Discrete Quantum Gravity Is Not Isometric
  We show that if a discrete quantum gravity is not classical, then it cannot be generated by an isometric dynamics. In particular, we show that if the quantum measure {\mu} (or equivalently the decoherence functional) is generated by an isometric dynamics, then there is no in- terference between events so the system describing evolving universes is classical. The result follows from a forbidden configuration in the path space of causal sets. 
Stan Gudder
gr-qc quant-ph