Gaussian pure loss channels model photon attenuation — the basic physical process of light fading through a medium. Reversing this loss means reconstructing the original quantum state from the attenuated one. The question: when does reversal become impossible?
There is an exact phase boundary. Below a critical squeezing-to-thermal ratio, standard reverse prescriptions work at modest cost. Above it, conventional methods become infeasible — a sharp discontinuity, not a gradual degradation. One side of the boundary is easy, the other is hard, and the transition between them has zero width.
Above the boundary, an optimal reversal strategy still exists (the covariance-aligned generator), but the cost can be severe. And at the extreme: reversing a pure quantum state is dynamically unattainable — the cost diverges as you approach perfect reversal. You can get asymptotically close, never there.
The asymmetry is one-directional. Loss is easy. Gain is hard. Below the critical ratio, both are manageable. Above it, only loss remains cheap. The universe has a preferred direction for Gaussian channels, and it's the direction of erasure.
The phase boundary means there is no smooth interpolation between reversible and irreversible regimes. Quantum reversibility doesn't degrade gracefully — it falls off a cliff.