Beyond Gottesman-Knill: Orbit Valence as a Finer Simulability Boundary

The Gottesman-Knill (GK) theorem establishes that Clifford circuits on stabiliser states are efficiently classically simulable. The simulability boundary is at zero magic: any state with positive Wigner negativity N(ρ) > 0 is beyond GK. This paper establishes that this boundary is not sharp enough.

Two states can both lie beyond GK — both have N > 0 — yet differ in computational power in a precise, measurable way. The distinguishing invariant is the Fano orbit valence label {p_L}: the decomposition of the state's Wigner characteristic function over the seven orbit valences of the symplectic polar space W(5,2). The orbit valence provides a strictly finer simulability classification than total negativity N.

The central new result is the magic valence mismatch theorem: injecting a magic state of the wrong orbit valence into a circuit that expects a different valence yields a winning probability of 0.472 in the Fano Line Verification Game — strictly below the 0.500 classical bound. Wrong-valence magic is not merely suboptimal; it is worse than no magic at all for the intended computation. Critically, this failure mode is invisible to syndrome-only decoders: CS₀₁, CS₀₂, and CS₁₂ states all produce identical stabiliser syndromes (+1)^7 but distinct orbit valence labels, detectable only by the ORBIT opcode.

A valence-aware magic state factory using the ORBIT opcode accepts states by orbit valence, achieving distillation overhead O(1/p) per accepted state rather than the O(1/p²) of standard T-gate distillation — a ~1000× overhead reduction at physical error rate p = 10⁻³ for CS-family computations.

The island graph picture of quantum circuit computation is introduced: entangled registers form islands connected by magic-carrying channels, and the computational power of the circuit is determined not by total magic N but by the magic current profile {I_L}. The ORBIT opcode serves as an ammeter for magic current: 7 Pauli measurements identify which orbit valences are flowing at each node.

Keywords

Gottesman-Knill Theorem, Classical Simulation, Magic State, Wigner Negativity, Fano Orbit, Magic Valence, Orbit Valence Label, W(5,2), Symplectic Polar Space, ORBIT Opcode, Magic State Distillation, CS Gate, Controlled-S Gate, Valence Mismatch, Island Graph, Magic Current, Fault-Tolerant Quantum Computation, Stabiliser Syndrome, Coherent Error Detection, Origami ISA, Magic-ISA, TriQ, SQU, Fano Plane, PG(2,2), Quantum Resource Theory, Simulability Boundary