Harrow-Hassidim-Lloyd algorithm without ancilla postselection

Abstract

Harrow-Hassidim-Lloyd algorithm (HHL) allows for the exponentially faster solution of a system of linear equations. However, this algorithm requires the postselection of an ancilla qubit to obtain the solution. This postselection makes the algorithm result probabilistic. Here we show conditions when the HHL algorithm can work without postselection of ancilla qubit. We derive expectation values for an observable $M$ on the HHL outcome state when ancilla qubit is measured in $\ket{0}$ and $\ket{1}$ and show condition for postselection-free HHL running. We provide an explicit example of a practically-interesting input matrix and an observable, which satisfy postselection-free HHL condition. Our work can improve the performance of the HHL-based algorithms.