Quantum simulation of Helmholtz equations via Schr{\"o}dingerization

The Helmholtz equation is a prototypical model for time-harmonic wave propagation. Numerical solutions become increasingly challenging as the wave number $k$ grows, due to the equation's elliptic yet noncoercive character and the highly oscillatory nature of its solutions, with wavelengths scaling as $1/k$. These features lead to strong indefiniteness and large system sizes. We present a quantum algorithm for solving such indefinite problems, built upon the Schr\"odingerization framework. This approach reformulates linear differential equations into Schr\"odinger-type systems by capturing the steady state of damped dynamics. A warped phase transformation lifts the original problem to a higher-dimensional formulation, making it compatible with quantum computation. To suppress numerical pollution, the algorithm incorporates asymptotic dispersion correction. It achieves a query complexity of $\mathcal{O}(\kappa^2\text{polylog}\varepsilon^{-1})$, where $\kappa$ is the condition number and $\varepsilon$ the desired accuracy. For the Helmholtz equation, a simple preconditioner further reduces the complexity to $\mathcal{O}(\kappa\text{polylog}\varepsilon^{-1})$. Our constructive extension to the quantum setting is broadly applicable to all indefinite problems.