Calculating a new view on quantum mechanics using quantum computers

By remotely accessing an IBM quantum computer through the Quantum Computer User Program (QCUP) - a quantum computing access program managed by the Oak Ridge Leadership Computing Facility, a Department of Energy Office of Science user facility located at DOE's Oak Ridge National Laboratory - a research scientist at Lawrence Berkeley National Laboratory successfully simulated a key process in particle physics: hadronization. Although based on a simplified model of quantum mechanics, the project lays the groundwork for how physicists can leverage the power of quantum computers to make large scientific calculations beyond the capabilities of classical supercomputers.

Hadronization occurs when two or more quarks-the subatomic building blocks of matter - bind together through the strong nuclear force to form composite particles called hadrons. The most familiar examples of hadrons are protons and neutrons, which form the nuclei of atoms. So, having a better understanding of the hadronization process means having a better understanding of the structure of matter and, in turn, the universe.

Physical experiments have not been able to reveal every step of the process, however. Researchers at the Large Hadron Collider (LHC) at CERN accelerate protons to near light speeds, guide them into collisions and study the resulting debris of quarks and antiquarks. But these particles can only be indirectly measured before they immediately undergo hadronization - hence the need for computer simulations to fill in the gaps of these scientific observations.

"In principle, we know the theory that describes hadronization, but we are unable to make predictions using it because the calculations have been too difficult for a classical computer. However, on a quantum computer, we should be able to directly make predictions for the details of how hadronization occurs, which will help with the searches for new physics performed at colliders such as the LHC," said Anthony Ciavarella, the Berkeley Lab research scientist who led the project. His findings were published in Physical Review D.

Quantum computing - a technology still in the early stages of development relative to classical supercomputers such as the OLCF's exascale-class Frontier - utilizes quantum bits, or qubits, to perform calculations. Unlike binary bits used by classical computers, qubits don't employ only ones and zeroes to encode information. Rather, they use a quantum superposition of combined ones and zeroes that may exponentially increase processing power for certain kinds of problems, such as the quantum mechanical interactions of subatomic particles.

Accurately simulating quantum chromodynamics (QCD) - the theory describing how the strong force binds quarks and gluons - overwhelms classical computers. The strong force binds and entangles the subatomic particles so that their representation and manipulation on classical computers requires exponential amounts of processing power and memory to predict observable results. This is because binary computers must separately represent all the different possible quantum states of the particles, which becomes an exponential scaling problem - the amount of memory needed doubles for every new particle or time step added to the simulation.

On the other hand, quantum computers are far more efficient at describing subatomic systems because their qubits can exist in multiple states just like the particles themselves. Furthermore, their computational power grows exponentially with each additional qubit.

"One of the original motivations for building quantum computers was that they naturally have this quantum phenomenology built into how they're constructed. And in these simulations of subatomic systems, we've got large amounts of entanglement and quantum correlations that you just can't efficiently represent on a regular computer," Ciavarella said.

Setting the template for quantum calculations

Ciavarella's project ultimately aims to develop the computational techniques needed to simulate the QCD of large subatomic systems on quantum computers of the near future. (Current quantum computers have limited numbers of qubits and are prone to high error rates, but the technology is rapidly evolving.) For this initial step, he simplified the simulation's parameters with a combination of his own techniques and ones that scientists have used for QCD simulations on classical computers. With the cloud access provided by QCUP, he applied them to a Heron processor on the IBM Quantum Platform, leveraging 104 of its 156 qubits.

First, Ciavarella used a heavy quark limit while simulating string breaking, which is a fundamental mechanism in the hadronization process. Quarks are linked by "strings" of gluons that stretch as the quarks collide and spin away, ultimately releasing enough energy to "snap" the gluon string apart as a new quark-antiquark pair bind together to form a hadron. Heavy quarks (with more mass) are easier to simulate because they don't spread out as much as light quarks, so they can fit more easily as points on a simulation grid. Researchers then extrapolate these heavy results down to the light quark behavior.

Second, Ciavarella used a "scalable circuit concurrent variational quantum solver," a computational technique that he co-developed as a graduate student at the University of Washington, to bring the quantum computer's qubits to a quantum vacuum state - the lowest energy level with the most stability.

"The idea is to optimize these vacuum preparation circuits on a small system size. Then you do it slightly bigger and slightly bigger and slightly bigger. So, by doing this, you can understand how the parameters of your circuit depend on the system size, and you can then extrapolate that out to doing it for a large system. For example, you can optimize this on up to 10-12 qubits and then extrapolate that out to hundreds if you choose to do so," Ciavarella said.

Finally, he limited his simulation to one dimension, with particles moving only left to right and back. Ciavarella plans on adding an additional dimension in the next iteration of his work, which he'll tackle once he can access improved quantum computers and algorithms. But this project succeeded in seeing how far the existing hardware can be pushed in string-breaking simulations, with results that matched previous work on classical supercomputers.

"One of the findings that we reproduced here is that, in the middle of the gluon string, it starts to look like it's gasifying at a finite temperature before it separates. This is exciting because, if we see this reproduced across a wide range of different simplified models, then it should be more likely it's an actual feature of QCD that describes the world we live in," Ciavarella said.

QCUP provides computational scientists with access to state-of-the-art commercial quantum computing resources for purposes of discovery and innovation in scientific computing applications. It is managed by the Oak Ridge Leadership Computing Facility. The OLCF is a DOE Office of Science user facility at ORNL that is supported by DOE's Advanced Scientific Computing Research program.

UT-Battelle manages ORNL for DOE's Office of Science. The Office of Science is the largest supporter of basic research in the physical sciences in the United States and is committed to addressing some of the most pressing challenges of our time. For more information, visit energy.gov/science. -Coury Turczyn