A team of researchers at Lawrence Berkeley National Laboratory (Berkeley
Lab) used a quantum computer to successfully simulate an aspect of particle
collisions that is typically neglected in high-energy physics experiments,
such as those that occur at CERN's Large Hadron Collider.

The quantum algorithm they developed accounts for the complexity of parton
showers, which are complicated bursts of particles produced in the
collisions that involve particle production and decay processes.

Classical algorithms typically used to model parton showers, such as the
popular Markov Chain Monte Carlo algorithms, overlook several quantum-based
effects, the researchers note in a study published online Feb. 10 in the
journal Physical Review Letters that details their quantum algorithm.

"We've essentially shown that you can put a parton shower on a quantum
computer with efficient resources," said Christian Bauer, who is Theory
Group leader and serves as principal investigator for quantum computing
efforts in Berkeley Lab's Physics Division, "and we've shown there are
certain quantum effects that are difficult to describe on a classical
computer that you could describe on a quantum computer." Bauer led the
recent study.

Their approach meshes quantum and classical computing: It uses the quantum
solution only for the part of the particle collisions that cannot be
addressed with classical computing, and uses classical computing to address
all of the other aspects of the particle collisions.

Researchers constructed a so-called "toy model," a simplified theory that
can be run on an actual quantum computer while still containing enough
complexity that prevents it from being simulated using classical methods.

"What a quantum algorithm does is compute all possible outcomes at the same
time, then picks one," Bauer said. "As the data gets more and more precise,
our theoretical predictions need to get more and more precise. And at some
point these quantum effects become big enough that they actually matter,"
and need to be accounted for.

In constructing their quantum algorithm, researchers factored in the
different particle processes and outcomes that can occur in a parton shower,
accounting for particle state, particle emission history, whether emissions
occurred, and the number of particles produced in the shower, including
separate counts for bosons and for two types of fermions.

The quantum computer "computed these histories at the same time, and summed
up all of the possible histories at each intermediate stage," Bauer noted.

The research team used the IBM Q Johannesburg chip, a quantum computer with
20 qubits. Each qubit, or quantum bit, is capable of representing a zero,
one, and a state of so-called superposition in which it represents both a
zero and a one simultaneously. This superposition is what makes qubits
uniquely powerful compared to standard computing bits, which can represent a
zero or one.

Researchers constructed a four-step quantum computer circuit using five
qubits, and the algorithm requires 48 operations. Researchers noted that
noise in the quantum computer is likely to blame for differences in results
with the quantum simulator.

While the team's pioneering efforts to apply quantum computing to a
simplified portion of particle collider data are promising, Bauer said that
he doesn't expect quantum computers to have a large impact on the
high-energy physics field for several years—at least until the hardware
improves.

Quantum computers will need more qubits and much lower noise to have a real
breakthrough, Bauer said. "A lot depends on how quickly the machines get
better." But he noted that there is a huge and growing effort to make that
happen, and it's important to start thinking about these quantum algorithms
now to be ready for the coming advances in hardware.

Such quantum leaps in technology are a prime focus of an Energy
Department-supported collaborative quantum R&D center that Berkeley Lab
is a part of, called the Quantum Systems Accelerator.

As hardware improves it will be possible to account for more types of bosons
and fermions in the quantum algorithm, which will improve its accuracy.

Such algorithms should eventually have broad impact in the high-energy
physics field, he said, and could also find application in
heavy-ion-collider experiments.

## Reference:

Benjamin Nachman et al. Quantum Algorithm for High Energy Physics
Simulations, Physical Review Letters (2021). DOI:
10.1103/PhysRevLett.126.062001

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