Quantum optics at Terahertz frequencies

研究问题：测量太赫兹辐射对于许多工程挑战至关重要，例如探测隐藏武器、制药业的质量控制以及识别爆炸物等危险。目前，探测太赫兹辐射的主要方法之一是将未知太赫兹辐射与短激光脉冲非线性相互作用产生的信号测量到特定晶体中。在这个项目中，我们的目标是研究一种方法来克服这种依赖于光的量子态特性的检测方案的当前限制。具体来说，压缩辐射可以通过为太赫兹探测提供更低噪声的探测脉冲来增加探测不灵敏度。方法：对于这个项目，我们有三个主要活动。1)新型太赫兹天线2)压缩态的产生和检测3)由于使用非经典态而导致的信噪比和灵敏度增强的量化。在第1部分中，学生将设计一种新的太赫兹发射器，该发射器根据可用激光器的特性进行优化。该活动包括数值模拟和对以往文献的研究。作为备用计划，学生将获得一个已经运行但未优化的太赫兹产生天线。第2部分将看到学生开发压缩辐射源和相关的检测工具。源将是II型非线性晶体的标准参数下转换。相反，检测将需要设计一个特定的电子放大器来驱动两个高量子效率的InGaAs光电二极管。放大器的设计应该使其能够检测到~100kHz的次脉冲噪声信号。第3部分将要求了解标准太赫兹时间分辨检测中的信噪比和动态范围，并将包括学生开发的设置的特征。因此，将使用双模压缩探针脉冲记录这些参数，并与经典情况进行比较。这项研究计划的新颖之处在于利用量子技术，如压缩态计量，来提高一项成熟的、具有广泛影响的技术的能力，即时域太赫兹光谱学。需要开发新的技术工具，例如时域光谱的非经典方案，以及所需的组件，例如太赫兹天线阵列和低噪声电流放大器。

Research questions: Measuring THz radiation is essential for many engineering challenges, such as the detection of concealed weapons, the quality control in the pharmaceutical industry and for the identification of hazards such as explosives. Currently, one of the main approaches to detect THz radiation relies on measuring the signal that originates from the nonlinear interaction between the unknown THz radiation and a short laser pulse into specific crystals. With this project, we aim to investigate a way to overcome the current limitation of this detection scheme relying on the properties of quantum states of light. Specifically, squeezed radiation can increase the detect insensitivity by providing a lower noise probe pulse for THz detection.Approach: For this project, we have three main activities.1) Novel THz antennas2) Generation and detection of squeezed states3) Quantification of the signal-to-noise and sensitivity enhancement due to the use of nonclassical statesFor part 1), the student will design a new THz emitter that is optimised on the characteristic of the available laser. This activity includes numerical simulation and the study of previous literature. As a backup plan, the student will have access to an already functioning, but not optimised THz generation antenna.Part 2 will see the student developing both the source of squeezed radiation and the relevant detection tools. The source will be a standard parametric down-conversion in type II nonlinear crystal. The detection, instead, will require to design a specific electronic amplifier to drive two, high-quantum efficiency, InGaAs photodiodes. The design of the amplifier should be such that it will enable detection of sub-shot-noise signals at ~100kHz.Part 3 will require understanding how the signal to noise ratio and dynamic range are in a standard THz time-resolved detection and will include the characterisation of the setup developed by the student. Hence, these parameters will be recorded using a two-mode squeezed probe pulse and compared to the classical case.Novel engineering and physical sciences contentThe novelty of this research program is in the use of quantum technologies, such as squeezed state metrology, to improve the capabilities of a well-established and broad impact technique, that is, time-domain THz spectroscopy. New technical tools will need to be developed, such as a non-classical scheme for time-domain spectroscopy, along with the required components, such as THz antenna arrays and low-noise current amplifiers.