a theorist of errors

To enter Dorit Aharonov’s office is to experience a sudden transitionbetween order and disorder. The

corridors of the computer-science buildingat the Hebrew University of Jerusalem arestark, white and neat. Aharonov’s office is a jumble of red-and-orange patterned cush-ions, article reprints and wicker furniture.It’s an appropriate setting for a theorist who has proved that when disorder reachesa certain level, the physics of the quantumrealm switches into the classical domain ofthe world we see every day.

Aharonov devotes herself to the theorybehind quantum computers.As-yet unbuilt, these machineswould harness the power ofquantum mechanics to performtasks that defeat conventionalcomputers — such as factoringlarge numbers. Aharonov, now34, has already made importantcontributions to this goal byshowing that a quantum com-puter could perform reliably and accuratelydespite a ‘noisy’environment.

Physics runs strong in Aharonov’s family.Her uncle, Yakir Aharonov, is a physicist at Tel Aviv University, and her father is a mathematician who taught her the beauty ofnumbers when she was little. She later chosephysics and mathematics for her undergrad-uate studies, but the quantum world did notinitially capture her imagination.She wantedinstead to use physics to study the brain.

A chance encounter“I wanted to solve the problem of conscious-ness,” she recalls. But she began to think thatthe problem was still beyond the reach oftoday’s science.“Then, one day, at a wedding,a friend asked me for advice about whatdirection to take in the study of the brain.I advised him to check out what people incomputer science were doing,” she says.

Realizing she should take her own advice,Aharonov went to the Hebrew University’scomputer-science building to find someoneto talk to.She was directed to Michael Ben-Orand, as she knocked on his door, she says thatshe had a strong feeling something importantwas going to happen. It did. Ben-Or told herabout quantum computation. “It fascinatedme. It was mathematics, physics and philoso-phy all in one package,”she says.

Back then, in 1994, the problem facingtheorists such as Ben-Or was how to prevent

a quantum computer from crashing. Allcomputers make errors when they operate,but quantum computers are more suscepti-ble to failure. This is because the quantumstates on which calculations depend are verydelicate: complex phenomena, such as thespin states of atomic nuclei, can store quan-tum information but this data can easily be lost if the particles interact with their sur-roundings.A computer can never be perfectlyisolated from its environment, so there willalways be ‘noise’ in the system and, inevitably,errors will arise. Moreover, correcting sucherrors is almost as difficult as doing the calcu-

lation in the first place. So will itever be possible to do a reliablequantum calculation?

“That was the problem Iposed to Dorit,” says Ben-Or,who became Aharonov’s disser-tation supervisor and later hercollaborator. Working with Ben-Or, Aharonov proved that at aconstant but low level of system

noise, a quantum computer can still produceaccurate results1.

“I consider her to be one of the most out-standing young people in this field,”says PeterZoller, a theoretical physicist atthe University of Innsbruck,Austria. Zoller wants to build aquantum computer,and he saysthat Aharonov has been instru-mental in laying the theoreticalfoundations on which a realmachine could be constructed.As well as her work on error tolerance, he cites an importantproof 2 Aharonov developedwith Oded Regev and otherswhile working at the Universityof California, Berkeley. Theproof showed that two existingmodels for quantum comput-ing are actually equivalent and,as a result, made writing quan-tum algorithms easier.

While at Berkeley,Aharonovextended her work on comp-uters to address a fundamentalpuzzle presented by quantummechanics — why its laws are evident in the world ofelementary particles, but not ineveryday life. At what pointdoes the world switch fromlooking quantum to looking

classical? Is it simply a matter of scale?Aharonov showed that for many noisy

quantum systems, there is a level of noiseabove which a transition to classical behav-iour is inevitable. Such transitions are muchsharper than expected from other theoriesthat predict a gradual shift away from quan-tum behaviour3.

Ben-Or says that what sets Aharonov apartis her boldness.As a graduate student she wasnot shy about contacting leading figures in thefield to discuss their work, he recalls. ZephLandau, a mathematician at the City Collegeof New York who collaborated with Aharonovon the model equivalence paper, says that sheis focused but not single-minded, findingtime to discuss other pursuits.

Aharonov says that balancing life andwork is essential to her research. Like manytheorists, she says that she has her best ideaswhen not thinking about work at all. Herdaily yoga session is particularly rewarding,she says: “It disperses the fog. My intuitionbecomes sharper. When there is less struggle,ideas become clear.”

Eastern ideas about the interconnected-ness of everything also influence her work.Forinstance,Aharonov is not fixated on the actualconstruction of a quantum computer. “Themost interesting thing that might come out ofan attempt to build one is the discovery thatwe can’t do it,”she says.By failing,she adds,wemight discover some entirely new physics. ■

Haim Watzman is a freelancer based in Jerusalem, Israel.1. Aharonov, D. & Ben-Or, M. Preprint at http://xxx.lanl.gov/

quant-ph/9611025 (1996).

2. Aharonov, D. et al. Preprint at http://xxx.lanl.gov/

quant-ph/0405098, (2004).

3. Aharonov, D. Phys. Rev. A 62, 062311 (2000).

Growing up on Einstein Street in Haifa, Israel, DoritAharonov was perhaps destined to study physics. Butshe pursued other interests before finally settling onquantum computation. Haim Watzman reports.

A theorist of errors

Relaxed: Dorit Aharonov finds her yoga inspires her theories.

“The most interestingthing that mightcome out of anattempt to build aquantum computer is the discovery thatwe can’t do it.”

— Dorit Aharonov

news featureC

. PA

NO

USS

IAD

OU

/PA

NO

S P

ICT

UR

ES

NATURE | VOL 433 | 6 JANUARY 2005 | www.nature.com/nature 9

6.1 News Feat Physics 23/12/04 3:24 pm Page 9

Nature Publishing Group© 2005

© 2005 Nature Publishing Group