Quantization of Space-time Noncommutative Theory
  General aspects of the quantization of field theories non-local in time are discussed. The path integral on the basis of Schwinger's action principle and the Bjorken-Johnson-Low prescription, which helps to recover the canonical structure from the results of the path integral, are used as the main machinery. A modified time ordering operation which formally restores unitarity in field theories non-local in time is analyzed in detail. It is shown that the perturbative unitarity and the positive energy condition, in the sense that only the positive energy flows in the positive time direction for any fixed time-slice in space-time, are not simultaneously satisfied for theories non-local in time such as space-time noncommutative theory. 
Kazuo Fujikawa