Squeezed states metrology for THz time-domain spectroscopy.

摘要。我与Clerici博士讨论的项目目标是利用量子光学来提高电磁辐射时间分辨测量的灵敏度。具体来说，它旨在开发量子增强计量工具来检测弱太赫兹频率（THz）波形。电磁波谱的太赫兹区域在技术上对光谱学（例如有害气体和爆炸物）、安全（通过包装材料成像）和制药（质量控制）的应用具有重要意义。然而，由于敏感探测器的可用性有限，难以进入该光谱区域。在我的博士项目中，我将研究使用量子光学技术来改善太赫兹时域光谱技术的现状。背景。电光采样（Electro-Optical Sampling， EOS）是利用短激光脉冲测量二阶非线性晶体（电光效应）中未知场引起的折射率变化的一种常用方法。该技术允许在时域直接记录太赫兹辐射，并通过获得傅里叶变换提供辐射频谱。该技术的灵敏度受到标准量子极限的限制，如果不使用非经典辐射就无法进一步提高。当使用平均光子数N测量数量时，在最好的情况下，即如果没有其他噪声源存在，我们的测量将受到与灵敏度成正比的误差的影响。经典计量学受到这个界限的限制，这个界限被称为标准量子极限。增加光子的数量，从而提高探针脉冲强度，提高测量的灵敏度。在基于eos的太赫兹探测的情况下，探针强度只能增加到非线性噪声的开始所施加的限制。为了进一步提高灵敏度，可以利用量子相关态将误差比例减小到，即海森堡极限。在我的博士项目中，我的目标是利用光的压缩状态实现海森堡有限太赫兹探测。方法。EOS是一种相位测量，通过在二阶非线性晶体中的非线性相互作用，记录探针脉冲上引起的相移，在特定的时间坐标上测量电场。我博士项目的一个主要目标是在太赫兹EOS中测试一种使用双光束[3]的实验方法。这些是光的量子态，其特征是两束光的光子数完全相关。它们是压缩光的可能实现之一，将作为克服太赫兹EOS灵敏度限制的工具进行测试。为了产生和检测双光束，我将扩展我在最后一年孟项目中实施的工作。我将使用Covesion有限公司提供的周期性极性铌酸锂晶体，设计用于ii型相匹配条件。双光束配置所获得的灵敏度的提高，将被量化，这要归功于我在孟最后一年的项目中设计并开始表征的亚射噪声平衡探测器。一种经典的提高相位测量灵敏度的方法包括使用激光腔。腔内相位干涉术（IPI）是标准干涉术的一种腔增强版本，通过用另一个未受扰动的梳子击打腔内的一个梳，可以直接获得发生在一个梳上的相移。具体地说，相位测量是通过记录通过干涉腔梳获得的拍音来完成的。这种技术从未用于测量太赫兹辐射，我的博士项目的进一步目标将是开发基于ipi的太赫兹探测方案。为了实现这一目标，我将在Clerici博士和UNO小组的指导下，我将遵循

Abstract. The project I have discussed with Dr Clerici targets the use of quantum optics to improve the sensitivity of time-resolved measurements of electromagnetic radiation. Specifically, it aims at developing quantum-enhanced metrology tools to detect weak Terahertz-frequency (THz) waveforms. The THz region of the electromagnetic spectrum is technologically important for applications in spectroscopy (for instance hazardous gasses and explosives), security (imaging trough packaging materials) and pharmaceutics (quality control). This spectral region is, however, hard to access due to the limited availability of sensitive detectors. In my PhD project I will investigate techniques to use quantum optics to improve the state of the art of THZ time-domain spectroscopy[1]. Background. Electro-Optical Sampling (EOS) is a commonly employed method where a short laser pulse is used to measure the change in refractive index caused by the unknown field in a second-order nonlinear crystal (electro-optical effect)[2]. This technique allows to record THz radiation directly in the time domain and delivers the radiation spectrum by obtaining the Fourier transform. The sensitivity of this technique is limited by the standard quantum limit and cannot be further improved without the use of non-classical radiation. When measuring a quantity using an average number of photons N, in the best case, i.e. if no other sources of noise are present, our measurement will be affected by an error that is proportional to the sensitivity. Classical metrology is limited by this bound, known as the standard quantum limit. Increasing the number of photons, and hence the probe pulse intensity, improves the sensitivity of the measurement. In the case of EOS-based THz detection, the probe intensity can only be increased up to a limit that is imposed by the onset of nonlinear noise. To further increase the sensitivity, quantum correlated states can be utilised to reduce the error proportionality to,which is the Heisenberg limit. In my PhD project I will aim at attaining a Heisenberg-limited THz detection using squeezed states of light. Methodology. EOS is a phase measurement where the electric field is measured at a specific temporal coordinate by recording the phase shift induced on the probe pulse, via a nonlinear interaction in a second-order nonlinear crystal. A primary goal of my PhD project will be to test an experimental approach inTHz EOS that uses twin-beams[3].These are quantum states of light characterised by two beams with a perfect correlation in their photon number. They are one of the possible realisations of squeezed light and will be tested as a tool to overcome the sensitivity limit of THz EOS. To generate and detect twin-beams, I will expand on the work that I have implemented during my final year MEng project. I will use a periodically poled Lithium Niobate crystal provided by Covesion Ltd. designed to work in a type-II phase matched condition. The improvement in sensitivity acquired with the twin-beam configuration, shall be quantified thanks to the sub-shot-noise balanced detector that I designed and started to characterise during my MEng final year project. A method to classically enhance the sensitivity of phase measurements, comprises of using a laser cavity. Intracavity phase interferometry (IPI) is a cavity enhanced version of standard interferometry that provides direct access to the phase shift occurring on one comb of a cavity by beating it with a second, unperturbed comb. Specifically, the phase measurement is performed by recording the beat note obtained by interfering the cavity combs. This technique has never been used to measure THz radiation, and a further goal of my PhD project will be to develop an IPI-based THz detection scheme. To achieve this goal, I will follow be supervised by Dr Clerici and the UNO group, and I will follow