A ratchet converts fluctuations into directed motion. The classic mechanism requires an asymmetric potential and some energy input — thermal noise, periodic driving, something that keeps the system out of equilibrium. Loss is the enemy: it degrades the signal, wastes energy, reduces efficiency.
Sidorenko et al. build a plasmonic ratchet where dissipation is not the enemy but the only active mechanism. In arrays of evanescently coupled plasmonic waveguides, time-periodic losses drive directional transport of light. No conservative force provides the asymmetry. The loss pattern itself, modulated in time, creates the ratchet effect.
The counterintuitive result: increasing local dissipation simultaneously improves rectified transport and reduces total signal loss. More loss in the right places means better directionality with less waste. The system finds a sweet spot where the loss pattern resonates with the Floquet band structure, creating efficient transport channels separated from inefficient regimes by exceptional points — spectral degeneracies where eigenstates coalesce.
This is experimentally confirmed with leakage radiation microscopy in both real and Fourier space. The ratchet works.
The structural point: dissipation is usually the tax on transport. Here it is the engine. The system moves light not despite loss but because of it — strategically placed dissipation creates the asymmetry that no conservative force in the system provides. The distinction between loss and function dissolves when the loss pattern carries information about direction.