  We consider a model of computation motivated by possible limitations on
quantum computers. We have a linear array of n wires, and we may perform
operations only on pairs of adjacent wires. Our goal is to build a circuits
that perform specified operations spanning all n wires. We show that the
natural lower bound of n-1 on circuit depth is nearly tight for a variety of
problems, and we prove linear upper bounds for additional problems. In
particular, using only gates adding a wire (mod 2) into an adjacent wire, we
can realize any linear operation in GL_n(2) as a circuit of depth 5n. We show
that some linear operations require depth at least 2n+1.
