Efficient quantum algorithms for solving quantum linear system problems

Abstract: We transform the problem of solving linear system of equations $A\mathbf{x}=\mathbf{b}$ to a problem of finding the right singular vector with singular value zero of an augmented matrix $C$, and present two quantum algorithms for solving this problem. The first algorithm solves the problem directly by applying the quantum eigenstate filtering algorithm with query complexity of $O\left( s\kappa \log \left( 1/\epsilon \right) \right) $ for a $s$-sparse matrix $C$, where $\kappa $ is the condition number of the matrix $A$, and $\epsilon $ is the desired precision. The second algorithm uses the quantum resonant transition approach, the query complexity scales as $O\left[s\kappa + \log\left( 1/\epsilon \right)/\log \log \left( 1/\epsilon \right) \right] $. Both algorithms meet the optimal query complexity in $\kappa $, and are simpler than previous algorithms.