Formation of bosonic $^{23}$Na$^{41}$K Feshbach molecules

In the coldest reaches of the laboratory, where temperatures drop to within a hair's breadth of absolute zero, atoms lose their individual chaos and begin to move as a single, unified wave. This is the realm of ultracold quantum gases, a field where scientists manipulate matter to reveal its most fundamental behaviors. Among the most exciting tools in this domain are Feshbach molecules, which are pairs of atoms held together not by the strong chemical bonds that make up water or air, but by a delicate, tunable magnetic force. By adjusting an external magnetic field, researchers can dial the strength of the attraction between atoms, turning them from free-floating particles into weakly bound pairs. These molecules are a crucial stepping stone. If scientists can cool them further and lock them into their lowest possible energy state, they become stable, strongly interacting particles with a permanent electric dipole. Such particles could act as tiny magnets that talk to one another over long distances, offering a new platform to simulate complex materials, test the laws of physics, and build future quantum technologies.

A team of researchers at Pohang University of Science and Technology in South Korea has now taken a significant step toward this goal by creating a specific type of these ultracold molecules using sodium and potassium atoms. While previous experiments have successfully created similar molecules using other combinations of elements, this group focused on a bosonic version of sodium-23 and potassium-41. This specific pairing is highly prized because the atoms involved have properties that make them exceptionally good candidates for forming large, stable clouds of quantum matter. The team's primary achievement was the successful creation of these molecules and a detailed map of how long they survive before falling apart, providing a clear roadmap for future experiments aiming to create a fully quantum-degenerate gas of these particles.

The process began with a mixture of ultracold sodium and potassium atoms trapped in a beam of laser light. The atoms were cooled to temperatures as low as 170 nanokelvins, a state where they move with extreme slowness. To bind them together, the researchers applied a specific magnetic field of about 73.6 gauss, a setting where the atoms are naturally inclined to pair up. They then used a precise burst of radio waves to gently nudge the atoms from their free state into a weakly bound molecular state. This technique, known as radio-frequency association, acts like a tuning fork that matches the energy difference between the free atoms and the new molecule, allowing the pair to snap together. In a single pulse, the team managed to create over 10,000 of these sodium-potassium molecules from the thermal cloud.

Once the molecules were formed, the researchers needed to understand their properties to ensure they could be used for further cooling. They measured the energy holding the molecules together by scanning the radio frequency and observing when the molecules appeared. This allowed them to pinpoint the exact magnetic field where the pairing occurs and to determine the width of this resonance, which turned out to be quite broad at 5.1 gauss. A broad resonance is a valuable feature because it means the scientists can tune the interactions between the atoms over a wide range without needing to hit a single, impossibly precise magnetic field value. This robustness makes the system much easier to control for future experiments.

The stability of these molecules is perhaps the most critical factor for their future use. The researchers measured how long the molecules lasted in the trap before colliding with other atoms and breaking apart. When the molecules were surrounded by the original mixture of sodium and potassium atoms, they survived for more than 2 milliseconds. However, when the researchers removed the leftover sodium atoms from the trap, the molecules lived significantly longer, surviving for about 7 milliseconds. This increase in lifetime is vital because it gives scientists enough time to perform the next steps of their experiment, such as transferring the molecules to their lowest energy state using laser pulses. The team found that this lifetime remained consistent across different magnetic field settings, suggesting the molecules are inherently stable against the types of collisions that usually destroy them.

To understand exactly how these molecules were formed and lost, the team developed a mathematical model that tracked the number of molecules and atoms over time. They found that the creation of the molecules was limited primarily by how fast the radio waves could bind the atoms together, rather than by the molecules immediately falling apart. Their model showed that the molecules were lost mostly when they collided with leftover sodium atoms, and that this loss happened much more frequently than collisions with potassium atoms. By adjusting the ratio of sodium to potassium atoms in the trap, they optimized the process to create the maximum number of molecules, reaching a peak of 11,000. This quantitative understanding allows them to predict exactly how many molecules they will get under different conditions, removing the guesswork from the process.

These results represent a solid foundation for the next phase of research. The team has demonstrated that they can produce a large number of these specific molecules with a high density, a prerequisite for cooling them further into a quantum degenerate gas. Because the molecules are stable for several milliseconds, there is ample time to apply the complex laser techniques needed to move them into their absolute ground state, where they become chemically stable and ready for advanced quantum simulations. With this successful creation and characterization of bosonic sodium-potassium molecules, the path is now clear to explore the rich physics of strongly interacting dipolar gases, potentially leading to new discoveries in how quantum matter behaves on a large scale.