装置的反击：动量守恒与空间叠加的代价
The Apparatus Strikes Back: Momentum Conservation and the Cost of Spatial Superpositions
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中文总结 AI 辅助
研究将大质量粒子制备成空间叠加时面临的问题，发现动量守恒和装置量子性质带来的约束，通过分析反冲位移与装置质心态相干长度关系得出定量约束，揭示其对相关实验和测试的影响及保守性。
中文摘要 AI 辅助
在量子科学中，将大质量粒子制备成相干空间叠加是核心目标，应用广泛。实现此类叠加的实验困难通常归因于环境退相干机制。本文发现由动量守恒和制备装置的量子性质产生的普遍约束。任何使质量为\(m\)的粒子处于间距为\(d\)的空间叠加的协议，都会使粒子与负责分裂的装置质心自由度纠缠。由此产生的反冲位移由质量偶极矩守恒确定，当装置不包含在量子系统中时会降低粒子的相干性。对于质量为\(M\)的装置，保持相干要求反冲位移小于装置质心态的相干长度，从而得出对其温度和固定于实验室框架刚性的定量约束。分析了该约束对当前物质波干涉测量实验、引力介导纠缠提议及普朗克尺度附近量子力学测试的影响。主要结果是，即使是重的宏观装置的反冲也会对质量远低于普朗克质量的粒子空间叠加相干性构成强约束。最后讨论了为何该限制应视为保守估计及在何种条件下可解释为假退相干实例。
英文摘要
Preparing massive particles in coherent spatial superpositions is a central objective of modern quantum science, motivated by applications ranging from fundamental tests of quantum mechanics and gravity to quantum-enhanced sensing. The experimental difficulty of realizing such superpositions is usually attributed to environmental decoherence mechanisms whose impact depends on the details of the experimental implementation. Here we identify a universal constraint arising from momentum conservation and the quantum nature of the preparation apparatus. Any protocol that places a particle of mass $m$ in a spatial superposition with separation $d$ necessarily entangles the particle with the center-of-mass degree of freedom of the apparatus responsible for the splitting. The resulting recoil displacement, fixed by conservation of the mass dipole moment, reduces the coherence of the particle when the apparatus is not included as part of the quantum system. For an apparatus of mass $M$, preserving coherence requires the recoil displacement to remain smaller than the coherence length of the apparatus center-of-mass state, leading to a quantitative bound expressible as a constraint on its temperature and on how rigidly the apparatus is anchored to the laboratory frame. We analyze the implications of this bound for current matter-wave interferometry experiments, proposals for gravitationally mediated entanglement, and tests of quantum mechanics near the Planck scale. Our main result, which may seem counterintuitive, is that the recoil of even heavy macroscopic apparatuses can pose a surprisingly strong constraint on the coherence of spatial superpositions of particles with masses well below the Planck mass. Finally, we discuss why the resulting limitation should be regarded as a conservative estimate and under which conditions it can be interpreted as an instance of false decoherence.
