Although I am no expert, I think your quantum computing comments are incorrect. To explore branches, retaining all histories, you need a "nondeterministic" computer that branches freely. This gives an exponential (2^n) speedup over a classical computer. Quantum computers apparently give only a polynomial one. For more detail, check out Scott Aaronson's blog "Schtetl-Optimized": http://scottaaronson.com/blog/