# In QAOA, why do we pick the initial Hamiltonian $B$ to be $\sigma_x$ applied to each qubit?

In QAOA 1, why do we pick the initial Hamiltonian $$B$$ to be $$\sigma_x$$ applied to each qubit? Would it be possible to pick $$B$$ to be an application of $$\sigma_z$$'s instead? Then $$C$$ and $$B$$ would be both diagonal in the Z-basis. What is preventing us from taking this choice for $$B$$? Thanks in advance!

We don't really need $$B = \sum \sigma_j^x$$ in our QAOA algorithm. As long as you pick it in such a way that it doesn't commute with $$C$$. One of the reason is if they are commute, then they share a common eigenvector. Then if you run into this type of situation, you will never get out, and you will be stuck in this state. You can think of $$U(\beta, B)$$ as a driver, it helps to navigate the Ansatze from getting stuck.
In term of the reason why $$B = \sum \sigma_j^x$$ in the first place is because QAOA is sort of a discretization of Quantum Annealing so that is why we see that the Ansatze of QAOA takes the form: $$U = e^{-i\beta_p B}e^{-i \gamma_p C} \cdots e^{-i\beta_1 B} e^{-i \gamma_1 C} = \prod_{i} e^{-i\beta_i B} e^{-i \gamma_i C}$$ which is a trotter approximation of the time evolution in the quantum annealing.
• I always thought that B and C do commute because we simulate $H(t) = (1 - s(t)) B + s(t) C$ as $e^{-i (1 - s (j\Delta t )) B\Delta t - is(j \Delta t ) C\Delta t }=e^{-i (1 - s (j\Delta t )) B\Delta t } e^{-is(j \Delta t ) C\Delta t }$ . Is that then not right? – Hannah Nov 28 '20 at 11:52