasadena, foresees such take the whole output of a quantum com- that," says Raymond Laflamme of Los networks having a widespread impact. puter and teleport it," says Christopher Alamos. "One could imagine a quantum Internet A. Fuchs, one of the Caltech researchers. In a manner analogous to the optical ex- in the future," he says. It would be a To achieve teleportation, scientists ex- periments, the researchers created a quan- more complex web than the one that ploit another of the quantum realm's tum conduit by entangling two atoms. currently spans the globe and would strange aspects. Known as entanglement, Then, they allowed another atom, which employ communication capabilities not it creates a correlation between quantum carried the spin message, to interact with possible with conventional technology. objects that, in theory, persists no matter one of the entangled pair, automatically af- "Such a network can do heroic how far apart those entities become. The fecting the other via the quantum link. things," he predicts. correlation arises because the objects Although no wires were involved, a occupy a joint quantum state. When the classical ingredient was still present. A entanglement ends-because of a mea- combination of radiofrequency pulses Less than 2 years ago, scientists suc- surement, for example-the once-entan- and quiescent periods guided the mole- ceeded for the first time in mak- gled states must adopt related values. cule into a final state that depends on the ing information appear to leap in- Two formerly entangled photons would arrangement of spins caused by the ini- stantaneously from one place to another take on predictable characteristics-for tial message interaction. Those influ- without passing through the intervening instance, opposite values. ences finally nudged the target atom into space. In independent experiments, scien- Kimble's team split a single laser beam the message's spin state. tists in two European laboratories trans- to create an entangled pair of beams. The Caltech and Los Alamos achieve- ferred a characteristic of one photon- One blazed into the sending station and ments are both "great experimental tours the elementary particle of light-to anoth- the other into the receiving station. de force in learning how to control these er photon via a technique called quan- When a new pulse, which can be thought things," says Charles H. Bennett of the tum teleportation (SN: 1/17/98, p. 41). of as the message, interacts with the IBM Thomas J. Watson Research Center Researchers say quantum teleportation sending half of the original beam, entan- in Yorktown Heights, N.Y. Bennett is one will be an essential ingredient of both glement requires that the receiving part of six theorists who in 1993 came up with quantum computers and networks. be affected, too. In essence, the receiver the idea of quantum teleportation (SN: Teleporting a photon's state is the gets a part of the message as quantum in- 4/10/93, p. 229).

220 SCIENCE NEWS, VOL. 155 APRIL3, 1999 Kimble,however, harbors grave doubts goes far beyond interatomicdistances, al- Early last year, however, a trio of com- about the NMRexperiment. "Inmy view, beit withoutcoming close to practicalnet- puter scientists examined how quantum it's not a demonstrationof teleportation, work dimensions. Moreover,"the same computers would solve the problem and it's a simulation,"he says. He contends procedurewe used for 3 photons can now reported that the number of bits needed that entanglements can't survive in the be generalized"to possibly as many as 10 would be significantly below n. disorderlysea of molecules constitutinga photons,Zeilinger says. "By using quantum bits, rather than typical liquid. NMRproponents counter classical bits, you can save on commu- that Kimbleand his fellow skeptics have nication," says Richard Cleve of the Uni- chosen to ignore other analyses showing ould quantum networks be versity of Calgary in Alberta, a member that entanglementscan survive. W worth the work that it will take of the research team. For this particular to develop the quantum links problem, however, he notes, the quan- that they will need?Theorists are probing tum treatment requires more exchanges Teleporting across a molecule-or how model networks might performand among the computers than a classical even a workbench-won't suffice comparing them with conventional, or solution does. for making practical networks. classical, computernetworks. An analysis of a more esoteric problem Researchers are building up to greater Each node of a quantum network by Ran Raz of the Weizmann Institute in leaps, however,by testing the notion that would be a quantumcomputer. Such ma- Rehovot, Israel, concludes that a quantum entanglementhas an unlimitedrange. chines would calculate and performlogi- interaction saves on both the number of WolfgangTittel and his colleaguesat