Evolving Quantum Error-Correcting Encodings for Molecular Simulation

Abstract

Useful quantum algorithms require many coupled discrete design choices. We study LLM-driven evolutionary program synthesis -- a language model edits a program, an external verifier scores the result, and high-scoring programs are retained and re-mutated -- as a tool for quantum-computing research. As a case study, we apply this loop to the Generalized Superfast Encoding (GSE), a fermion-to-qubit encoding whose prior molecular constructions reach code distance $3$. The search discovered interpretable constructor programs whose codes have \emph{exact} distance $5$ on the molecular instances tested, and distance $6$ on one $20$-mode instance, under strict stabilizer-coset semantics. To our knowledge these are the first GSE/superfast encodings beyond distance $3$ for dense molecular Hamiltonians. A second search, guided by verifier analysis of the first artifact, found a circulant constructor that reaches a five-qubits-per-mode floor on the tested $12$-, $14$-, $16$-, and $20$-mode instances, with certified dense-rule fallback at the failing $18$-mode case. As secondary resource descriptors, in a code-capacity \emph{memory} comparison at $p=10^{-3}$ the resulting encodings use $4.2$--$5.0\times$ fewer data qubits than a scoped per-mode Jordan--Wigner $+$ $[[25,1,5]]$ surface route and have $3.4$--$8.2\times$ lower logical-failure rates under finite-weight decoding tables with explicit truncation brackets; we claim no circuit-level fault-tolerance or Trotter-cost advantage. The search trajectory illustrates a general operating lesson: rewarding distance alone selects trivial dense graphs, whereas holding verified distance fixed and rewarding compression selects structured rules.