纠缠是一种不可避免的现实特性

据2017年9月1日报道，英国科学家研究发现纠缠是一种不可避免的现实特性。纠缠是关于量子力学理论最著名的预测[1] 。它描述了两个粒子互相纠缠，即使相距遥远距离，一个粒子的行为将会影响另一个的状态[1] 。当其中一颗被操作（例如量子测量）而状态发生变化，另一颗也会即刻发生相应的状态变化[1] 。量子纠缠是粒子在由两个或两个以上粒子组成系统中相互影响的现象，虽然粒子在空间上可能分开。但是，纠缠是描述物理世界真正必须的吗？有一些后量子理论（post-quantum theory）没有纠缠有可能吗?为了回答这些问题，英国的科学家在一项新的研究中,他们在数学上已经证明，有经典局限意义的任何理论即在必须包含纠缠的一定条件下，通过恢复的经典理论可以描述我们对经典世界的观察。所以尽管纠缠违背古典直觉,但纠缠必须是一种不可避免的特性，不仅量子理论,而且包括任何非经典的理论,甚至包括那些尚未被开发的理论。

英国伦敦帝国理工学院（Imperial College London）和伦敦大学学院（University College London）物理学家乔纳森·李申斯（Jonathan G. Richens）、伦敦帝国理工学院和牛津大学（University of Oxford）的约翰·塞尔比（John H. Selby）以及诺丁汉特伦特大学（Nottingham Trent University）的萨布里·阿尔萨菲（Sabri W. Al-Safi）合作将他们的研究结果于2017年8月24日已经在《物理评论快报》（Physical Review Letters）网站发表——Jonathan G. Richens, John H. Selby, and Sabri W. Al-Safi. Entanglement is Necessary for Emergent Classicality in All Physical Theories. Physical Review Letters, 2017, 119(8), 080503 – Published 24 August 2017. DOI: 10.1103/PhysRevLett.119.080503.

ABSTRACT

One of the most striking features of quantum theory is the existence of entangled states, responsible for Einstein’s so called “spooky action at a distance.” These states emerge from the mathematical formalism of quantum theory, but to date we do not have a clear idea of the physical principles that give rise to entanglement. Why does nature have entangled states? Would any theory superseding classical theory have entangled states, or is quantum theory special? One important feature of quantum theory is that it has a classical limit, recovering classical theory through the process of decoherence. We show that any theory with a classical limit must contain entangled states, thus establishing entanglement as an inevitable feature of any theory superseding classical theory.

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"Quantum theory has many strange features compared to classical theory," Richens told Phys.org. "Traditionally we study how the classical world emerges from the quantum, but we set out to reverse this reasoning to see how the classical world shapes the quantum. In doing so we show that one of its strangest features, entanglement, is totally unsurprising. This hints that much of the apparent strangeness of quantum theory is an inevitable consequence of going beyond classical theory, or perhaps even a consequence of our inability to leave classical theory behind."

Although the full proof is very detailed, the main idea behind it is simply that any theory that describes reality must behave like classical theory in some limit. This requirement seems pretty obvious, but as the physicists show, it imparts strong constraints on the structure of any non-classical theory.

Quantum theory fulfills this requirement of having a classical limit through the process of decoherence. When a quantum system interacts with the outside environment, the system loses its quantum coherence and everything that makes it quantum. So the system becomes classical and behaves as expected by classical theory.

Here, the physicists show that any non-classical theory that recovers classical theory must contain entangled states. To prove this, they assume the opposite: that such a theory does not have entanglement. Then they show that, without entanglement, any theory that recovers classical theory must be classical theory itself—a contradiction of the original hypothesis that the theory in question is non-classical. This result implies that the assumption that such a theory does not have entanglement is false, which means that any theory of this kind must have entanglement.

This result may be just the beginning of many other related discoveries, since it opens up the possibility that other physical features of quantum theory can be reproduced simply by requiring that the theory has a classical limit. The physicists anticipate that features such as information causality, bit symmetry, and macroscopic locality may all be shown to arise from this single requirement. The results also provide a clearer idea of what any future non-classical, post-quantum theory must look like.

"My future goals would be to see if Bell non-locality can likewise be derived from the existence of a classical limit," Richens said. "It would be interesting if all theories superseding classical theory must violate local realism. I am also working to see if certain extensions of quantum theory (such as higher order interference) can be ruled out by the existence of a classical limit, or if this limit imparts useful constraints on these ''post-quantum theories.''"