Scattering phase shift in quantum mechanics on quantum computers: non-Hermitian systems and imaginary-time simulations

To overcome the fast oscillatory behavior of correlation functions for extracting scattering phase shift in real-time quantum simulations encountered in Ref.\cite{Guo:2026qkx}, we propose and test two solutions in the present work. One is to simulate Hermitian systems in imaginary time, the other is to simulate non-Hermitian systems in real time. We demonstrate that both approaches lead to the problem of non-unitary quantum evolution which can be solved by combining two quantum algorithms: block encoding and Hadamard test. The combined quantum algorithm does not require mid-circuit measurements or adjustment of the input parameters of the Hamiltonian, and can be easily implemented on quantum computers. Both the size and length of quantum circuits grow linearly with evolution time. Numerical tests on quantum simulators show that both approaches agree with exact solutions for a sufficiently long time before the signal is lost in statistical fluctuations. The results bode well for using non-Hermitian and imaginary-time simulations to circumvent oscillations inherent in real-time simulation of other quantum systems.

💡 Research Summary

The paper addresses a fundamental obstacle in extracting scattering phase shifts from real‑time quantum simulations: the rapid oscillations of the integrated correlation function (ICF) that quickly drown the signal in statistical noise. Building on their previous work, the authors propose two complementary strategies to eliminate these oscillations. The first strategy retains the original Hermitian Hamiltonian but rotates the time axis to imaginary time (τ = −i t), converting the evolution operator from e^{−iĤt} to the non‑unitary e^{−Ĥτ}. The second strategy rotates the spatial dimension (L → i L), which renders the Hamiltonian non‑Hermitian, Ĥ(iL) = Ĥ₁ + iĤ₂, and the real‑time evolution factorizes into a unitary part e^{−iĤ₁t} and a non‑unitary part e^{Ĥ₂t}. In both cases the core computational task becomes the evaluation of the trace of a non‑unitary operator, Tr