December 19, 2025, 6:58 AM EST. New work by Aaron Z. Goldberg and colleagues shows that stabilizer expectation values, a staple benchmark for GKP states, do not reliably quantify proximity to an ideal GKP state. Specifically, stabilizers provide only an upper bound on fidelity, meaning a harmonic-oscillator qubit can display excellent stabilizer signals while remaining far from the ideal GKP state under fidelity-based measures. This overturns the idea that strong stabilizer performance guarantees high-quality logical qubits and prompts a rethink of benchmarking for GKP codes. The findings have broad implications for bosonic quantum error correction, especially in optical/microwave platforms, where state preparation and measurement imperfections affect code performance. The work underscores the need for more reliable, fidelity-based metrics and improved strategies for preparing high-fidelity GKP states and cluster states, and resilience to photon loss.