Add support for initializing Simulator to a specific state about projectq

[thomashaener]

So by state you mean individual amplitudes, rather than initializing to a classical bit-string?

[Strilanc]

Yes.

An alternative approach, for this use case, is a "PermutationSimulator" which only allows classical gates and then spits out the final permutation.

[thomashaener]

Ok the idea was actually to use the cheat function for this. Unfortunately, the default pybind export of std::vector performs a copy each time a switch between Python and C++ is made, which is why this currently does not work. At some point, we should implement a custom version of this and update the C++ export.

[thomashaener]

Oh and we'll definitely need a permutation simulator.

[hsim13372]

I'm currently working on testing out different initial states for the VQE so being able to initialize the ProjectQ Simulator to a specific state would be very useful!

[thomashaener]

I'll add an extra function over the weekend since I currently don't see a quick fix for the pybind issue with cheat.

[damiansteiger]

This feature has been added to the Simulator in the develop branch, see
http://projectq.readthedocs.io/en/latest/projectq.backends.html#projectq.backends.Simulator.set_wavefunction

Thanks @thomashaener