Quantum Algorithms for Highly Structured Search Problems

Tad Hogg

A quantum algorithm for a class of highly structured combinatorial search problems is introduced. This algorithm finds a solution in a single step, contrasting with the linear growth in the number of steps required by the best classical algorithms as the problem size increases, and the exponential growth required by classical and quantum methods that ignore the problem structure. In some cases, the algorithm can also guarantee that insoluble problems, in fact, have no solutions, unlike previously proposed quantum search algorithms.

Lov K. Grover

Lov K. Grover

Tad Hogg

The structure of satisfiability problems is used to improve search algorithms for quantum computers and to reduce their required coherence times by using only a single coherent evaluation of problem properties. The structure of random k-SAT allows the determination of the asymptotic average behavior of these algorithms, showing they improve on the quantum algorithms, such as amplitude amplification, that ignore detailed problem structure but remain exponential for hard problem instances. Compared to good classical methods, the algorithm performs better, on average, for weakly and highly constrained problems, but worse for hard cases. The analytic techniques introduced here apply also to other quantum algorithms, supplementing the limited evaluation possible with classical simulations, and showing how quantum computing can use the ensemble properties of NP search problems.

Tad Hogg

A previously developed quantum search algorithm for solving 1-SAT problems in a single step is generalized to apply to a range of highly constrained k-SAT problems. We identify a bound on the number of clauses in satisfiability problems for which the generalized algorithm can find a solution in a constant number of steps as the number of variables increases. This performance contrasts with the linear growth in the number of steps required by the best classical algorithms, and the exponential number required by classical and quantum methods that ignore the problem structure. In some cases, the algorithm can also guarantee that insoluble problems in fact have no solutions, unlike previously proposed quantum search algorithms.

Tad Hogg