From its seemingly non-intuitive and puzzling nature, most evident in numerous EPR-like gedankenexperiments to its almost ubiquitous presence in quantum technologies, entanglement is at the heart of modern quantum physics. First introduced by Erwin Schrödinger nearly a century ago, entanglement has remained one of the most fascinating ideas that came out of quantum mechanics. Here, we attempt to explain what makes entanglement fundamentally different from any classical phenomenon. To this end, we start with a historical overview of entanglement and discuss several hidden variables models that were conceived to provide a classical explanation and demystify quantum entanglement. We discuss some inequalities and bounds that are violated by quantum states thereby falsifying the existence of some of the classical hidden variables theories. We also discuss some exciting manifestations of entanglement, such as N00N states and the non-separable single particle states. We conclude by discussing some contemporary results regarding quantum correlations and present a future outlook for the research of quantum entanglement.

中文翻译：

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纠缠：量子还是经典？

从其看似不直观和令人费解的性质（在众多类似 EPR 的 gedanken 实验中最为明显）到其在量子技术中几乎无处不在的存在，纠缠是现代量子物理学的核心。大约一个世纪前，由 Erwin Schrödinger 首次提出，纠缠一直是量子力学中最引人入胜的想法之一。在这里，我们试图解释是什么使纠缠与任何经典现象有着根本的不同。为此，我们从纠缠的历史概述开始，并讨论几个隐藏变量模型，这些模型旨在提供经典解释并揭开量子纠缠的神秘面纱。我们讨论了量子态违反的一些不等式和界限，从而证伪了一些经典的隐藏变量理论的存在。我们还讨论了纠缠的一些令人兴奋的表现，例如 N00N 状态和不可分离的单粒子状态。最后，我们讨论了有关量子相关性的一些当代成果，并提出了量子纠缠研究的未来展望。