TR20-101 Authors: Uma Girish, Ran Raz, Wei Zhan

Publication: 7th July 2020 19:03

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The Forrelation problem, first introduced by Aaronson [AA10] and Aaronson and Ambainis [AA15], is a well studied computational problem in the context of separating quantum and classical computational models. Variants of this problem were used to give tight separations between quantum and classical query complexity [AA15]; the first separation between poly-logarithmic quantum query complexity and bounded-depth circuits of super-polynomial size, a result that also implied an oracle separation of the classes BQP and PH [RT19]; and improved separations between quantum and classical communication complexity [GRT19]. In all these separations, the lower bound for the classical model only holds when the advantage of the protocol (over a random guess) is more than $\approx 1/\sqrt{N}$, that is, the success probability is larger than $\approx 1/2 + 1/\sqrt{N}$. This is unavoidable as $\approx 1/\sqrt{N}$ is the correlation between two coordinates of an input that is sampled from the Forrelation distribution, and hence there are simple classical protocols that achieve advantage $\approx 1/\sqrt{N}$, in all these models.

To achieve separations when the classical protocol has smaller advantage, we study in this work the XOR of $k$ independent copies of (a variant of) the Forrelation function (where $k\ll N$). We prove a very general result that shows that any family of Boolean functions that is closed under restrictions, whose Fourier mass at level $2k$ is bounded by $\alpha^k$ (that is, the sum of the absolute values of all Fourier coefficients at level $2k$ is bounded by $\alpha^k$), cannot compute the XOR of $k$ independent copies of the Forrelation function with advantage better than $O\left(\frac{\alpha^k}{{N^{k/2}}}\right)$. This is a strengthening of a result of [CHLT19], that gave a similar statement for $k=1$, using the technique of [RT19]. We give several applications of our result. In particular, we obtain the following separations:

Quantum versus Classical Communication Complexity: We give the first example of a partial Boolean function that can be computed by a simultaneous-message quantum protocol with communication complexity $\mbox{polylog}(N)$ (where Alice and Bob also share $\mbox{polylog}(N)$ EPR pairs), and such that, any classical randomized protocol of communication complexity at most $\tilde{o}(N^{1/4})$, with any number of rounds, has quasipolynomially small advantage over a random guess. Previously, only separations where the classical protocol has polynomially small advantage were known between these models [G16, GRT19].

Quantum Query Complexity versus Bounded Depth Circuits: We give the first example of a partial Boolean function that has a quantum query algorithm with query complexity $\mbox{polylog}(N)$, and such that, any constant-depth circuit of quasipolynomial size has quasipolynomially small advantage over a random guess. Previously, only separations where the constant-depth circuit has polynomially small advantage were known [RT19].