Swedish university demos quantum chemistry simulation

Researchers at Chalmers University in Sweden have used a quantum computer to simulate a chemical reaction.

Quantum chemistry is often modelled using supercomputers, which apply the laws of quantum mechanics to simulate electrons and atoms. Since the laws of quantum mechanics describe the behaviour of nature on a subatomic level, many researchers believe that a quantum computer should be better equipped at running molecular calculations than a conventional supercomputer that relies on binary logic.

Martin Rahm, associate professor in theoretical chemistry at Chalmers’ department of chemistry and chemical engineering, who has led the study, said: “Quantum computers could in theory be used to handle cases where electrons and atomic nuclei move in more complicated ways. If we can learn to utilise their full potential, we should be able to advance the boundaries of what is possible to calculate and understand.”

This field of research is still young and the small model calculations that are run are complicated by noise from the quantum computer’s surroundings. The researchers at Chalmers University developed a new method, called Reference-State Error Mitigation (REM), that handles the noise inherent in the circuits that make up a quantum computers. This combines calculations from both a quantum computer and a conventional computer to take into account errors that occur due to noise.

REM starts with a simpler simulation of the chemical reaction, which acts as reference state that can be run both on a conventional and a quantum computer. This reference state represents a simpler description of a molecule than the original problem intended to be solved by the quantum computer. A conventional computer can solve this simpler version of the problem quickly.

By comparing the results from both computers, the researchers said they can obtain an estimate for the amount of error caused by noise on the quantum computer. The difference between the two computers’ results for the reference problem is then used to correct the original, more complex, problem when it is run on the quantum processor.

When run on Chalmers’ Särimner quantum computer, the researchers succeeded in calculating the intrinsic energy of small example molecules such as hydrogen and lithium hydride. While these calculations can be carried out more quickly on a conventional computer, they said that the new method represents an important development. It is also the first demonstration of a quantum chemical calculation on a quantum computer in Sweden.

“The study is a proof-of-concept that our method can improve the quality of quantum-chemical calculations. It is a useful tool that we will use to improve our calculations on quantum computers moving forward,” Rahm added.

“It is only by using real quantum algorithms that we can understand how our hardware really works and how we can improve it. Chemical calculations are one of the first areas where we believe that quantum computers will be useful, so our collaboration with Martin Rahm’s group is especially valuable,” said Jonas Bylander, associate professor in quantum technology at the department of microtechnology and nanoscience, Chalmers University.