Researchers from Monash University in Australia have described a previously unknown state of quantum matter in which two fundamentally different types of particles hold each other in a stable bond. This refers to so‑called quantum droplets formed by a mixture of bosons and fermions.

In particle physics, all objects are divided into two categories depending on the spin quantum number. Fermions, such as protons and electrons, form matter as such. Bosons, which include photons, act as carriers of interactions between fermions. It is extremely difficult to combine these two families in a single stable structure, since the Pauli exclusion principle does not allow fermions to occupy the same quantum state. It is this limitation that creates a kind of pressure that pushes the system outward from within. Scientists have shown that the attraction between particles can precisely compensate for this pressure.

Monash University PhD student Sam Foster noted that quantum systems can behave in ways that seem impossible in the everyday world, and these two very different types of particles can perfectly balance each other, creating a stable droplet that effectively holds itself together.

The research is based on an ansatz – a trial mathematical form of the wave function that physicists select in advance and then check for consistency with the equations. This approach allows them to bypass the impossibility of finding an exact solution to the many‑body problem. In addition to the model itself, the team proposed a roadmap for the experimental search for droplets. Previously, it was believed that in such boson–fermion mixtures, particles interact only very weakly, and any stronger bonds would destroy the system. New calculations indicate the opposite: the region of strong interaction turns out to be noticeably more stable than previously assumed. This fundamentally changes the assessment of the chances of observing such droplets in the laboratory.

The researchers note that the proposed scheme is within the capabilities of current setups. Modern experiments with ultracold atoms already make it possible to obtain the configurations necessary for the formation of such states. Such systems are cooled almost to absolute zero, where quantum effects manifest themselves on macroscopic scales. Calculations also predict that similar phenomena may occur in systems with strong light–matter coupling. The authors see practical applications in ultra‑precise sensors and next‑generation quantum computers.