wai-quantum 0.3.35: time evolution of spin chains by TDVP, held to exact answers #160

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dcharlot merged 2 commits from wai-quantum-tdvp into main 2026-10-05 15:54:10 -04:00

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2026-10-05

Transaction Science

wai-quantum 0.3.35: time evolution of spin chains by TDVP, held to exact answers ...

quantum_tdvp evolves a matrix-product state by the two-site time-dependent
variational principle, a symmetric second-order sweep:
- each bond goes forward half a step under its effective Hamiltonian, and the
  new centre goes back half a step under its one-site one;
- the exponentials are Lanczos with full re-orthogonalisation, and a step
  that does not converge is halved;
- bonds are split by a Jacobi SVD, so amplitudes resolve to rounding;
- every bond's basis is completed with null vectors up to max_bond, which
  removes the projection error a product state's small bonds would cause.

The referees are exact Krylov evolution of all amplitudes (up to 20 sites) and
the free-fermion Ising quench at any length, including the Loschmidt echo.

Checked:
- complete bases are exact to rounding at any step size;
- at 100 sites and bond 64 the critical quench stays within 1e-9 of the exact
  answer through t = 3, and past that the error follows the reported
  discarded weight;
- runs are bit-identical across thread counts and natively vs wasm32-wasip2
  (new pin);
- the CLI command tdvp prints the same bytes natively and under WASI.

A fourth-order Yoshida composition was measured as less accurate and slower,
and was removed; the module docs record why.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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2026-10-05 11:13:38 -04:00

Transaction Science

wai-quantum: the DMRG engine generic over real and complex scalars ...

The contraction engine, the density-matrix split and the canonical form
now run on any Scalar: f64 for ground states, the crate's complex type
for time evolution. Real arithmetic keeps its exact order, so the DMRG
pin does not move. A Hermitian eigensolver (Householder to a real
tridiagonal, the shared QL iteration, reflections carried back) serves
the complex split.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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2026-10-05 10:02:51 -04:00