涡旋光子穿过双缝,看到的是天使还是魔鬼?

光子是高速、大容量信息处理极好的信息载体,因其能以极低的损耗和没有任何已知的退相干机制在自由空间中长距离传输。光子作为一个基本粒子具有波粒二象性。在双缝干涉中,每个光子经过某一路径到达观测点并发生自干涉,从而表现出粒子性;同时大量光子干涉的行为统计表现出概率分布的波动性。

光子的一个特殊性质,即轨道角动量(OAM),由螺旋波前为exp(ilΦ)的涡旋光束所携带,其中l表示螺旋模式指数或拓扑荷,而Φ表示垂直于传播方向平面上的方位角坐标。与光的不同物理维度,如偏振、波长和时间一样,OAM作为一种新的自由度和独立的物理维度也可以用来承载独立的信息通道。量子力学上,涡旋光束携带的OAM即是每光子具有的离散值lℏ (是约化普朗克常数)。特别地,OAM在理论上具有无限维的正交性和完备性,这些特性为光学和量子超高容量信息处理打开了大门。但是,携带角动量的涡旋光子在双缝干涉中的量子特性尚不清楚。

南京大学的王慧田教授和南开大学李勇男教授课题组在Chinese Optics Letters 2020年第18卷第10期(W. Qi, et al., Double-slit interference of single twisted photons)报道了携带轨道角动量的单光子在双缝干涉中的基本特性。此项工作基于平均光子轨迹(APTs)从理论上描述了携带OAM涡旋单光子在双缝干涉中的干涉行为。

光子平均轨迹描述了双缝干涉中涡旋光子的粒子行为(左侧)和波动特性(右侧)。

不确定性原理表明,人们不可能讨论量子粒子的确定轨迹,因为任何位置测量都不可撤销地干扰动量,反之亦然。然而,它允许定义一组量子粒子的平均轨迹。因此,平均光子轨迹可以间接地描述光子的类粒子行为。

模拟结果表明,平均光子轨迹具有明显的螺旋结构,这意味着涡旋光子的传播速度低于既定的光速。平均光子轨迹的端点概率变为周期性弯曲条纹,反映了涡旋光子的波动特性。

实验上,该项工作利用非线性晶体中自发参量下转换效应产生的关联光子对作为单光子源,其中一个光子经调制后携带轨道角动量并入射到双缝上作为信号光子,另一个光子作为闲频光子入射至单光子探测器产生电信号用以触发增强电荷偶合检测(ICCD)时间门控相机。ICCD相机最大限度保证光子被单独探测并有效降低噪声,当一个闲频光子进入预报单光子探测器的路径并触发ICCD相机时才会记录数据。在较短曝光时间下,ICCD相机记录的探测点在空间随机分布,说明光子在空间传播的粒子性。持续增加曝光时间,探测点的空间分布逐渐表现出规律性的弯曲条纹状与理论模拟结果相符。该工作为理解微观量子行为提供了一种直观的方法。

该方法也可用于探索其它量子粒子的微观行为。借助于更先进、更灵敏的量子检测技术,螺旋粒子的类螺旋性质有望应用于量子精密测量。

Double-slit interference of single twisted photons

Photons are excellent information carrier for high-speed and large-capacity information processing as they can be transmitted over a long distance with very low loss and without any known decoherence mechanism in free space. As one of fundamental particles, photons have wave-particle duality demonstrated in double-slit experiment. In this experiment, each photon interferes only with itself due to the quantum uncertainty of which path through the optical apparatus it takes, which associates to a particle-like behavior. The interference pattern, which is the accumulated sum of many single photon interference events, accords to a wave-like behavior.

A special property of photons is their orbital angular momentum (OAM) carried by the vortex beam with a helical wavefront of exp(ilΦ), where l denotes the helical mode index or the topological charge and Φ refers to the azimuthal coordinate that is in the plane transverse to the propagation direction, respectively. The different physical dimensions of light, such as polarization, wavelength, and time, OAM is a new degree of freedom of light and an independent physical dimension that can be utilized to carry independent information channels. In quantum mechanics, the amount of OAM carried by the vortex beam is discrete value of lℏ per photon ( is the reduced Planck's constant). In particular, OAM has opened the door for boosting optical and quantum ultrahigh-capacity information processing, due to its theoretically unbounded orthogonality and completeness. But the quantum property of single twisted photons carrying the OAM in double-slit is unclear.

The research group led by Prof. Huitian Wang from Nanjing University and Prof. Yongnan Li from Nankai University studies the fundamental property of the OAM at the single-photon level in the double-slit interference. The research results are published in Chinese Optics Letters, Volume 18, No. 10, 2020 (W. Qi, et al., Double-slit interference of single twisted photons). The interference behavior of single twisted photons carrying OAM in the Young's double-slit has been theoretically described by average photon trajectories (APTs).

The double-slit interference process of twisted photons. The averaged photon trajectories demonstrate the helical particle-like behavior (left) and wave character (right) of twisted photons.

The double-slit interference process of twisted photons. The averaged photon trajectories demonstrate the helical particle-like behavior (left) and wave character (right) of twisted photons.

The uncertainty principle suggests that one may not discuss the definite trajectory of a quantum particle, because any measurement of position irrevocably disturbs the momentum, and vice versa. However, it is permitted to define a set of average trajectories for an ensemble of quantum particles. So the APTs can describe indirectly the particle-like behavior of photons.

The simulations show that the APTs exhibit obvious helical structures, which means that the twisted photon travels slower than the established speed of light. The probability of the end-point of the APTs becomes periodic bent-fringed pattern, which reflects the wave character of twisted photons.

Experimentally, they use the correlated photon pairs generated from nonlinear crystal based on spontaneous down conversion as the single photon source, where the signal photon is modulated to carry OAM and then incident to the double-slit, while the idler photon incident on the single-photon detector produces the electrical signal to trigger the intensified Charge-Coupled Detector (ICCD) camera. The time-gated ICCD camera is used to detect individually the signal photons and to reduce the noise. When an idler photon enters into the path of the heralding single-photon detector, the triggered ICCD camera will only record the event of the signal photon. Under the short exposure time of the ICCD camera, the signals recorded by the ICCD camera are randomly distributed in space, indicating the particle nature of photon propagation in space. As the exposure time increases, the spatial distribution of the events detected by the ICCD camera gradually exhibits a regular bent-fringes pattern, just as the theoretical simulation results. This work provides us a more visual and intuitive method for understanding of quantum behaviors.

The method presented in this work can also be used to explore the microscopic behavior of other quantum particles and the helical particle-like property can be applied to quantum precision measurement with help of more advanced and sensitive quantum detection technology.