Quantum computers are still years away, but a trio of

theorists has already figured out at least one talent they may have. According

to the theorists, including one from NIST, physicists might one day use quantum

computers to study the inner workings of the universe in ways that are far

beyond the reach of even the most powerful conventional supercomputers.

Quantum computers require technology that may not be

perfected for decades, but they hold great promise for solving complex

problems. The switches in their processors will take advantage of quantum

mechanics—the laws that govern the interaction of subatomic particles. These

laws allow quantum switches to exist in both on and off states simultaneously,

so they will be able to consider all possible solutions to a problem at once.

This unique talent, far beyond the capability of today’s

computers, could enable quantum computers to solve some currently difficult

problems quickly, such as breaking complex codes. But they could look at more

challenging problems as well.

“We have this theoretical model of the quantum

computer, and one of the big questions is, what physical processes that occur

in nature can that model represent efficiently?” said Stephen Jordan, a

theorist in NIST’s Applied and Computational Mathematics Division. “Maybe

particle collisions, maybe the early universe after the Big Bang? Can we use a

quantum computer to simulate them and tell us what to expect?”

Questions like these involve tracking the interaction of

many different elements, a situation that rapidly becomes too complicated for

today’s most powerful computers.

The team developed an algorithm—a series of instructions

that can be run repeatedly—that could run on any functioning quantum computer,

regardless of the specific technology that will eventually be used to build it.

The algorithm would simulate all the possible interactions between two

elementary particles colliding with each other, something that currently

requires years of effort and a large accelerator to study.

Simulating these collisions is a very hard problem for

today’s digital computers because the quantum state of the colliding particles

is very complex and, therefore, difficult to represent accurately with a feasible

number of bits. The team’s algorithm, however, encodes the information that

describes this quantum state far more efficiently using an array of quantum

switches, making the computation far more reasonable.

A substantial amount of the work on the algorithm was done

at the California Institute of Technology, while Jordan was a postdoctoral fellow.

His coauthors are fellow postdoctoral researcher Keith S.M. Lee (now a postdoctoral

researcher at the University

of Pittsburgh) and

Caltech’s John Preskill, the Richard P. Feynman Professor of Theoretical

Physics.

The team used the principles of quantum mechanics to prove

their algorithm can sum up the effects of the interactions between colliding

particles well enough to generate the sort of data that an accelerator would

provide.

“What’s nice about the simulation is that you can raise

the complexity of the problem by increasing the energy of the particles and

collisions, but the difficulty of solving the problem does not increase so fast

that it becomes unmanageable,” Preskill says. “It means a quantum

computer could handle it feasibly.”

Though their algorithm only addresses one specific type of

collision, the team speculates that their work could be used to explore the

entire theoretical foundation on which fundamental physics rests.

“We believe this work could apply to the entire

standard model of physics,” Jordan says. “It could allow

quantum computers to serve as a sort of wind tunnel for testing ideas that

often require accelerators today.”