When “Leakiness” Helps: Physicists Predict a New Kind of Superfluid

Superfluids are the showboats of quantum physics: liquids that can flow without friction, as if the usual drag of the world has been switched off. Normally, though, superfluidity is a delicate trick. Add disturbance-heat, disorder, or loss-and the spell breaks. A new theoretical study from the Institute of Science Tokyo argues something more counterintuitive: in a certain quantum setting, loss and one-way motion don’t just fail to destroy superfluidity-they can actually stabilize a new superfluid phase, one that contains strange built-in “singularities” known as exceptional points. The team calls the predicted phase an “exceptional fermionic superfluid.” Superfluidity, but in the messy real world Most classic textbook quantum models assume a clean, sealed system-nothing leaks out, nothing preferentially flows one way, and the math behaves nicely. But many real experiments aren’t like that. Particles can be lost. Motion can be biased. The result is an “open” quantum system that must be described with a broader mathematical framework called non-Hermitian physics. Non-Hermitian systems have become famous for producing unusual effects, including exceptional points-special conditions where two energy levels and their corresponding quantum states collapse into one another. You can think of an exceptional point as a kind of quantum “knot” in the system’s behavior: not just a crossing, but a place where the identity of states becomes inseparable. Until now, exceptional points in superfluid theories typically showed up near the breaking point -at the boundary where superfluidity fails. The new work claims something bolder: exceptional points can live deep inside a stable superfluid phase, coexisting with the frictionless flow rather than marking its collapse. The key ingredient: “spin depairing” To reach that result, the researchers studied a standard workhorse model in condensed-matter physics-the attractive Hubbard model -but in a non-Hermitian form. Their twist is a specific kind of dissipation they call spin depairing , where particles with opposite “spins” preferentially hop in opposite directions. In plain terms: the system is set up so that the two partners that normally pair up to create a superfluid are being pushed to move with a built-in directional mismatch. That sounds like it should wreck pairing. Instead, the team finds it can do the opposite-under the right conditions, the same mechanism actively supports a robust superfluid state that is unique to the non-Hermitian setting. “We studied the NH attractive Hubbard model to find that spin depairing stabilizes the superfluid state unique to the NH system, which we refer to as the exceptional fermionic superfluidity,” says physicist Akihisa Koga, one of the study’s authors. “This phase is characterized not only by a finite order parameter but also by the emergence of EPs in the momentum space.” Translated: the model still shows the telltale signature of superfluidity-paired particles forming a collective frictionless state-but now the superfluid’s “map” of possible motions includes exceptional points as an intrinsic feature. Why geometry matters: 2D vs 3D Another headline result is that the phase’s stability depends strongly on the underlying geometry-the “lattice” the particles effectively live on. In a two-dimensional square lattice,...

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