Fiber source of entangled photons with giant tunable frequency separation

光的非经典态是许多量子技术中不可或缺的工具。最需要的是纠缠光子对（双光子），它们用于量子通信和量子计算以及量子成像、传感和光谱学的单光子的先兆生成。纠缠光子对的各种应用对其生产提出了具有挑战性的要求。对频率范围的可调性和模态内容的可调性都有很高的要求。特别是，量子成像需要可靠的多模光源；相反，纯单光子的生产预示着需要单模双光子。近年来，基于纠缠光子的“诱导相干”效应的“未探测光子”成像和光谱学引起了人们的极大兴趣。由于“诱导相干性”，人们可以通过观察纠缠在一个光子上的光子，对任何一个光子频率的物质进行成像或光谱分析，而这个光子的频率可能是非常不同的。这些方法可以获得“困难”的光谱范围，如中红外（MIR）和太赫兹。未探测光子的成像、传感和光谱学需要光子对内具有较大光谱分离的纠缠光子源。在这个方案中，我们的目标是产生频率间隔超过三个八度的信号光子和空闲光子的纠缠光子对，其中一个在紫外光谱范围内，另一个在红外光谱范围内。在这里，我们计划利用三阶非线性气体填充空心光子晶体光纤。与依赖二阶非线性的源相比，这些源具有几个优点。首先，泵浦的波长不需要比子光子的波长短。我们期望实现与泵浦波长有很大距离的相位匹配的窄边带。此外，系统的分散可以通过填充气体的压力进行精细调节，这使得这样的系统非常通用。这将允许不仅调谐所产生的边带的波长，而且还完全控制所产生的纠缠态的频率模式内容。作为测试，我们将利用获得的光纤源进行时间幽灵成像和诱导相干实验。这样的项目处于两个学科的前沿：非线性光纤和量子光学。

Nonclassical states of light are an indispensable tool in many quantum technologies. Most demanded are pairs of entangled photons (biphotons), which are used for the heralded generation of single photons for quantum communication and quantum computation, and for quantum imaging, sensing, and spectroscopy. The diverse applications of entangled photon pairs lead to challenging requirements to their production. Highly demanded is the tunability of both the frequency range and the mode content. In particular, quantum imaging requires reliable multimode sources; on the contrary, heralded production of pure single photons dictates a need for single-mode biphotons. Recently a lot of interest is attracted to imaging and spectroscopy ‘with undetected photons’, based on the ‘induced coherence’ effect with entangled photons. Due to ‘induced coherence’, one can perform imaging or spectroscopy of any material at the frequency of one photon by looking at the photon entangled to it, which can be at a very different frequency. These methods give access to ‘difficult’ spectral ranges like mid-infrared (MIR) and terahertz.Imaging, sensing, and spectroscopy with undetected photons require sources of entangled photons with large spectral separation within the pairs. In this proposal, we aim at the generation of entangled photon pairs with the signal and idler photons separated in frequency by more than three octaves, one of them being in the UV range and the other in the IR range of spectrum. Here we plan to exploit third-order nonlinearity in gas-filled hollow-core photonic crystal fibers. These offer several advantages over the sources relying on second-order nonlinearity. First of all, the pump does not need to have a shorter wavelength than the daughter photons. We expect to achieve phase-matched narrow sidebands with a very large separation from the pump wavelength. Additionally, the dispersion of the system can be finely adjusted through the pressure of the filling gas, which makes such a system extremely versatile. This will allow not only tuning of the wavelength of the generated sidebands, but also a full control of the frequency mode content of the generated entangles states. As a test, we will carry out time ghost imaging and induced coherence experiments using the obtained fiber source. Such a project is at the frontier of two disciplines: nonlinear fiber optics and quantum optics.