Native SU(8) Measurement Structure in Pseudo-Photon-Number-Resolving Detection: A Proposal for Decoherence Suppression in Gaussian Boson Sampling

We identify a native 8-dimensional measurement space embedded in the pseudo-photon-number-resolving (pseudo-PNR) detection architecture of the Jiuzhang 3.0 photonic quantum processor. Each ofthe 72 fiber-loop detection units produces exactly 8 distinguishable output states from the Cartesianproduct of 4 temporal bins and 2 spatial path bins. We argue that this output space — which is isomorphicas a Hilbert space to ℂ⁸, the carrier space of the fundamental representation of SU(8) — enables a four-layer decoherence suppression scheme: (1) decoherence-free subspace (DFS) encoding in the j=0 singletsector; (2) dynamical decoupling via a 63-generator pulse sequence; (3) a [[8,1,3]]₈ stabilizer quantumerror-correcting code exploiting the 3-bit PNR syndrome information per mode; and (4) an optimalLinear-Quadratic-Gaussian (LQG) feedback controller on the 63-dimensional Bloch vector. We carefullydistinguish the measurement space (dual of SU(8)) from the transformation space (SU(8) itself), adistinction central to the falsifiability of our claims. Conservative estimates yield Γ_eff/Γ₀ ~ 10⁻⁸ to 10⁻¹²under realistic operating conditions; the theoretical ideal of 10⁻¹⁴ requires further hardware development.Layers 1 and 4 of the scheme require no hardware modification to JZ3; layers 2 and 3 require electro-opticmodulators and ancilla detection channels respectively. We state explicit falsifiable predictions testableagainst existing Jiuzhang 3.0 data.