现代量子技术的进步:任意波长的单个光子的频率转换

诸平

A spatio-temporal hologram of molecular vibrations is created in the gas by stimulated Raman scattering. This hologram is then used for highly efficient, correlation-preserving frequency conversion of single photons. Credit: Nicolas Joly/Max Planck Institute for the Science of Light

据德国马克斯·普朗克光科学研究所（Max Planck Institute for the Science of Light）2022年5月6日提供的消息，现代量子技术的进步,可以实现对任意波长的单个光子的频率转换（A step forward in modern quantum technology: Frequency conversion of single photons at arbitrary wavelengths）。

光量子即光子（photons），构成了现代密码网络中量子密钥分配的基础。然而，在量子技术的巨大潜力完全实现之前，仍存在一些挑战。现在已经找到了其中一个问题的解决方案。

2022年5月5日在《科学》（Science）杂志网站上发表的一篇论文——R. Tyumenev, J. Hammer, N. Y. Joly, P. St. J. Russell, D. Novoa. Tunable and state-preserving frequency conversion of single photons in hydrogen. Science, 5 May 2022, 376(6593): 621-624. DOI: 10.1126/science.abn1434. https://www.science.org/doi/10.1126/science.abn1434

在此文中，由大卫·诺沃亚（David Novoa）、尼古拉斯·乔利（Nicolas Joly）和菲利普·拉塞尔（Philip Russell）领导的团队，报告了基于充满氢气的空心光子晶体光纤(photonic crystal fiber简称PCF)在单个光子频率上转换方面的突破。首先，通过受激拉曼散射在气体中产生了分子振动的时空全息图。然后，这个全息图被用于对单个光子进行高效的、保持相关的频率转换。该系统在一个压力可调的波长下运行，这使得它在量子通信中具有潜在的吸引力。在量子通信中，难以分辨的单光子的有效来源无法在与现有光纤网络兼容的波长上使用。上述由马克斯·普朗克光科学研究所尼古拉斯·乔利提供的图片，是通过受激拉曼散射，在气体中产生了分子振动的时空全息图。然后，这个全息图被用于对单个光子进行高效的、保持相关的频率转换。

此方法结合了量子光学（quantum optics）、基于气体的非线性光学（nonlinear optics）、空心PCF和分子振动物理学，形成了一种有效的工具，可以在从紫外到中红外的任何光谱波段工作，这是现有技术无法实现的超宽工作范围。这些发现可能用于开发基于光纤的工具，如量子通信（ quantum communications）和量子增强成像技术（quantum-enhanced imaging）。

参与此项研究的除了来自马克斯·普朗克光科学研究所的研究人员之外，还有来自德国埃朗根的弗雷德里希-亚历山大大学（Friedrich-Alexander-Universität, Erlangen, Germany）和西班牙毕尔巴鄂的巴斯克地区大学(University of the Basque Country, Bilbao, Spain)以及西班牙巴斯克科学基金会（Basque Foundation for Science, Spain）的研究人员。

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定制单光子:光控光子作为新技术的关键（Tailored single photons: Optical control of photons as the key to new technologies）

A quantum frequency converter

Practical quantum technologies require interfacing of several subsystems operating in widely different spectral regions. However, most current approaches offer only very small frequency shifts and limited tunability. Tyumenev et al. demonstrate efficient (up to 70%) quantum state–preserving photon frequency up-conversion by molecular modulation in hydrogen gas–filled antiresonant photonic crystal fibers (see the Perspective by Sokolov). By combining a high-intensity coherent pump pulse with a (low-intensity) quantum source based on parametric down-conversion (generating pairs of signal and idler photons), the authors show that the idler photons can be up-converted by 125 terahertz using a Raman-scattering process. The process is general and should be applicable to other quantum sources. —ISO

Abstract

In modern quantum technologies, preservation of the photon statistics of quantum optical states upon frequency conversion holds the key to the viable implementation of quantum networks, which often require interfacing of several subsystems operating in widely different spectral regions. Most current approaches offer only very small frequency shifts and limited tunability, while suffering from high insertion loss and Raman noise originating in the materials used. We introduce a route to quantum-correlation–preserving frequency conversion using hydrogen-filled antiresonant-reflecting photonic crystal fibers. Transient optical phonons generated by stimulated Raman scattering enable selective frequency up-conversion by 125 terahertz of the idler photon of an entangled pair, with efficiencies up to 70%. This threshold-less molecular modulation process preserves quantum correlations, making it ideal for applications in quantum information.