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Itsushi Sakata

Publications and source records attributed to Itsushi Sakata.

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Physics-inspired transformer quantum states via latent imaginary-time evolution

Neural quantum states (NQS) are powerful ans\"atze in the variational Monte Carlo framework, yet their architectures are often treated as black boxes. We propose a physically transparent framework in which NQS are treated as neural approximations to latent imaginary-time evolution. This viewpoint suggests that standard Transformer-based NQS (TQS) architectures correspond to physically unmotivated effective Hamiltonians dependent on imaginary time in a latent space. Building on this interpretation, we introduce physics-inspired transformer quantum states (PITQS), which enforce a static effective Hamiltonian by sharing weights across layers and improve propagation accuracy via Trotter-Suzuki decompositions without increasing the number of variational parameters. For the frustrated $J_1$-$J_2$ Heisenberg model, our ans\"atze achieve accuracies comparable to or exceeding state-of-the-art TQS while using substantially fewer variational parameters. This study demonstrates that reinterpreting the deep network structure as a latent cooling process enables a more physically grounded, systematic, and compact design, thereby bridging the gap between black-box expressivity and physically transparent construction.

cond-mat.dis-nn

Resolvent-Based Singular-Value Diagnostics for Data-Driven Koopman Finite Sections

Finite-dimensional Koopman eigenvalues do not characterize resolvent growth, particularly for nonnormal compressions. We study the singular-value structure of empirical Koopman finite sections in the inner product induced by the data. Resolvent Dynamic Mode Decomposition (Resolvent DMD) removes unresolved Gram directions and then forms the Koopman or generator compression in a Gram-orthonormal basis. At a prescribed spectral parameter, the reciprocal of the smallest shifted singular value is the resolvent norm on the retained observable space. The associated right and left singular vectors give a minimum-residual pseudomode and the corresponding optimal forcing direction. For deterministic unregularized data, the squared ResDMD residual admits an orthogonal decomposition into a projected shifted residual and an invariance defect. The finite-section singular value is therefore a lower bound for the minimum ResDMD residual on the same space. We then identify stability assumptions that connect the finite sections with the underlying Koopman operator. Pointwise stability gives one-sided inclusion of inverse-resolvent sublevel sets. Stability on a punctured isolating neighborhood obtains local Hausdorff convergence near an isolated eigenvalue, while multiplicity stability preserves the algebraic count. Contractive, measure-preserving, and compact settings provide concrete sufficient conditions. The numerical examples examine coordinate dependence, finite-data residuals, contour calculations, and pseudomode separation for a noisy two-frequency signal.

math.DS