Quantum algorithms for highly non-linear Boolean functions
Martin Rötteler · 2008
Attempts to separate the power of classical and quantum models of computation have a long history. The ultimate goal are exponential separations for computational (promise) problems, however, these do not come a dime a dozen: while there were some early successes in the form of hidden subgroup problems for abelian groups–which generalizes Shor’s factoring algorithm perhaps most faithfully–only for a handful of non-abelian groups efficient quantum algorithms were found. Recently, problems have gotten increased attention that seek to identify hidden sub-structures of other combinatorial and algebraic objects besides groups, notably the quantum algorithms by Childs, Schulman, and Vazirani for shifted subset problems for spheres over a finite field. In this paper we provide new examples for exponential separations by considering hidden shift problems that are defined for several classes of highly non-linear Boolean functions. These so-called bent functions arise in cryptography, where their property of having perfectly flat Fourier spectra on the Boolean hypercube gives them resilience against certain types of attack. We present quantum algorithms that solve the hidden shift problems for several well-known classes of bent functions in polynomial time with a constant number of queries, while the classical query complexity is shown to be exponential. Our approach uses a technique that exploits the duality between bent functions and their Fourier transforms. We also give a quantum algorithm that solves the hidden shift problem for quadratic forms that might be of independent interest. This algorithm can also be used to find approximating polynomials and shifts for functions with large Gowers U3 norm. 1 1