Physicists Predict a New Quantum Droplet That Holds Itself Together

Researchers at Monash University have predicted an unusual new form of quantum matter that could overturn long-held assumptions about how ultracold particles behave. Their calculations, published in Physical Review Letters, suggest that under the right conditions, two fundamentally different classes of quantum particles — bosons and fermions — can combine to create stable, self-bound "quantum droplets." Scientists had previously considered such droplets unlikely to form in strongly interacting Bose-Fermi systems.

"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," said lead author Sam Foster, a Monash PhD candidate in the School of Physics and Astronomy. Unlike ordinary drops of liquid, the droplets' stability comes from quantum mechanics: an attractive force pulling the particles together is precisely counteracted by pressure produced by the fermions.

Foster said the work also solves a longstanding theoretical problem. "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." The calculations indicate the predicted droplets could be produced using ultracold atom experiments that already exist, giving researchers a realistic path toward testing the prediction.

The team also found signs of additional unusual quantum behaviour resembling the transition between a liquid and a gas, suggesting these systems may contain a broader range of quantum phases than previously recognised. The study was conducted by Foster, Associate Professor Jesper Levinsen and Professor Meera Parish of the Monash School of Physics and Astronomy, together with collaborators at Heidelberg University.