The authors report the creation and coherence of Greenberger-Horne-Zeilinger (GHZ) states with up to 14 qubits in an ion-trap quantum computer. They observe a coherence decay that scales quadratically with the number of qubits, consistent with a theoretical model of correlated Gaussian phase noise. This superdecoherence is a significant limitation for large-scale quantum computing and metrology, as it can lead to high error probabilities in multi-qubit systems. The study uses a Hamiltonian model to describe the collective phase fluctuations affecting the quantum register and measures the fidelity of the GHZ states over time. The results show that the coherence of an N-qubit GHZ state decays by a factor of \(N^2\) faster than a single qubit, highlighting the importance of noise-insensitive states for future quantum information processing. The experiment also demonstrates the potential of generating genuine multiparticle entangled states with up to 14 qubits, which could enable advanced quantum simulations and applications in quantum metrology.The authors report the creation and coherence of Greenberger-Horne-Zeilinger (GHZ) states with up to 14 qubits in an ion-trap quantum computer. They observe a coherence decay that scales quadratically with the number of qubits, consistent with a theoretical model of correlated Gaussian phase noise. This superdecoherence is a significant limitation for large-scale quantum computing and metrology, as it can lead to high error probabilities in multi-qubit systems. The study uses a Hamiltonian model to describe the collective phase fluctuations affecting the quantum register and measures the fidelity of the GHZ states over time. The results show that the coherence of an N-qubit GHZ state decays by a factor of \(N^2\) faster than a single qubit, highlighting the importance of noise-insensitive states for future quantum information processing. The experiment also demonstrates the potential of generating genuine multiparticle entangled states with up to 14 qubits, which could enable advanced quantum simulations and applications in quantum metrology.