Researchers reveal microscopic quantum correlations of ultracold molecules

Newswise – Physicists are increasingly using ultracold molecules to study the quantum state of matter. Many researchers argue that molecules have advantages over other methods, such as trapped ions, atoms or photons. These advantages suggest that molecular systems will play an important role in emerging quantum technologies. But, for a while now, the research of molecular systems has progressed so far due to the long-term problems of preparing, controlling and observing molecules in the quantum regime.

Now, as stated in a study published this week on Nature, Princeton researchers have achieved great success by microscopically studying molecular gases on a scale that has never been achieved by previous research. The Princeton team, led by Waseem Bakr, assistant professor of physics, was able to cool the molecules to extremely cold temperatures, putting them inside an artificial crystal of light known as ” optical lattice”, and studied their collective quantum behavior with high spatial resolution. each molecule could be seen.

“We fixed the molecules in the gas in a well-defined internal and vibrating quantum field. The strong interactions between the molecules produced subtle quantum interactions that we were able to detect with the first time,” said Bakr.

This experiment has profound implications for basic physics research, such as many-body physics, which looks at the emergent behavior of ensembles of quantum particles. The research could also accelerate the development of large-scale quantum computing systems.

In the effort to create large-scale quantum systems, for quantum computing and general scientific applications, researchers have used a variety of different alternatives – everything from trapped ions and atoms to electrons trapped in “quantum dots.” The goal is to convert these different types of particles into what are called qubits, which are the building blocks of a quantum computer system.

Although so far no single type of qubit has emerged as a precursor, Bakr and his team believe that molecular systems, although less explored than other planets, have some promise.

Another important advantage of using molecules in experimental settings—especially as potential qubits—is the fact that molecules can store quantum information in many new ways that are not yet possible. found in single atoms. For example, even for a simple molecule made of only two atoms, which can be seen as a small dumbbell, quantum information can be stored in the rotational motion of the dumbbell or the motion of the constituent atoms its related to others. Another advantage of molecules is that they often have long-range interactions; they can interact with other molecules, many distances apart in optical light, while atoms, for example, can only interact if they stay in the same place.

When using molecules to study many-body physics, it is expected that these advantages will help researchers explore new fascinating aspects of matter in this artificial system. However, the biggest problem, which Bakr and his team were able to overcome in this experiment, is the microscopic appearance of these quantum states.

“The ability to analyze gas at the level of individual molecules is a new area of ​​our research,” said Bakr. “When you can look at individual molecules, you can get a lot of information about a multi-body system.”

What Bakr means by extracting more information is the ability to observe and record the subtle relationships that characterize molecules in the quantum realm—for example, the relationships between their positions in a lattice or their orbital positions. .

“Researchers had prepared molecules in the ultracold regime before, but they couldn’t measure their interactions because they couldn’t see individual molecules,” said Jason Rosenberg, the student who discovered degrees in Princeton’s Department of Physics and co-author. paper. “By seeing each molecule, we can better identify and analyze the different quantum phases that are expected to appear.”

Although researchers have been studying many-body physics with atomic quantum gases for more than two decades, molecular quantum gases have been more difficult to control. Unlike atoms, molecules can store energy by vibrating and rotating in many different ways. These various excitations are known as “degrees of freedom”—and their large number is a characteristic that makes molecules difficult to manipulate and manipulate experimentally.

“In order to study molecules in the quantum regime, we need to control all their degrees of freedom and place them in a well-defined quantum mechanical environment,” said Bakr.

The researchers achieved this precise level of control by first cooling the two atomic gases of sodium and rubidium to extremely low temperatures measured in nanokelvins, or billionths of a degree Kelvin. At these very cold temperatures, each of the two gases transitions to a state known as the Bose-Einstein condensate. In this ultracold environment, researchers combine atoms to form sodium-rubidium molecules in a well-defined quantum state. Then they use lasers to transfer the molecules to their ground state where all rotation and vibration of the molecules is frozen.

To preserve the quantum behavior of molecules, they are isolated in a vacuum chamber and captured by an optical beam made of standing waves of light.

“We interfere with a set of laser beams together and, from this, we create a corrugated surface that resembles an “egg box” in which the molecules sit,” he said. Rosenberg.

In the experiment, the researchers captured about a hundred molecules in the “egg box”. Then the researchers pushed the system out of equilibrium—and followed what happened in the dynamic interaction system.

Lysander Christakis, a graduate student and lead author of the paper, said: “We gave the system a sudden ‘action’. We allowed the molecules to interact and create a quantum entanglement. This confusion manifests itself in subtle relationships, and the ability to examine the system at this subtle level allows us to uncover these relationships—and learn from them.”

Seizures are one of the most fascinating and perplexing aspects of the body. It describes a part of the subatomic world where quantum particles—whether molecules, electrons, photons, or whatever—are inextricably bound together no matter how far apart they are. Scaling is very important in quantum computing because it works like a form of multiplication. It is an important combination based on exponential speed to solve problems with quantum computers.

The unprecedented control that researchers have achieved in preparing and detecting molecules has clear implications for quantum computing. But the researchers emphasize that, in the end, the experiment is not necessarily about creating the most advanced qubits. Rather, it is, most importantly, a major step forward in basic physics research.

“This research opens up many opportunities to study really interesting problems in many-body physics,” said Christakis. “What we have demonstrated here is a complete platform for using ultracold molecules as a means of studying complex quantum phenomena.”

Rosenberg agreed. “In this experiment, the molecules were frozen in one place of the fabric and the quantum information was stored only in the rotating parts of the molecules. Going forward, it will be interesting to explore another scene interesting things that happen when you let molecules “jump” from one place to another. Our research has opened up the possibility of investigating unusual situations that can be repaired with these molecules. , and now we can identify them very well,” he concluded.

Other members of the Princeton team are graduate student Ravin Raj; postdoctoral research assistant Zoe Yan; undergraduate Sungjae Chi; and theorists Alan Morningstar, postdoctoral fellow at Stanford University, and David Huse, Cyrus Fogg Brackett Professor of Physics at Princeton. The research was supported by the National Science Foundation and the David and Lucile Packard Foundation.

Lecture, “Exploring spatially resolved correlations in the spin system of ultracold molecules,” by Lysander Christakis, Jason S. Rosenberg, Ravin Raj, Sungjae Chi, Alan Morningstar, David A. Huse, Zoe Z. Yan, and Waseem S .Bakr was published online by Natureon February 1, 2023. DOI: 10.1038/s41586-022-05558-4.

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