科学家在Franson干涉实验中使用真热光源完成二阶干涉实验

干涉现象是光具有波动性的最重要证明。波的相位是解释叠加光束的干涉相长和相消的关键概念。此外,把光看作粒子或光子时,单个光子也会自相干。虽然光兼具波动性和粒子性,但在一阶干涉情况下,光波场振幅的线性叠加与光子概率振幅的线性叠加是相同的。

量子干涉存在于经典物理学之外,它是指由于光的量子特性而产生的多光子干涉。双光子干涉是多光子干涉中最简单的一种,它是一种二阶干涉现象。在实验量子光学中,多光子干涉是量子光学研究和量子信息应用(如量子通信、量子模拟、量子计算和量子计量)的核心。

1989年,Franson利用参量下转换过程中纠缠光子对的发射时间不确定性原理,提出了一种双光子干涉实验方案。Franson干涉利用非平衡干涉仪中两个双光子振幅的相位差进行重合检测,对理解纠缠的非局域性和时间—能量纠缠光子的特性具有重要意义。

到目前为止,已经利用纠缠光子对验证了部分Franson干涉实验,也进一步表明了Franson干涉是具有时间—能量纠缠光子的光子源非经典性质的最好证明。近日,韩国釜山国立大学Han Seb Moon教授带领的研究团队在Photonics Research 2021年第1期上(Jiho Park, Heonoh Kim, Han Seb Moon. Second-order interference of true thermal light from a warm atomic ensemble in two independent unbalanced interferometers[J]. Photonics Research, 2021, 9(1): 01000049)首次报道了使用热原子系综发射的真热光,并且在两个空间上分离的非平衡迈克尔逊干涉仪中传播的二阶干涉实验。

Franson型干涉仪中源于多普勒展宽级联式87Rb原子真热光的二阶干涉示意图

热光分为真热光和赝热光。常见的真热光有白炽灯、太阳光等,而赝热光是激光照射旋转毛玻璃产生的散斑光场,其统计性质与真热光场相似,但相干时间远大于真热光场,因此常用于各类实验。

实验中,他们利用集体双光子相干性,使用源于多普勒展宽级联式87Rb原子的超辐射热光。由于原子系综热光具有长相干时间,从而通过时间分辨重合探测可以观测到它在两个独立的非平衡迈克尔逊干涉仪中的二阶干涉。

最有趣的是,两个空间分离干涉仪中干涉热光的时域波形与Franson干涉法中时间—能量纠缠光子对的时域双光子波形相似。

Han Seb Moon教授认为,该研究对量子信息科学及其相关应用领域有重大意义。

Second-order interference of true thermal light from warm atomic ensemble in two independent unbalanced interferometers

The interference phenomenon forms the most important evidence of the wave-like property of light. The phase of the wave is the key concept underlying the interpretation of constructive and destructive interferences of superposed light beams. On the other hand, it is known that even when light is considered as a particle or photon, a single photon can interfere with itself. Although light exhibits both wave- and particle-like properties, the linear superposition of the field amplitudes for light waves is the same as that of the probability amplitudes of a photon in the case of first-order interference.

However, beyond the classical physics perspective, we can consider quantum interference, which refers to multi-photon interference due to the quantum nature of light. The simplest multi-photon interference is two-photon interference, which is a second-order interference phenomenon. In experimental quantum optics, multi-photon interference lies at the heart of quantum optics research and quantum information applications such as quantum communication, quantum simulation, quantum computing, and quantum metrology. In particular, Franson interference beyond the single-photon coherence length in twin nonlocal unbalanced interferometry experiments is considered a counterintuitive phenomenon from the viewpoint of classical physics. The famous Franson interference experiment is important for understanding both the nonlocal nature of entanglement and the characteristics of time–energy entangled photons via coincidence detection according to the phase difference between the two two-photon amplitudes in unbalanced interferometers. Thus far, several Franson interference experiments have been demonstrated with the use of entangled photon pairs, keeping in mind that Franson interference is regarded as evidence of the nonclassical nature of photon sources with time–energy entangled photons.

Recently, the research group led by Prof. H. S. Moon from the Pusan National University of South Korea reported, for the first time to the best of our knowledge, a second-order interference experiment using real thermal light emitted from a warm atomic ensemble and propagated along two spatially separated unbalanced Michelson interferometers (UMIs), which is published in Photonics Research, Vol.9, No.1, 2021(Jiho Park, Heonoh Kim, Han Seb Moon. Second-order interference of true thermal light from a warm atomic ensemble in two independent unbalanced interferometers[J]. Photonics Research, 2021, 9(1): 01000049).

Second-order interference with real thermal light from Doppler-broadened cascade-type 87Rb atoms in Franson-type interferometer

They used superradiantly emitted thermal light from Doppler-broadened cascade-type 87Rb atoms by exploiting collective two-photon coherence. The long coherence time of the thermal light from the atomic ensemble enables the observation that its second-order interference in the two independent UMIs by means of time-resolved coincidence detection.

A most interesting aspect is that the temporal waveforms of the interfering thermal light in the two spatially separated interferometers exhibit similarities with the temporal two-photon waveforms of time–energy entangled photon pairs in Franson interferometry.

Prof. H. S. Moon believes that their study makes a significant contribution to the literature because of its immediate bearing on quantum information science and related fields of application.