Explicit near-optimal quantum algorithm for solving the advection-diffusion equation

Abstract: An explicit near-optimal quantum algorithm is proposed for modeling dissipative initial-value problems. This method, based on the Linear Combination of Hamiltonian Simulations (LCHS), approximates a target nonunitary operator as a weighted sum of Hamiltonian evolutions, thereby emulating a dissipative problem by mixing various time scales. We propose an efficient encoding of this algorithm into a quantum circuit based on a simple coordinate transformation that turns the dependence on the summation index into a trigonometric function and significantly simplifies block-encoding. The resulting circuit has high success probability and scales logarithmically with the number of terms in the LCHS sum and linearly with time. We verify the quantum circuit and its scaling by simulating it on a digital emulator of fault-tolerant quantum computers and, as a test problem, solve the advection-diffusion equation. The proposed algorithm can be used for modeling a wide class of nonunitary initial-value problems including the Liouville equation and linear embeddings of nonlinear systems.