An Invariant-Anchored Benchmark for Quantum Processors from the Parity-Entanglement Bound

The Parity–Entanglement Bound (PEB) and its ℤₙ / non-abelian extensions fix the clock–system mutual information I(C;S) of a symmetry-constrained pure state to an absolute, calculable value: 2.000 bits for the uniform ℤ₂ ground state, 2log₂n for ℤₙ, and representation-dependent values (e.g. 3.84 bits for S₃). These targets depend only on the symmetry group: they are independent of qubit number, clock size, clock position, and physical platform. Here we convert these results into a platform-agnostic benchmarking protocol for quantum processors. The primary readout is the rank-2 subspace weight w₂, a linear functional of the reduced clock state whose target is 1.000 for every clock size while an unconstrained register of k qubits would give 2^(1−k); because w₂ is linear it needs no purity assumption, no entropy estimator and no noise model. It is paired with the mandatory complementary-basis coherence check. On IBM hardware we report a clock-size scan at k = 1, 2, 3 giving w₂ = 1.000, 0.969, 0.920 against naive values 1.000, 0.500, 0.250, and a two-sided Schmidt-balance sweep confirming the exact complementarity V² + P² = 1 to 1.1% constancy. A separable control state is included: a single-basis clock-marginal readout returns 1.9991 bits on it while the tomographic readout returns 0.835 bits, which is why the coherence pairing is mandatory. The PEB thus supplies an absolute reference for calibration, SPAM-and-coherence diagnosis, and cross-platform comparison — a tool that assists quantum computation without being a fault-tolerance code, and complementary to (not a replacement for) randomized and cross-entropy benchmarking, which characterize average-gate quantities.