New experiments designed to test the mysterious quantum realm

Mapping the quantum frontier, one layer at a time. Artist’s concept.

Researchers are designing new experiments to map and test the mysterious quantum realm.

A cardiac surgeon does not need to understand quantum mechanics to perform successful operations. Even chemists don’t always need to know these fundamentals to study chemical reactions. But for Kang-Kuen Ni, Morris Kahn Associate Professor of Chemistry and Chemical Biology and Physics, quantum caving is, like space exploration, a quest to discover a vast and mysterious new realm.

Today, much of quantum mechanics is explained by the Schrödinger equation, a sort of master theory that governs the properties of everything on Earth. “Even though we know that, in principle, quantum mechanics governs everything,” Ni said, “to actually see it is difficult and actually calculate it is almost impossible.

With a few well-reasoned assumptions and innovative techniques, Ni and his team can achieve the near-impossible. In their lab, they test current quantum theories about chemical reactions against real experimental data to get closer to a verifiable map of the laws that govern the mysterious quantum realm. And now with ultra cold chemistry – in which atoms and molecules are cooled to temperatures just above absolute zero where they become highly controllable – Ni and members of his lab have collected real experimental data from a hitherto unexplored quantum frontier, providing strong evidence for what the theoretical model is doing right (and wrong), and a roadmap for further exploration of the next shadow layers of quantum space.

“We know the underlying laws that govern everything,” Ni said. “But since almost everything on Earth is made up of three or more atoms or more, these laws quickly become far too complex to solve.”

Mapping the quantum frontier

Kang-Kuen Ni, right, and Matthew A. Nichols, a postdoc, conduct a hands-on consultation in their lab. Ni and his team are using ultra-cold chemistry to test quantum theory against real experimental data and create a verifiable map of the quantum laws that govern everything on earth. Credit: Jon Chase / Harvard Staff Photographer

In their study reported in Nature, Ni and his team set out to identify all the possible outcomes of the energy state, from start to finish, of a reaction between two molecules of potassium and rubidium – a reaction more complex than that previously studied. in the quantum domain. This is no small feat: at its most basic level, a reaction between four molecules has a considerable number of dimensions (the electrons revolving around each atom, for example, could be in an almost infinite number of locations simultaneously). This very high dimensionality makes it impossible to calculate all the possible reaction paths with current technology.

“Calculating exactly how energy is redistributed in a reaction between four atoms is beyond the power of today’s best computers,” Ni said. A quantum computer could be the only tool capable of performing such a complex calculation one day.

In the meantime, calculating the impossible requires some well-reasoned assumptions and approximations (choosing a location for one of these electrons, for example) and specialized techniques that give Ni and his team ultimate control over their reaction.

One of these techniques was another recent discovery from the Ni laboratory: in a study Posted in Nature Chemistry, she and her team exploited a reliable characteristic of molecules – their very stable nuclear spin – to control the quantum state of reacting molecules through to products. They also discovered a way to detect the products of a single collision reaction event, a difficult feat when 10,000 molecules could react simultaneously. Using these two new methods, the team was able to identify the unique spectrum and quantum state of each product molecule, the kind of precise control needed to measure the 57 pathways their potassium-rubidium reaction could take.

For several months during the COVID-19[female[feminine pandemic, the team conducted experiments to collect data on each of these 57 possible response channels, repeating each channel once per minute for several days before moving on to the next. Fortunately, once the experiment is set up, it can be performed remotely: lab members can stay at home, keeping the lab reoccupied to COVID-19 standards, while the system is running.

“The test,” said Matthew Nichols, postdoctoral researcher at the Ni Lab and author of the two papers, “indicates good agreement between the measurement and the model for a subset containing 50 pairs of states, but reveals significant deviations in several pairs of states. . “

In other words, their experimental data confirmed that previous predictions based on statistical theory (a far less complex one than Schrödinger’s equation) are correct – for the most part. Using their data, the team was able to measure the likelihood of their chemical reaction taking each of the 57 reaction channels. Then they compared their percentages with the statistical model. Only seven of the 57 showed a discrepancy large enough to challenge the theory.

“We have data that pushes that frontier,” Ni said. “To explain the seven divergent channels, we have to calculate the Schrödinger equation, which is still impossible. So now the theory must catch up and come up with new ways to efficiently perform these exact quantum calculations.

Next, Ni and his team plan to scale down their experiment and analyze a reaction between just three atoms (one molecule and one atom). In theory, this reaction, which has far fewer dimensions than a four-atom reaction, should be easier to calculate and study in the quantum domain. And yet, already, the team has discovered something strange: the middle phase of the reaction lasts many orders of magnitude longer than theory predicts.

“There is already a mystery,” Ni said. “It’s up to theorists now.”

Reference: “Statistical Dynamics Accuracy Test with State-to-State Ultra-Cold Chemistry” by Yu Liu, Ming-Guang Hu, Matthew A. Nichols, Dongzheng Yang, Daiqian Xie, Hua Guo and Kang-Kuen Ni, May 19, 2021, Nature.
DOI: 10.1038 / s41586-021-03459-6


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