1. T. D. Newton and E. P. Wigner, Rev. Mod. Phys. 21, 400 (1949).
2. A. S. Wightman, Rev. Mod. Phys. 34, 845 (1962); T. O. Philips, Phys. Rev. 136, B893 (1964); W. O. Amrein, Helv. Phys. Acta 42, 149 (1969); S. T. Ali and G. G. Emch, J. Math. Phys. 15, 176 (1974).
3. For a recent review article on this subject, see S. T. Ali, Riv. Nuovo Cimento 8, No. 11 (1985).
4. C. K. Hong and L. Mandel, Phys. Rev. Lett. 56, 58 (1986).
5. D. Han, Y. S. Kim, and M. E. Noz, Phys. Rev. A 35, 1682 (1987).