Free massive fermions inside the quantum discrete sine-Gordon model
  We extend the notion of space shifts introduced by L. D. Faddeev and A. Yu. Volkov (Phys. Lett. B 315 (1993)) for certain quantum light cone lattice equations of sine-Gordon type at root of unity. As a result we obtain a compatibility equation for the roots of central elements within the algebra of observables (also called current algebra). The equation which is obtained by exponentiating these roots is exactly the evolution equation for the "classical background" as described in V. Bazhanov, A. Bobenko, N. Reshetikhin (Comm. Math. Phys. 175 (1996)).   As an application for the introduced constructions, a one to one correspondence between a special case of the quantum light cone lattice equations of sine-Gordon type and free massive fermions on a lattice as constructed in Destri and de Vega (Nucl. Phys. B 290 (1987)) is derived. 
Nadja Kutz