“These quantum computing algorithms were originally developed in a completely different context. We have used them here for the first time to calculate the electron densities of molecules, in particular also their dynamic evolution after excitation by a light pulse,” explains Annika Bande, who heads a theoretical chemistry group at HZB. Together with Fabian Langkabel, who is doing her doctorate with Bande, she has now shown in a study how well it works.

## error-free quantum computer

“We developed an algorithm for a fictitious, completely error-free quantum computer and ran it on a classical server simulating a ten Qbit quantum computer,” says Fabian Langkabel. The scientists limited their study to smaller molecules so that they could perform the calculations without a real quantum computer and compare them to classical calculations.

## Faster calculation

Indeed, the quantum algorithms produced the expected results. Unlike conventional calculations, however, quantum algorithms are also suitable for calculating much larger molecules with future quantum computers: “It has to do with computation times. They increase with the number of atoms that make up the molecule,” says Langkabel. While computation time multiplies with each additional atom for classical methods, this is not the case for quantum algorithms, making them much faster.

## Photocatalysis, light reception and more

The study thus shows a new way of calculating the densities of electrons and their “response” to light excitations in advance with a very high spatial and temporal resolution. This makes it possible, for example, to simulate and understand ultrafast decay processes, which are also crucial in quantum computers made of so-called quantum dots. Predictions about the physical or chemical behavior of molecules are also possible, for example during the absorption of light and the subsequent transfer of electrical charges. This could facilitate the development of photocatalysts for the production of green hydrogen with sunlight or help understand processes in light-sensitive receptor molecules in the eye.