Implement a better method to reset qubits in pyqrack runtime.

[weinbe58]

          I need to make sure this will work in general. I know using `force_m` can create some issues. Lets leave it for now but I will revisit how to properly reset a qubit with pyqrack.

Originally posted by @weinbe58 in #207 (comment)

[david-pl]

@weinbe58 out of curiosity: what issues are there with using force_m? Would using an explicit matrix form of a projector work?

[cduck]

force_m() is the wrong operation. This is post-selecting for a specific measurement outcome, not resetting the qubit to a specific state. https://pyqrack.readthedocs.io/en/latest/autoapi/pyqrack/qrack_simulator/index.html#pyqrack.qrack_simulator.QrackSimulator.force_m This is why the simulator would sometimes crash because the |0> state had zero amplitude.

Instead of force_m() do this (pseudo-code):

if measure(qubit) == 1:
    x_gate(qubit)

[david-pl]

Ah, I see, I had the wrong idea about the method then. The projector has the same issue of course, but your code should do the trick, thanks