Energy translation and Proper-Time Eigenstates
  The usual quantum mechanics describes the mass eigenstates. To describe the proper-time eigenstates, a duality theory of the usual quantum mechanics was developed. The time interval is treated as an operator on an equal footing with the space interval, and the quantization of the space-time intervals between events is obtained. As a result, one can show that there exists a zero-point time interval. 
Zhi-Yong Wang, Cai-Dong Xiong, Ai-Lin Zhang