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 the ancilla qubit. We derive expectation values for an observable $M$ on the HHL outcome state when ancilla qubit is measured in $|0\rangle$ and $|1\rangle$ and show a condition for postselection-free HHL running. We provide an explicit example of a practically interesting input matrix and an observable, which satisfies the postselection-free HHL condition. Our work can improve the performance of the HHL-based algorithms.

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• Quantum algorithms
• Quantum computation