Study of a self-adjoint operator indicating the direction of time within   standard quantum mechanics
  In [J. Math. Phys. 51 (2010) 022104] a self-adjoint operator was introduced that has the property that it indicates the direction of time within the framework of standard quantum mechanics, in the sense that as a function of time its expectation value decreases monotonically for any initial state. In this paper we study some of this operator's properties. In particular, we derive its spectrum and generalized eigenstates, and treat the example of the free particle. 
Y. Strauss, J. Silman, S. Machnes, L.P. Horwitz
quant-ph math-ph math.MP