### Quantum computers to simulate particle collisions

Quantum computers may still be years away, but that hasn't stopped scientists from figuring out at least one potential use for the advanced systems.

According to a trio of theorists, including one from the National Institute of Standards and Technology (NIST), physicists might one day harness the raw horsepower of 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.

Although quantum computers require technology that might not be perfected for decades, they still hold significant potential for solving complex problems.

Indeed, the switches in their processors will take advantage of quantum mechanics (quantum switches exist in both on and off states simultaneously), allowing them to consider all possible solutions to a problem at once.

This unique capability, far beyond the realm of current HPC computing, could enable quantum systems to solve conventional problems such as breaking complex codes rather quickly. But they could also be used to analyze more challenging and complex problems.

"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.

As such, the team developed an algorithm that can 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.

As noted above, simulating such collisions is challenging for today's digital computers as the quantum state of the colliding particles is quite complex and 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.

Essentially, the team used the principles of quantum mechanics to prove their algorithm was sufficiently capable of summing up the effects of the interactions between colliding particles, which would allow it to generate the sort of data rendered by an accelerator.

"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," Caltech's Professor John Preskill says. "It means a quantum computer could handle it feasibly."

Though their algorithm only addresses one specific type of collision, the team speculates 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... It could allow quantum computers to serve as a sort of wind tunnel for testing ideas that often require accelerators today," Jordan added.