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Ancilla-train quantum algorithm for simulating non-Markovian open quantum systems

Hans Michael Christensen, Johannes Agerskov, and Frederik Nathan

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Phys. Rev. A 114, 042422 – Published 13 October, 2026

DOI: https://doi.org/10.1103/dc52-844x

Abstract

We present a quantum algorithm for simulating open quantum systems coupled to Gaussian environments valid for any configuration and coupling strength. The algorithm is applicable to problems with strongly coupled, or non-Markovian, environments; problems with multiple environments out of mutual equilibrium; and problems with time-dependent Hamiltonians. We show that the algorithm can reproduce the true dynamics of such problems at arbitrary accuracy and, for a broad range of problems, only adds a minor resource cost relative to Trotterized time evolution; the cost is low-degree polynomial in the inverse target accuracy. The algorithm is based on the insight that any Gaussian environment can be represented as a train of ancillary qubits that sequentially interact with the system through a time-local coupling, given by the convolution square root of the bath correlation function; this is a secondary result of our work. In numerical simulations of a spin-boson model, we find accurate simulations can be achieved with moderate (101–103) ancilla counts in the intermediate to strong-coupling regimes. Our results open up applications of quantum computers for efficient simulation of nonequilibrium and open quantum systems.

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Phys. Rev. A 114, 042422– Published 13 October, 2026
  • Received 14 November 2025
  • Accepted 19 August 2026
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