Kabel Viktoria, de la Hamette Anne-Catherine, Apadula Luca, Cepollaro Carlo, Gomes Henrique, Butterfield Jeremy, Brukner Časlav
Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, A-1090 Vienna, Austria.
University of Vienna, Faculty of Physics, Vienna Doctoral School in Physics and Vienna Center for Quantum Science and Technology (VCQ), Boltzmanngasse 5, A-1090 Vienna, Austria.
Commun Phys. 2025;8(1):185. doi: 10.1038/s42005-025-02084-3. Epub 2025 Apr 30.
The study of quantum reference frames (QRFs) is motivated by the idea of taking into account the quantum properties of the reference frames used, explicitly or implicitly, in our description of physical systems. Like classical reference frames, QRFs can be used to define physical quantities relationally. Unlike their classical analogue, they relativise the notions of superposition and entanglement. Here, we explain this feature by examining how configurations or locations are identified across different branches in superposition. We show that, in the presence of symmetries, whether a system is in "the same" or "different" configurations across the branches depends on the choice of QRF. Hence, sameness and difference - and thus superposition and entanglement - lose their absolute meaning. We apply these ideas to the context of semi-classical spacetimes in superposition and use coincidences of four scalar fields to construct a comparison map between spacetime points in the different branches. This reveals that the localisation of an event is frame-dependent. We discuss the implications for indefinite causal order and the locality of interaction and conclude with a generalisation of Einstein's hole argument to the quantum context.
量子参考系（QRFs）的研究源于这样一种理念，即在我们对物理系统的描述中，明确或隐含地考虑所使用参考系的量子特性。与经典参考系一样，量子参考系可用于以关系方式定义物理量。与它们的经典类似物不同，它们使叠加和纠缠的概念相对化。在这里，我们通过研究如何在叠加态的不同分支中识别构型或位置来解释这一特征。我们表明，在存在对称性的情况下，一个系统在各分支中是处于“相同”还是“不同”构型取决于量子参考系的选择。因此，相同和不同——进而叠加和纠缠——失去了它们的绝对意义。我们将这些想法应用于叠加态中的半经典时空背景，并利用四个标量场的重合来构建不同分支中时空点之间的比较映射。这表明事件的定位取决于参考系。我们讨论了对不确定因果顺序和相互作用局域性的影响，并以将爱因斯坦的空穴论证推广到量子背景作为结论。
