We propose a mechanism for coherent densification in a tilted optical lattice. The starting point is a well-known property of Wannier-Stark ladders: a homogeneous resonant coupling between neighboring Wannier-Stark states restores coherent translation. When the coupling amplitude is position dependent and has a node, the local transport velocity vanishes at the node. A single wave packet is thereby transported toward this point, while a cloud distributed around the node undergoes a reduction of its spatial extent. The aim is to provide a reversible density-redistribution tool in a lattice of fixed period, complementary to matter-wave lenses, accordion lattices, box traps, and coherent-transport protocols. We introduce a nearest-neighbor effective model in which the coupling varies linearly with the site index. This model admits an exact analytic solution via a generating function and the method of characteristics; in the continuum limit it predicts exponential contraction of distances to the node. We discuss a primary implementation based on a spatially shaped Raman or Bragg coupling, and an alternative based on temporal modulation of the lattice depth. Estimates for cesium indicate that densification times ranging from a few tens to a few hundreds of milliseconds are achievable with realistic parameters. The mechanism is not dissipative cooling, but a coherent compression step that can be halted, reversed, or followed by a controlled expansion stage.

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• Cold gases in optical lattices