the cal operations using delicate strings of bits exchanged and the number of ex- Universityof Genevahold the worldrecord entangled qubits, each in a superposi- changes. Raz is scheduled to present his for extendingentanglement across space. tion of many states. So far no more than work in May in Atlanta at the 31st Annual Usingexisting optical fibers that had been three-qubit, rough-hewn calculating ex- Association for Computing Machinery's installedfor telecommunications,the sci- perimentshave appearedin labs. Not un- Symposium on Theory of Computing. entificteam splitup entangledpairs of pho- til that qubit number grows to thirty or Another example of a possible quan- tons producedin Genevaand sent them on forty and a robust technology emerges tum-network edge emerged at a January separate paths to Bellevue and workshop on algorithms in quan- Bernex, two villages outside Geneva TjTHEY 4IoWEP 14OWARiTHUR(P L-AN A5K MGRLMTO tum information processing at De- (SN: 2/10/96, p. 90). The Swiss re- ffiAI6- method, two computers, named remained correlated throughout the Arthur and Merlin after the leg- photons' journeys. In the May 1998 endary king and sorcerer, have a PHysIcALREVIEW A, the researchers ~ID chat. Merlin, who is wise but not al- concluded, borrowing a phrase from ways honest, presents the proof to Albert Einstein, that the "spooky ac- Arthur. Limited both in brains and tion at a distance" between their en- [R~~~~~~~0 time, Arthur queries Merlin in an at- tangled photons does not break tempt to verify the proof. down across the 10.9 kilometers be- Daunted by the proof itself, tween the villages. Arthur asks Merlin to carry out re- Scientists are planning to test lated calculations instead. By check- longer entanglements. Anton Zeil- ing for consistency among Merlin's inger of the University of Vienna Quantumnetworks could quicklydetermine whether answers, Arthur can discern if the says that his group is gearing up Merlin'sproof is valid. proof is valid. for exDeriments at distances be- In the case of conventional com- yond 20 km. He led one of the firstexperi- will quantum computers begin to make puters, the number of rounds of question- ments in photon teleportationmore than their mark,scientists say. and-answer grows as the mathematical a year ago, while at the University of At that point, teleportationwould pro- statement or equation under considera- Innsbruck in Austria. His newly estab- vide the thread to tie the computers to- tion becomes more complicated. lished Viennalab will collaborate on the gether. How far those threads might At the DePaul conference, however, project with the Genevaresearchers and stretch remains to be seen. Tittel thinks John Watrous of the University of Mon- other scientists. 50 km is possible with current optical treal reported that a quantum Arthur and To go beyond entanglement,the Vien- fibertechnology. Merlin duo could determine the validity na researchers are also laying plans for If quantum computers start talking to of the proof, no matter how convoluted the first teleportation of a photon's char- each other, what would come next? its mathematical expression, in only two acteristics across kilometer distances, Computer scientists have begun to ad- rounds of questioning. Watrous says that Zeilingersays. dress that question, although on a very the finding rests on a widely accepted as- Those experimentswould follow on an abstract level. sumption that there is a certain type of achievement completed while he was In one sort of problem that they pon- complexity in the mathematics. still at Innsbruckbut just reported in the der, several very busy people try to Watrous's new evidence of latent quan- Feb. 15 PHYSIcALREVIEW LETTERS. The Inns- arrangeto meet for lunch. They let their tum network power has impressed Bennett. bruck team simultaneously entangled computers,which knowtheir jam-packed "This is another major step along the way," three photons, with distances of roughly schedules, interactwith each other in or- he says. For more than two computers, Wa- 20 centimetersbetween each of them. der to find a time slot in which all three trous says, he expects a quantum approach It'snot the first report of three-particle people are free. also to yield a bonus, but he hasn't yet ana- entanglement. Laflamme and his col- For conventional computers, re- lyzed that situationin detail. leagues made that claim in a NMRexperi- searchers had already proved years ago If researchers ultimately find ways to ment last year involvingthree atoms in a that for schedules with some number,n, make quantum information leap far molecule (SN: 9/12/98, p. 165). of time slots, no fewer than n bits would enough and wide enough, a quantum leap The Innsbruck experiment, however, have to be exchanged by the machines. for networks may not be far behind. DG

APRIL3, 1999 SCIENCE NEWS, VOL. 155 221