Their calculations 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.

The findings offer researchers a new theoretical framework for future experiments and could improve scientists' understanding of quantum materials relevant to emerging technologies, including ultra-precise sensors and quantum computing.

A Quantum Droplet That Holds Itself Together

Lead author and Monash PhD candidate Sam Foster from the School of Physics and Astronomy said the results create opportunities to investigate entirely new 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."

These quantum droplets are fundamentally different from ordinary drops of liquid. Their stability comes from the unusual laws of quantum mechanics. An attractive force pulling the particles together is precisely counteracted by pressure produced by the fermions, keeping the droplet from collapsing.

Foster said the new work also addresses a theoretical problem that researchers have struggled with for years.

"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."

Quantum Droplets Could Be Tested in the Lab

Importantly, the calculations indicate that these predicted droplets could be produced using ultracold atom experiments that already exist. That means researchers may have a realistic path toward testing the prediction experimentally.

The team also found signs of additional unusual quantum behavior. Their results point to phenomena resembling the transition between a liquid and a gas, suggesting that these systems may contain a much broader and more complex range of quantum phases than previously recognized.

Foster said the implications could eventually reach beyond the field of atomic physics.

"Understanding how matter organizes itself under extreme quantum conditions gives us new tools for designing and controlling quantum systems. While this is fundamental research, discoveries like this often become the foundation for tomorrow's quantum technologies."

The study was conducted by Sam Foster, Associate Professor Jesper Levinsen and Professor Meera Parish from the Monash School of Physics and Astronomy, together with collaborators at Heidelberg University.

The paper, 'Quantum droplets in a resonant Bose-Fermi mixture', is published in Physical Review Letters.