XEKE NM6 of the week

Computation Takes a Quantum Leap Custom-assemble five fluorine atoms

with a few other atoms, and the product is a molecule. It’s also a computer. The p rob lem that it’s solved is a simple one, but the exercise provides experimental evidence that a quantum computer can handle cer- tain mathematical problems more effi- ciently than can a conventional computer.

Though a practical quantum computer still may be decades away, “this result gives us a great deal of confidence in un- derstanding how quantum computing can evolve into a future technology,” says Isaac L. Chuang of the IBM Almaden Re- search Center in San Jose, Calif.

He and his collaborators announced the feat last week at the Hot Chips 2000 conference at Stanford University.

A quantum computer exploits the quan- tum mechanical nature of tiny particles,

Structure of molecule used as a quantum computer to solve a simple problem.

such as electrons or atomic nuclei, to en- code information as quantum bits, or qubits. Whereas an ordinary bit has at any

HArF! Argon’s not so noble after all Noble gases-radon, xenon, krypton,

argon, neon, and helium-are snobs. Their atoms typically shun liaisons with other elements because they already have all the electrons they need, but none to share. Only when chemists en- gage in forced matchmaking do some of these gases react with other elements to form stable, neutral compounds.

Researchers at the University of Helsinki in Finland report in the Aug. 24 NATURE that the formerly aloof argon has been coerced into the chemical equiva- lent of a shotgun wedding.

The scientists made the new com- pound, argon fluorohydride (HArF), by shining a strong ultraviolet light on frozen argon that contained a small amount of hydrogen fluoride. The light split some of the hydrogen fluoride molecules into hydrogen and fluorine atoms, which then combined with ar- gon to form the new compound, says Markku Rasanen, an author of the re- port. The resulting mixture absorbed wavelengths of infrared light that theo- rists had predicted would be absorbed by hydrogen-argon and fluorineargon bonds, thereby confirming the pres- ence of the new molecule.

The HArF molecules are marginally stable and remain intact only when iso- lated within the matrix of frozen argon, admits W a n e n . If they warm above 27 kelvins or if they touch one another, they readily break apart.

“This Is a remarkable achievement and yet is only a half step toward truly synthesizing an argon compound,” says Gernot Frenking, a chemist a t Philipps-

Universitat Marburg in Germany. The true success, he contends, is making a molecule that can survive a t room temperatures and be handled as other chemical compounds are. Although the- oretically possible, such molecules may be extremely difficult to produce.

Scientists have created stable neutral compounds that include xenon and krypton. They’ve also made molecules that contain radon atoms, but these compounds are short-lived because of radon’s 3.84day half-life.

Now, the only remaining loners are neon and helium. Their atoms are small- er than their noble brethren’s and there- fore hold onto their outer-shell electrons more tightly. Frenking expects that the experimental techniques used to make HArF could be applied to manufacture stable molecules that incorporate the two noble holdouts, but finding the right chemical partners may be difficult.

Even though fluorine is highly reac- tive, Frenking says the bond between fluorine and neon probably wouldn’t be strong enough to hold the two atoms together. Instead, chemists may have t o turn to chlorine. Although less reactive than fluorine, chlorine won’t be a s strongly repelled by neon because of the configuration of the atoms’ outer- shell electrons.

Any neon o r helium compounds that chemists produce are unlikely to be sta- ble a t room temperature. Frenking says that these molecules-like HArF-would have to spend their lives isolated within a frozen matrix, maintaining an eternal cold shoulder. 4. Perkins

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time a value of either 0 or 1, a qubit can also take on both values at once. Because a quantum computer can act on these multiple states simultaneously, it’s poten- tially many times as powerful as a conven- tional computer (SN 1/14/95, p. 30).

For their quantum computer, Chuang’s team designed a molecule in which the nuclei of five fluorine atoms interact with each other to produce a five-qubit sys- tem (SN: 1/18/97, p. 37). The researchers report that they used radio pulses to “program” the nuclei into specific quan- tum states. Nuclear magnetic resonance (NMR) instruments could then detect the results.

The investigators tackled a permuta- tion problem. Roughly speaking, comput- ing the order of a permutation is akin to finding the shortest path through a hid- den maze of rooms, each with precisely one exit and one entrance, connected by oneway passages. The goal is to get back to the starting point most quickly.

Last year, Richard Cleve of the Uni- versity of Calgary in Alberta showed theoretically how a quantum computer could solve a particular version of this problem in fewer steps than a conven- tional computer.

Chuang and his coworkers tackled a four-room version of the permutation problem. They used radio pulses to pro- gram the molecules, which were dis- solved in a liquid, so that two qubits en- coded the starting room, and three qubits represented, in effect, the eight possible systems of oneway passageways.

Then, the researchers obtained the molecules’ NMR spectrum, and its pat- tern of lines gave the correct answer in each test. In essence, the result emerged in one computational step, whereas a conventional computer would have re- quired more steps.

“We anticipate more-complex algo- rithms, involving a few extra qubits, may be possible using our current approach,” comments team member Lieven M.K. Vandersypen of Stanford.

However, “such implementations do not provide sufficient information about critical issues for the future of quantum computation, such as how much control we actually can achieve over a quantum computer,” says Emanuel Knill of the Los Alamos (N.M.) National Laboratory.

Earlier this year, Knill and his coworkers put together a sevenqubit quantum com- puter made up of six hydrogen and four carbon atoms. Although the researchers didn’t perform a complete computation at that time, they demonstrated a reliable, ef- ficient procedure for setting up qubits for a computation. 4 Peterson

AUGUST 26,2000