Neural Quantum State (NQS)

Restricted Boltzmann Machine wavefunction ψ_θ(σ) = e^{Σaᵢσᵢ} Πⱼ cosh(bⱼ+Σᵢ Wᵢⱼσᵢ). Variational parameters {a,b,W} optimized via gradient descent to minimize ⟨H⟩. Watch hidden unit activations and energy convergence.

NQS Parameters

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RBM Wavefunction

Carleo-Troyer (2017): ψ_θ(σ) encodes quantum state amplitudes as an RBM. Parameters θ={aᵢ,bⱼ,Wᵢⱼ} are complex-valued for sign structure.

log ψ(σ) = Σᵢ aᵢσᵢ + Σⱼ log cosh(Λⱼ(σ))
where Λⱼ = bⱼ + Σᵢ Wᵢⱼσᵢ

SR optimization: Stochastic reconfiguration minimizes ⟨H⟩ using the quantum geometric tensor Sₖₗ = ⟨O_k* O_l⟩ - ⟨O_k*⟩⟨O_l⟩ as metric.

Expressibility: α hidden units per spin: RBM captures area-law states exactly; with α→∞ can represent any state. Critical systems need α ~ poly(N).

TFI exact: Critical at h=1 (J=1); E₀/N = -1.0 at h→∞ (product state).