Researchers predict that strongly interacting bosons and fermions can form stable quantum droplets that may be testable with current experiments. Two very different kinds of quantum particles may be able to join together in a stable form of matter that physicists once considered unlikely. Researchers at Monash University predict that ultracold bosons and fermions can combine under the right conditions to create self-bound “quantum droplets.” Bosons and fermions follow fundamentally different quantum rules, yet the new theoretical work indicates that strongly interacting mixtures of the two can remain bound together. The result challenges the long-held expectation that stable droplets would be difficult to form in strongly interacting Bose-Fermi systems. The prediction gives experimental physicists a new state of matter to search for and could improve understanding of quantum materials relevant to technologies ranging from ultra-precise sensors to quantum computing. Lead author and Monash PhD candidate Sam Foster from the School of Physics and Astronomy said the findings make it possible to investigate previously unexplored quantum states. “Quantum systems can behave in ways that seem impossible in our everyday world. We’ve shown that these two very different types of particles can balance each other perfectly to create a stable droplet that effectively holds itself together.” Quantum pressure prevents the droplets from collapsing A quantum droplet is held together differently from an ordinary liquid droplet. Here, attraction between the particles is balanced by pressure generated by the fermions, stopping the system from collapsing even as the particles bind together. Sam said the new theoretical framework addresses a long-standing limitation in describing these systems. “Previous theories could only describe these systems when the particles interacted relatively weakly. Our new approach lets us explore what happens when those interactions become much stronger, which is where the most interesting physics emerges.” Existing experiments could test