Physicists Predict Bosons and Fermions Can Bind Into Stable 'Quantum Droplets'

Physicists have theoretically predicted that two fundamentally different kinds of quantum particles — bosons and fermions — can bind together into a stable droplet of matter under extreme, strongly interacting conditions. Researchers at Monash University say the finding challenges the long-held expectation that such droplets would be difficult to form in strongly interacting Bose-Fermi mixtures.

Bosons and fermions are the two basic categories of quantum particle, yet the new work shows a strongly interacting mixture of the two can still remain bound together in a self-bound "quantum droplet." Unlike an ordinary liquid droplet, it holds itself together because attraction between the particles is balanced by pressure generated by the fermions, which stops the system from collapsing. "Quantum systems can behave in ways that seem impossible in our everyday world," said lead author Sam Foster, a PhD candidate in Monash University's School of Physics and Astronomy. "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."

Previous theories could only describe these systems when the particles interacted relatively weakly, Foster said. "Our new approach lets us explore what happens when those interactions become much stronger, which is where the most interesting physics emerges." The team also found signs of quantum behavior resembling the transition between a liquid and a gas, suggesting the system can support a wider and more complex range of quantum phases than the droplets alone. Understanding how matter organizes itself under extreme quantum conditions, Foster added, gives new tools for designing and controlling quantum systems.

The calculations indicate the droplets should be possible to create using ultracold atom experiments that already exist, making experimental verification a realistic next step. The work, "Quantum Droplets in a Resonant Bose-Fermi Mixture," by Foster with Olivier Bleu, Jesper Levinsen and Meera M. Parish, was published in Physical Review Letters on 14 August 2026.