There are high expectations that quantum computers could deliver important new possibilities for simulating chemical processes. Developments could have a major impact, from the development of new pharmaceuticals to new materials.
Conventionally, detailed chemical studies are performed using super computers. These are based on logical circuits. There is a limit for which calculations such conventional computers can handle. This is especially the case with the laws of quantum mechanics. These describe the behaviour of nature on a subatomic level.
Quantum computing offers new approaches to explore these issues further. To model how things may evolve in the future, researchers at Chalmers University have used a quantum computer called Särimner (a quantum processor with five qubits) in order to undertake calculations within a real-life case in chemistry.
This is based on using the laws of quantum mechanics to understand which quantum chemical reactions are possible. Furthermore, these inquiries can assess which structures and materials could be developed, as well as further understanding characteristics they possess.
This led to a method is called Reference-State Error Mitigation (REM). This works by correcting for the errors that occur due to noise by utilising the calculations from both a quantum computer and a conventional computer.
The principle behind the method is to first consider a reference state by describing and solving the same problem on both 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, an exact estimate can be made for the amount of error caused by noise. The difference between the two computers’ solutions for the reference problem can then be used to correct the solution for the original, more complex, problem when it is run on the quantum processor.
By combining this new method with data from Chalmers’ quantum computer Särimner the researchers succeeded in calculating the intrinsic energy of small example molecules such as hydrogen and lithium hydride. The significance of this was the first demonstration of a quantum chemical calculation on a quantum computer in Sweden.
Commenting on the research, Martin Rahm, Associate Professor in Theoretical Chemistry at the Department of Chemistry and Chemical Engineering says: “Quantum computers could in theory be used to handle cases where electrons and atomic nuclei move in more complicated ways.”
Rahm adds: “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”.
The research has been written up into a paper in the Journal of Chemical Theory and Computation, which is titled “Reference-State Error Mitigation: A Strategy for High Accuracy Quantum Computation of Chemistry”.
