Physicists Predict New Self-Bound Quantum Droplet State of Matter

Researchers at Monash University have predicted a new type of quantum matter that challenges decades of thinking about how ultracold particles behave. The theoretical study, published in the journal Physical Review Letters, shows that under the right conditions, mixtures of bosons and fermions—two fundamentally different types of quantum particles—can form stable, self-bound "quantum droplets." Scientists had believed such droplets were unlikely to exist in strongly interacting boson-fermion mixtures.

Unlike an ordinary liquid droplet, which holds together through surface tension, a quantum droplet exists because of the rules of quantum mechanics: an attractive force between the particles is exactly balanced by pressure generated by the fermions, preventing the system from collapsing. "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," said lead author Sam Foster, a Monash PhD candidate in the School of Physics and Astronomy.

Foster said earlier theories could describe such boson-fermion systems only when the particles interacted weakly. "Our new approach lets us explore what happens when those interactions become much stronger, which is where the most interesting physics emerges," he said. Beyond predicting the droplets, the researchers found evidence of a transition resembling the change between a liquid and a gas, pointing to a richer landscape of quantum phases than expected.

The team, which included Associate Professor Jesper Levinsen and Professor Meera Parish from Monash's School of Physics and Astronomy along with collaborators from Heidelberg University, found that the predicted droplets should be achievable using existing ultracold-atom experiments, making experimental confirmation a realistic next step.