For decades, physicists believed that mixtures of two fundamentally different quantum particles, bosons and fermions, couldn’t form stable droplets when interacting strongly. Bosons are particles that like to crowd together (photons, for example), while fermions obey the Pauli exclusion principle, meaning no two can occupy the same state (like electrons). Their behaviors are so different that combining them into a single, self-bound state seemed impossible. Researchers at Monash University have now refuted that assumption. The study, led by PhD candidate Sam Foster, predicts that under the right conditions, bosons and fermions can form stable, self-bound ‘quantum droplets’. The droplets are held together not by normal forces but by the fine balance of quantum mechanics itself. A quantum droplet differs from a drop of water, held together by surface tension, in that it results from a balance between attraction and quantum pressure: the bosons and fermions attract one another, but the fermions produce a sort of pressure that stops collapse. What is obtained in this way is a perfectly balanced and self-contained droplet. Foster explained: “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.” Physicists accurately measured the mass of the W boson Previous theories could only describe Bose-Fermi mixtures when the particles interacted weakly. But the Monash team developed a new approach that works in the strongly interacting regime, precisely where the most exotic physics emerges. This advance goes beyond predicting droplets, as it also shows a wide variety of quantum phases, some of which exhibit behavior similar to the liquid-gas transition. The prediction is not just theoretical speculation. The researchers argue that these droplets should be achievable in existing ultracold