Dismantling the wave–particle duality
Dismantling the wave-particle duality.
Wave and particle are two fundamental properties of nature. The debate of whether light is a wave or a particle is an ancient problem. For examples, Newton regarded that light is a particle, while Huygens, Thomas Young, Maxwell, etc. believed that light is a wave. After the appearance of quantum theory, people came to realize that light can be a contradictory quantum entity, namely, it can be either a wave or a particle, depending upon the circumstances of the experiments. Such a surprising phenomenon is well-known as the principle of wave-particle duality for light.
In 1923, the French physicist Louis de Broglie boldly generalized the wave–particle duality from light to any other matters, and this talented idea has thoroughly changed our viewpoint to the physical world, and meanwhile stimulated the establishment of the fundamental equation in quantum mechanics, i.e., Schrödinger's equation.
Nowadays, the wave–particle duality has been deeply rooted in people's hearts. It has led to a common-sense perception that wave property and particle property coexist simultaneously in a quantum entity. The wave-particle duality seems to tell us that these two physical attributes are mutually bound together, thus cannot be dismantled in a quantum object.
This gives rise to a fundamental question: Can one completely separate the wave and particle properties for a single photon, thus successfully dismantling the wave–particle duality?
In order to answer this question, P. Chowdhury from Indian Institute of Technology Guwahati, Prof. A. K. Pati from Harish-Chandra Research Institute, and Prof. Jing-Ling Chen from Nankai University, China, have proposed a thought experiment to demonstrate that the wave and particle properties can be spatially separated in a single photon. This significant result was published as a cover in Photonics Research, Vol. 9, Issue 7, 2021 (Pratyusha Chowdhury, Arun Kumar Pati, Jing-Ling Chen. Wave and particle properties can be spatially separated in a quantum entity. Photonics Research, 2021, 9 (7): 1379-1383).
To show such a separation, the team started with the wave-particle box that can generate a wave-particle superposition state of a single photon. Then the team designed an experimental setup based on a similar technology of quantum Chesire cat--a quantum paradox for which a cat (i.e., quantum object) and its grin (i.e., physical property) can be spatially separated.
The proposed setup includes a modified Mach-Zehnder interferometer, one may calculate various weak values for suitable observables by choosing the pre- and post-selected states. As a result, the team found that the particle property of light is zero in the left arm of the interferometer, whereas the wave property is zero in the right arm, thus realizing the separation of the wave and the particle properties in the modified Mach-Zehnder interferometer.
Furthermore, the team found that, although the wave-particle duality was broken, the complementarity principle still holds under the wave-particle separation. The result may find potential applications in quantum foundations and quantum communications.
Prof. Jing-Ling Chen believes that dismantling the wave–particle duality is an important contribution for quantum foundations, and it will stimulate some new experimental investigations. For instances, it would be interesting to find out if the interference fringes on the screen vanish when one adopts the lights with solely particle property to perform the Young-type double-slit experiments, and also if the electron diffraction effects disappear when one adopts the electrons with solely particle property to perform the corresponding experiments.
Prof. A. K. Pati believes that an interesting proposal can be carried out in the follow-up work. For a quantum Cheshire cat, it implies a bipartite separation between quantum object and one of its physical properties. For dismantling the wave–particle duality, it indicates a bipartite separation between two physical properties in a quantum object.
By combining quantum Cheshire cat and the dismantlement of the wave–particle duality, one may consequently consider a tri-separation, i.e., the separation among the quantum object itself, the wave property, and the particle property. Once such a separation is achieved, then one will obtain a quantum Cheshire "Super Cat."
思想实验：拆分波粒二象性
拆分波粒二象性
波动性与粒子性是大自然的两个基本属性。关于光是波还是粒子的争论是一个古老的问题。例如，牛顿认为光是一个粒子，而惠更斯、托马斯·杨、麦克斯韦等人则认为光是一个波。
量子理论出现后，人们开始意识到光可以是一个相互矛盾的量子实体，即它可以是波或者粒子，取决于具体的实验环境。这种令人惊讶的现象就是众所周知的光的波粒二象性原理。
1923年，法国物理学家路易斯·德布罗意大胆地将波粒二象性从光推广到任意其他物质，此天才的思想彻底改变了我们对物理世界的看法，同时也启发了量子力学基本方程的建立，即薛定谔方程。如今，波粒二象性已经深深扎根于人们的心中。它导致了一种常识性的看法，即波动属性和粒子属性在一个量子实体中同时共存。
波粒二象性似乎告诉我们，波动性与粒子性这两个物理属性是相互绑定在一起，因此不能在一个量子实体中被拆分。这就引起了一个根本问题：人们能否完全分离单个光子的波动性与和粒子性，从而成功拆分波粒二象性？
为了解决上述问题，印度理工学院的P. Chowdhury、哈里什-钱德拉研究所的A. K. Pati教授和南开大学的陈景灵教授提出了一个思想实验，用以展示单个光子的波动性与粒子性可以在空间中分离。这一重大成果作为封面文章发表于Photonics Research 2021年第9卷第7期 (Pratyusha Chowdhury, Arun Kumar Pati, Jing-Ling Chen. Wave and particle properties can be spatially separated in a quantum entity. Photonics Research, 2021, 9 (7): 1379-1383)。
为了展示这种分离，该研究团队首先使用波粒盒子来产生单个光子的波粒叠加状态，然后根据量子柴郡猫的类似技术设计了一个实验装置。量子柴郡猫是一种量子悖论，其表示猫（量子物体）和它的笑容（物理属性）可以在空间中分离。
此研究工作所提议的实验装置包括改进的马赫-曾德尔干涉仪，人们可以通过选择合适的预选和后选状态来计算某些可观察量的弱值。研究团队发现光的粒子特性在干涉仪的左臂为零，而波的特性在右臂为零，从而在改进的马赫-曾德尔干涉仪中实现了波粒属性的空间分离。
此外，他们还发现，虽然光的波粒二象性被打破，但在波粒属性空间分离的情况下互补性原理仍然成立。该研究成果在量子基础和量子通信领域具有潜在的应用价值。
陈景灵教授认为，拆分波粒二象性是量子基础的一个重要贡献，其将激发一些新的实验研究。例如，当采用“纯粒子特性”的光进行杨氏双缝实验时，屏幕上的干涉条纹是否会消失；同时，当采用具有“纯粒子特性”的电子进行相应的衍射实验时，电子的衍射效应是否会消失。
Pati教授认为可以在后续工作中实施一个有趣的提议。对于量子柴郡猫来说，它意味着量子物体与其物理特性之间的双方分离。对于拆分波粒二象性而言，它表示量子物体中两个物理特性之间的双方分离。 通过联合量子柴郡猫以及波粒二象性的拆分，人们可以进一步考虑三方分离，即量子物体本身、波动属性和粒子属性之间的三方分离。一旦实现这样的分离，那么人们将获得量子的“超级柴郡猫”。