A diode passes current in one direction. It needs no energy to enforce the asymmetry — the semiconductor's band structure does the work. But in quantum mechanics, unidirectional transport typically requires either external driving (pumping energy into the system) or dissipation (losing energy to the environment). Coherent, energy-conserving, unidirectional quantum transport has not been straightforward to achieve.
Bilokon et al. (arXiv:2602.20508) demonstrate it using interacting bosons in an optical lattice with an asymmetric barrier. No driving. No dissipation. The directionality emerges from many-body interactions alone.
The mechanism: when bosons interact, the system's quantum state projects asymmetrically onto sectors of Hilbert space that support transport and sectors that forbid it. The interactions engineer an effective one-way boundary — an event horizon in Hilbert space. A quantum state can cross in one direction but not return, because the reverse process would require accessing Hilbert space sectors that the interactions have made inaccessible.
The analogy to black holes is precise at the formal level. An astrophysical event horizon is a boundary in spacetime that permits only inward passage. This Hilbert-space horizon is a boundary in the space of quantum states that permits only unidirectional transport. In both cases, the asymmetry is built into the geometry (of spacetime or of the interaction-engineered Hilbert space), not imposed by external forcing.
The practical result is coherent rectification — one-way quantum flow in atomtronic circuits, the quantum analog of electronic diodes. The bosons flow one way not because they are pushed but because the landscape of their interactions admits no return path.
What makes this conceptually interesting beyond the specific application is the inversion of how we think about irreversibility. In thermodynamics, irreversibility comes from many-body interactions through statistical mechanics — too many degrees of freedom to track, information lost to entropy. Here, many-body interactions produce irreversibility at the quantum level, without entropy, without decoherence, without losing information. The state is still pure. The evolution is still unitary. But the transport is one-way.
Interactions do not just make systems complex. They build doors, and some of those doors only open from one side.