An international team of scientists from the Tokyo Institute of Science has discovered a fundamentally new quantum state: a stable fermionic superfluid with internal exceptional points. The results were published in the journal Physical Review Letters (PRL).

What exceptional points are and why they matter

In conventional quantum mechanics, energy levels and the corresponding states are described by Hermitian operators. Their eigenvalues are always real, and the states are orthogonal. In open quantum systems, where particle loss or directional asymmetry is possible, the equations become non-Hermitian. In such systems, exceptional points can arise. These are special conditions under which several energy levels and their corresponding wave functions merge simultaneously.

Previously, it was believed that exceptional points appear only at phase transition boundaries, for example when a superfluid state is destroyed by an external influence. The new study shows that this is not necessarily true. Exceptional points can be an intrinsic internal property of the superfluid phase itself.

How the new phase was discovered

The researchers studied a non-Hermitian version of the attractive Hubbard model with broken spin-pair symmetry. In this model, particles with opposite spins preferentially move in different directions, which makes dissipation asymmetric. Instead of destroying Cooper pairs, this spin-dependent imbalance stabilizes a new type of superfluid state.

Calculations showed that on a square lattice, exceptional superfluidity appears even for arbitrarily weak attractive interactions. In three dimensions, exceptional points form lines in momentum space, while in two dimensions they remain isolated points.

The project leader, Associate Professor Akihisa Koga, explained that this new phase is characterized not only by a finite order parameter, like ordinary superfluidity, but also by exceptional points embedded directly in momentum space. He emphasized that this feature fundamentally distinguishes it from previously known non-Hermitian superfluid states.

Experimental realization

The authors believe that this phase can be realized experimentally using ultracold fermionic gases such as lithium-6 or potassium-40. In such systems, scientists can precisely control particle losses and engineer the required forms of dissipation.

The discovery shows that dissipation does not have to be a purely destructive factor. Instead, it can serve as a powerful tool for creating new quantum states of matter. This opens a new direction in the study of nonequilibrium, strongly correlated quantum systems.

In brief

Physicists from the Tokyo Institute of Science have discovered a stable superfluid phase with internal exceptional points, which are singularities where energy levels and quantum states merge. This is the first known phase in which such points are an intrinsic property of the phase itself rather than markers of a phase transition boundary. The phase is stabilized by spin-dependent asymmetric dissipation and can potentially be realized in experiments with ultracold atoms. The discovery changes the understanding of the role of dissipation in quantum matter and opens a new research direction.