Universal quantum computation on the power of quantum non-demolition   measurements
  In this paper we investigate the linear and nonlinear models of optical quantum computation and discuss their scalability and efficiency. We show how there are significantly different scaling properties in single photon computation when weak cross-Kerr nonlinearities are allowed to supplement the usual linear optical set. In particular we show how quantum non-demolition measurements are an efficient resource for universal quantum computation. 
Kae Nemoto and W. J. Munro