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Quantum Science and Technology

Exact finite-dimensional reduction of Wigner dynamics for open quantum systems using EGQPDs

Peng Guo

Published Jul 28, 2026• 0 citations
Publisher

Abstract

Abstract Simulating non-Gaussian quantum dynamics in open continuous-variable systems is notoriously challenging: exact methods scale exponentially, while approximations sacrifice non-classical features such as Wigner negativity. Here we introduce an exact finite-dimensional reduction that overcomes this curse of dimensionality. We prove that the algebra of extended Gaussian quasi-probability densities (EGQPDs) is closed under Lindblad evolution with linear jump operators, reducing the infinite-dimensional Wigner PDE to a closed system of ODEs that scales polynomially with the number of modes. A necessary and sufficient condition for closure is established: the algebra remains closed iff every jump operator is at most linear; otherwise the extended algebra with polynomial prefactors is required. We further show that the minimal number of Gaussian components defines a non-Gaussian rank—a discrete complexity measure monotonically non-increasing under the dynamics. The framework yields explicit ODEs for the component parameters, admits a rigorous density theorem and error propagation bound, and provides closed-form algebraic criteria for Gaussianity, purity, and entanglement. Seven numerical experiments validate the method across Gaussian and non-Gaussian states, entanglement decay, PT-symmetric exceptional-point dynamics, and complexity scaling, demonstrating machine-precision accuracy and exponential speedups over grid and Fock methods. The EGQPD framework provides a systematic, exact, and efficient toolbox for non-Gaussian open quantum dynamics.

