Neutrino oscillations as weight-preserving quantum circuits: what the qg symmetry filter reads, and what no oscillation circuit can see

A neutrino is produced and detected in a flavour state and propagates in mass states, so flavour oscillation is a rotation. We write it in the measurement-level units of the open-source library qang, where a qubit is described by qg_Z = <Z>, and place every flavour on its own qubit with a single excitation (the weight-1 sector). Every gate of the oscillation circuit then conserves the Hamming weight, and the qg symmetry filter (discard the shots that left the sector) applies. Six results follow, all on the noise models of IBM devices, each pre-registered and reported as passed or failed. (i) In vacuum qg_Z = cos²2θ + sin²2θ cos Δ is the spherical law of cosines, and at the oscillation maximum the mixing angle is a quarter of the qang angle arccos(qg_Z), with the θ-independent error bar 1/(4√(N+½)). (ii) Three flavours on three qubits: the filter cuts the error of the transition probabilities about 3× and the CP asymmetry at a DUNE-like baseline survives. (iii) Earth matter enters as a weight-preserving Trotter circuit, and matter fakes CP violation. (iv) In dense neutrino gases the conserved number is the number of ν₂, not the flavour number, so the filter applies in the mass basis (2.2-2.8×). (v) For solar neutrinos the phases are lost before detection, so the whole Sun is one rotation and the filter, though it works, is not needed. (vi) The mass ordering is read correctly through matter on three device models; Majorana phases change no oscillation probability, and the transpiler deletes them; a sterile neutrino is a fourth qubit in the weight-1 sector, where the filter cuts the error 5.5×. The link between neutrinos and qang is modelling, not new physics: the oscillation is a rotation, and qang reads rotations.