There is no classical algorithm for $n$-bit TQBF with better than $O(2^n)$ complexity. Is that also the best known bound for quantum algorithms / circuits?

Edit: As pointed out by Huck Bennett, in the full alternation subset of TQBF we can in fact beat $O(2^n)$ via randomized tree search. This is the case I care about most, so I would also be curious for the best exponent for a quantum algorithm if there are all $n-1$ quantifier flips.

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    $\begingroup$ I would be surprised if you can't get some speedup. Intuition: If you have no quantifier alternations you can get a $2^{n/2}$-time quantum algorithm via Grover, and if you have $n-1$ quantifier alternations you can get a $2^{0.793n}$-time classical algorithm, as noted in cstheory.stackexchange.com/a/5337/969. So it seems likely that there's a way to interpolate between the two regimes (although of course maybe this intuition is wrong). $\endgroup$ Commented Jan 26, 2021 at 16:42
  • $\begingroup$ Ah, very true, I was momentarily forgetting that alpha/beta works even in the worst case. I’m most curious about the full-alternation case, so I’ll edit appropriately. $\endgroup$ Commented Jan 26, 2021 at 20:06

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The best quantum algorithm for QBF on n variable formulas of size s runs in about $2^{n/2}poly(s)$ steps, regardless of quantifier alternations. This is due to a series of advances on quantum algorithms for game tree search in the early 2000s.

Here is the first reference I found from googling; there are more: https://arxiv.org/abs/0907.1623


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