Hybrid divide-and-conquer approach for tree search algorithms : possibilities and limitations

Abstract

One of the challenges of quantum computers in the near- and mid- term is the limited number of qubits we can use for computations. Finding methods that achieve useful quantum improvements under size limitations is thus a key question in the field. In this vein, it was recently shown that a hybrid classicalquantum method can help provide polynomial speed-ups to classical divide-andconquer algorithms, even when only given access to a quantum computer much smaller than the problem itself. In this work, we study the hybrid divide-andconquer method in the context of tree search algorithms, and extend it by including quantum backtracking, which allows better results than previous Groverbased methods. Further, we provide general criteria for threshold-free polynomial speed-ups in the tree search context, and provide a number of examples where polynomial speed ups, using arbitrarily smaller quantum computers, can be obtained. We provide conditions for speedups for the well known algorithm of DPLL, and we prove threshold-free speed-ups for the PPSZ algorithm (the core of the fastest exact Boolean satisfiability solver) for well-behaved classes of formulas. We also provide a simple example where speed-ups can be obtained in an algorithm-independent fashion, under certain well-studied complexitytheoretical assumptions. Finally, we briefly discuss the fundamental limitations of hybrid methods in providing speed-ups for larger problems.