A Spin Chain from String Theory

We study the semiclassical spectrum of bosonic string theory on AdS_3 x S^1 in the limit of large AdS angular momentum. At leading semiclassical order, this is a subsector of the IIB superstring on AdS_5 x S^5. The theory includes strings with K spikes which approach the boundary in this limit. We show that, for all K, the spectrum of these strings exactly matches that of the corresponding operators in the dual gauge theory up to a single universal prefactor which can be identified with the cusp anomalous dimension. We propose a precise map between the dynamics of the spikes and the classical SL(2,R) spin chain which arises in the large-spin limit of N=4 Super Yang-Mills theory.