Quantum computing provides new insight into photochemical processes research

Quantum computing has provided new insights into a fundamental aspect of photochemical reactions that previously proved difficult to study. The findings could improve scientists’ understanding of light-driven processes such as photosynthesis, smog formation and ozone destruction.

Photochemical processes occur when atomic nuclei and their electrons assume different shapes after a photon has been absorbed. Some of these interactions are guided by a quantum phenomenon called conic crossing, in which the potential energy surfaces that describe a molecule in its ground and excited states converge. In these cases, quantum mechanical interference can prevent certain molecular transitions from occurring, a limitation known as geometric phasing. This limits the path a reaction can take and affects the outcome of the reaction. The geometry has been known since the 1950s, but due to femtosecond timescales, it has not been directly observed in a molecular system.

Now, two research teams working independently of each other have shown how the geometrical phase can be measured using quantum simulators.

“It is very difficult to see this phase in real particles because it occurs far from the ground state and requires a clean quantum state with little thermal interference,” says Kenneth Brown, a quantum systems researcher from Duke University in North Carolina, US.

“We’ve built a quantum system that has some properties of the system we want to study,” Brown says. Designing this quantum simulator allowed the Duke team to measure the effect “in a much easier-to-read time scale,” Brown says.

The researchers used a laser to manipulate a series of five trapped ytterbium ions in a way that mimics the quantum behavior of atoms at a conical junction. Since the quantum dynamics of trapped ions is much slower than the dynamics of a molecule, the team was able to measure how the geometric phase directly affects the spatial distribution of the ion’s wavefunction.

“Our experiment is one of the first demonstrations of how to perform electronic vibrational coupling of trapped ions,” Brown says. He notes that understanding the engineering phase could provide chemists with “another way to control” which products are made during multiple product reactions.

The Duke team’s findings are published alongside similar work led by researchers at the University of Sydney, Australia. The group, led by Yvan Casale, used an analog quantum simulator based on a captured ytterbium ion.

“One of the most important things here is that we were able to observe, in real time, the geometric phase interference in this system that behaves at the same speed as the molecular system,” says Vanessa Agudelo, a doctoral student in Casale’s lab. I worked on the project.

engineering stage

The quantum computer allowed them to slow down the chemical dynamics of the system they were studying from femtoseconds to milliseconds, allowing for meaningful observations.

“Here you have a real video of a single atom splitting in half, devastatingly interfering with itself when it goes on the other side of the conic cross,” Casale explains. “This simulates the entry of a photon and how the molecule interacts on femtosecond timescales.”

“And this is especially important in things like atmospheric chemistry — why does smog form?” How is the ozone layer formed? Or how is it destroyed? he adds.

Despite the different technical approaches taken by the two teams, their findings are consistent. Casale notes that this work highlights how quantum computing can help solve complex chemistry problems. “The goal of using quantum computers in chemistry is to be able to simulate any kind of chemical process… like drug discovery, or better materials discovery,” he says.

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