In quantum metrology and quantum simulation, a coherent non-classical state must be manipulated before unwanted interactions with the environment lead to decoherence. In atom interferometry, the non-classical state is a spatial superposition, where each atom coexists in multiple locations as a collection of phase-coherent partial wavepackets. These states enable precise measurements in fundamental physics and inertial sensing. However, atom interferometers usually use atomic fountains, where the available interrogation time is limited to around 3 s for a 10 m fountain. Here we realize an atom interferometer with a spatial superposition state that is maintained for as long as 70 s. We analyse the theoretical and experimental limits to coherence arising from collective dephasing of the atomic ensemble. This reveals that the decoherence rate slows down markedly at hold times that exceed tens of seconds. These gains in coherence may enable gravimetry measurements, searches for fifth forces or fundamental probes into the non-classical nature of gravity.

在量子计量和量子模拟中，在与环境的不需要的相互作用导致退相干之前，必须操纵相干的非经典态。在原子干涉测量中，非经典状态是空间叠加，其中每个原子作为相位相干部分波包的集合共存于多个位置。这些状态使得基础物理和惯性传感中的精确测量成为可能。然而，原子干涉仪通常使用原子喷泉，对于 10 米的喷泉来说，可用的询问时间仅限于 3 秒左右。在这里，我们实现了一种原子干涉仪，其空间叠加态可以维持长达70秒。我们分析了原子系综集体失相所产生的相干性的理论和实验限制。这表明，当保持时间超过数十秒时，退相干速率显着减慢。这些相干性的提高可以实现重力测量、寻找第五力或对重力的非经典性质进行基本探索。