Hello. I thought that simply calling the exp_U_first with U = X would do but it doesn’t. The circuit does not product the right amplitudes. Any hints?
Hi @qreg, the circuits from the previous H.6 exercises implement the first/second approximations to e^{tU} for U a unitary. But here, we are trying to implement e^{itX}; so the input to exp_U_first needs to be adjusted slightly to take into account the different prefactor.
Feel free to let us know if there’s anything else we can clarify! 
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Thank you! I have finally solved it.