Quantum metrology with non-classical states offers a promising route to improved precision in physical measurements. The quantum effects of Schrödinger-cat superpositions or entanglements enable measurement uncertainties to reach below the standard quantum limit. However, the challenge of maintaining a long coherence time for such non-classical states often prevents full exploitation of the quantum advantage in metrology. Here we demonstrate a long-lived Schrödinger-cat state of optically trapped ¹⁷³Yb (I = 5/2) atoms. The cat state, a superposition of two oppositely directed and furthest-apart spin states, is generated by a nonlinear spin rotation. Protected in a decoherence-free subspace against inhomogeneous light shifts of an optical lattice, the cat state persists for a coherence time of 1.4(1) × 10³ s. A magnetic field is measured using Ramsey interferometry, demonstrating a scheme of Heisenberg-limited metrology for atomic magnetometry, quantum information processing and searching for new physics beyond the Standard Model.

具有非经典态的量子计量为提高物理测量精度提供了一条有前途的途径。薛定谔猫叠加或纠缠的量子效应使测量不确定性能够达到标准量子极限以下。然而，为此类非经典态保持较长相干时间的挑战通常会阻止在计量学中充分利用量子优势。在这里，我们展示了一个由光学俘获的 ¹⁷³Yb （I = 5/2） 原子组成的长寿命薛定谔-cat 状态。cat 态是两个方向相反且相距最远的自旋态的叠加，由非线性自旋旋转产生。在无退相干子空间中受到保护，免受光晶格的不均匀光偏移的影响，cat 状态持续 1.4（1） × 10³ s。使用 Ramsey 干涉测量法测量磁场，展示了用于原子磁力测量、量子信息处理和寻找标准模型之外的新物理学的海森堡极限计量方案。