DOI
10.1088/2058-9565/ae9184

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Abstract Simulating non-Gaussian quantum dynamics in open continuous-variable systems is notoriously challenging: exact methods scale exponentially, while approximations sacrifice non-classical features such as Wigner negativity. Here we introduce an exact finite-dimensional reduction that overcomes this curse of dimensionality. We prove that the algebra of extended Gaussian quasi-probability densities (EGQPDs) is closed under Lindblad evolution with linear jump operators, reducing the infinite-dimensional Wigner PDE to a closed system of ODEs that scales polynomially with the number of modes. A necessary and sufficient condition for closure is established: the algebra remains closed iff every jump operator is at most linear; otherwise the extended algebra with polynomial prefactors is required. We further show that the minimal number of Gaussian components defines a non-Gaussian rank—a discrete complexity measure monotonically non-increasing under the dynamics. The framework yields explicit ODEs for the component parameters, admits a rigorous density theorem and error propagation bound, and provides closed-form algebraic criteria for Gaussianity, purity, and entanglement. Seven numerical experiments validate the method across Gaussian and non-Gaussian states, entanglement decay, PT-symmetric exceptional-point dynamics, and complexity scaling, demonstrating machine-precision accuracy and exponential speedups over grid and Fock methods. The EGQPD framework provides a systematic, exact, and efficient toolbox for non-Gaussian open quantum dynamics.
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The Fokker-Planck equation models rare events across sciences, but blue{direct solution of the PDE is intractable for classical computers due to } its high-dimensional nature. Classical stochastic methods circumvent this curse-of-dimensionality, and serve as the de facto standard for practicing computational scientists. Quantum algorithms for such non-unitary dynamics often suffer from exponential decay in success probability. We introduce a quantum algorithm that overcomes this bottleneck for estimating reaction rates {and dynamical correlation functions more generally}. Using a sum-of-squares representation, we develop a Gaussian linear combination of Hamiltonian simulations (Gaussian-LCHS) to represent the non-unitary propagator with O(t∥H∥log⁡(1/ε))O\left(\sqrt{t\|H\|\log(1/ε)}\right)O(t∥H∥log(1/ε)​) queries to its block encoding. Crucially, we pair this with {a} novel technique to directly estimate matrix elements without exponential decay. For ηηη pairwise interacting particles discretized with NNN plane waves per degree of freedom, we estimate reactive flux to error εεε using O~((η5/2tβαV+η3/2t/βN)/ε)\widetilde{O}\left((η^{5/2}\sqrt{tβ}α_V + η^{3/2}\sqrt{t/β}N)/ε\right)O((η5/2tβ​αV​+η3/2t/β​N)/ε) quantum gates, where αV=max⁡r∣V′(r)/r∣α_V = \max_{r}|V'(r)/r|αV​=maxr​∣V′(r)/r∣. We further prove that under comparable worst-case analytical guarantees, the sharpest classical bounds for estimating reaction rates via simulation of the associated overdamped Langevin dynamics scale as O(tη2eΩ(η)/ε4)O(tη^2 e^{Ω(η)}/ε^4)O(tη2eΩ(η)/ε4), yielding an exponential improvement in ηηη, a quartic speedup in εεε, and quadratic speedup in the time horizon ttt. While classical algorithms may outperform these bounds in practice, this work demonstrates a rigorous route toward quantum advantage for high-dimensional dissipative dynamics.
We consider finite-dimensional many-body quantum systems described by time-independent Hamiltonians and Markovian master equations, and present a systematic method for constructing smaller-dimensional, reduced models that e x a c t l y reproduce the time evolution of a set of initial conditions or observables of interest. Our approach exploits Krylov operator spaces and their extension to operator algebras, and may be used to obtain reduced linear models of minimal dimension, well-suited for simulation on classical computers, or reduced quantum models that preserve the structural constraints of physically admissible quantum dynamics, as required for simulation on quantum computers. Notably, we prove that the reduced quantum-dynamical generator is still in Lindblad form. By introducing a new type of observable-dependent symmetries , we show that our method provides a non-trivial generalization of techniques that leverage symmetries, unlocking new reduction opportunities. We quantitatively benchmark our method on paradigmatic open many-body systems of relevance to condensed-matter and quantum-information physics. In particular, we demonstrate how our reduced models can quantitatively describe decoherence dynamics in central-spin systems coupled to structured environments, magnetization transport in boundary-driven dissipative spin chains, and unwanted error dynamics on information encoded in a noiseless quantum code.
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We investigate the algebraic structure underlying the Lindblad equation for finite-dimensional open quantum systems. By introducing a suitable operator representation of the Liouville superoperator, we show that the dynamics can be formulated in terms of a closed algebra of Hermitian operators that is independent of the particular physical model. This formulation reveals that dissipative dynamics requires a substantially richer algebraic structure than purely unitary evolution, thereby providing a clear characterization of the additional complexity introduced by the Lindbladian. The resulting framework naturally leads to parametrizations of the dynamical map and to differential equations governing its evolution. We further derive recursion relations that enable the efficient construction of the algebra for systems of increasing dimension. Because the algebraic basis is universal, while all model-dependent information enters through a single set of coefficients, the proposed approach significantly reduces the computational cost of constructing the Liouville superoperator compared with direct methods. To facilitate the implementation of the method, we provide a Mathematica notebook containing a one-qubit example that can be systematically extended to an arbitrary number of qubits. The proposed framework therefore provides both a general mathematical description of finite-dimensional Lindblad dynamics and a practical foundation for efficient analytical and numerical implementations.
Nonlinear dynamics generate non-Gaussian states and operations, but they also make continuously monitored quantum systems difficult to track. A quantum filter performs this task by compressing a noisy measurement record into a set of evolving variables that predict future observables. For Gaussian dynamics, this compression is exact and finite. In nonlinear systems, conventional moment equations generally expand without bound, but the failure of one representation does not prove that every finite description is impossible. Here we formulate quantum filtering as a realization problem for general quantum input--output maps. We introduce the linearized observable Hankel operator KOK_OKO​, which maps changes in the past measurement record to changes in a future conditional observable. Any finite-dimensional smooth filter forces KOK_OKO​ to have finite rank, so infinite rank provides a coordinate-independent proof that no exact finite-dimensional filter exists. Applied to single-mode polynomial bosonic systems under continuous quadrature monitoring, this yields a sharp boundary: Gaussian dynamics and the measurement-aligned Conditional Momentum Moment class admit finite realizations, whereas dynamics outside these classes produce infinitely many response directions and admit no finite-dimensional smooth exact filter. Exact impossibility, however, need not imply large practical complexity. The singular values of KOK_OKO​ quantify how many response directions matter at a chosen local accuracy, defining the effective memory of an observable. For the Kerr and Duffing oscillators, these singular values decay rapidly despite infinite exact rank, showing that infinite exact memory can coexist with low finite-accuracy complexity. The framework therefore separates exact realizability from effective complexity in nonlinear quantum filtering.