Otte Eileen, Rosales-Guzmán Carmelo, Ndagano Bienvenu, Denz Cornelia, Forbes Andrew
Institute of Applied Physics, University of Muenster, Muenster D-48149, Germany.
School of Physics, University of the Witwatersrand, Wits 2050, South Africa.
Light Sci Appl. 2018 May 4;7:18009. doi: 10.1038/lsa.2018.9. eCollection 2018.
It is well known that the entanglement of a quantum state is invariant under local unitary transformations. This rule dictates, for example, that the entanglement of internal degrees of freedom of a photon remains invariant during free-space propagation. Here, we outline a scenario in which this paradigm does not hold. Using local Bell states engineered from classical vector vortex beams with non-separable degrees of freedom, the so-called classically entangled states, we demonstrate that the entanglement evolves during propagation, oscillating between maximally entangled (purely vector) and product states (purely scalar). We outline the spin-orbit interaction behind these novel propagation dynamics and confirm the results experimentally, demonstrating spin-orbit coupling in paraxial beams. This demonstration highlights a hitherto unnoticed property of classical entanglement and simultaneously offers a device for the on-demand delivery of vector states to targets, for example, for dynamic laser materials processing, switchable resolution within stimulated emission depletion (STED) systems, and a tractor beam for entanglement.
众所周知，量子态的纠缠在局部酉变换下是不变的。例如，这条规则表明，光子内部自由度的纠缠在自由空间传播过程中保持不变。在此，我们概述了一种该范例不成立的情形。利用由具有不可分离自由度的经典矢量涡旋光束构建的局部贝尔态，即所谓的经典纠缠态，我们证明了纠缠在传播过程中会发生演化，在最大纠缠态（纯矢量态）和乘积态（纯标量态）之间振荡。我们概述了这些新颖传播动力学背后的自旋 - 轨道相互作用，并通过实验证实了结果，展示了傍轴光束中的自旋 - 轨道耦合。这一演示突出了经典纠缠迄今未被注意到的特性，同时提供了一种将矢量态按需输送到目标的装置，例如用于动态激光材料加工、受激辐射损耗（STED）系统中的可切换分辨率以及用于纠缠的牵引光束。
