The anomaly-free quantization of two-dimensional relativistic string. I
  An anomaly-free quantum theory of a relativistic string is constructed in two-dimensional space-time. The states of the string are found to be similar to the states of a massless chiral quantum particle. This result is obtained by generalizing the concept of an ``operator'' in quantum field theory. 
S.N. Vergeles (The Landau Institute for Theoretical Physics, Russian   Academy of Sciences.)