A Multilevel Blocking Approach to the Sign Problem in Real-Time Quantum   Monte Carlo Simulations
  We propose a novel approach toward the general solution of the sign problem in real-time path-integral simulations. Using a recursive multilevel blocking strategy, this method circumvents the sign problem by synthesizing the phase cancellations arising from interfering quantum paths on multiple timescales. A number of numerical examples with one or a few explicit degrees of freedom illustrate the usefulness of the method. 
C.H. Mak and R. Egger