Quantizing Open Spin Chains with Variable Length: an example from Giant   Gravitons
  We study an XXX open spin chain with variable number of sites, where the variability is introduced only at the boundaries. This model arises naturally in the study of Giant Gravitons in the AdS/CFT correspondence. We show how to quantize the spin chain by mapping its states to a bosonic lattice of finite length with sources and sinks of particles at the boundaries. Using coherent states, we show how the Hamiltonian for the bosonic lattice gives the correct description of semiclassical open strings ending on Giant Gravitons. 
David Berenstein, Diego H. Correa, Samuel E. Vazquez