Compact Terahertz Clock

近几十年来，原子钟已经从纯粹的研究仪器发展成为不可或缺的基础设施关键设备。原子钟现在广泛用于全球导航卫星系统（GNSS）、数据中心、电力和移动的网络、金融市场的交易时间戳以及研究和开发。目前，许多需要高精度定时的应用依赖于GNSS信号。然而，这使得关键的基础设施容易受到全球导航卫星系统干扰和故障的影响，造成严重的社会经济后果。因此，需要本地高性能原子钟来防止这种情况。其他应用程序需要在拒绝全球导航卫星系统的环境中运行，例如潜艇导航或电子战和其他安全情况。目前的便携式时钟稳定性和准确性有限，或者对于移动的平台上的应用来说过于庞大和敏感。虽然在高精度光学原子钟的激光系统和光谱单元的升级方面取得了巨大进展，但仍有两个主要挑战需要克服：参考激光器需要高精细度光腔和光频梳（OFC）在这里，我们提出采用拉曼跃迁来创建一个高度稳定的精确的原子钟与光学原子钟相比，原子参考稳定性不转移到单个激光器的频率，而是编码在两个拉曼激光器之间的频率差中。为了实现太赫兹时钟，我们提出了利用射频离子阱中捕获的钙离子和D3/2能级与D5/2能级之间的拉曼跃迁来实现太赫兹时钟。这两个状态之间的频率分裂为1.819 THz，并且时钟的预期分数频率精度优于10-14（系统精度优于1 e-15），外形尺寸为20升，明显小于当前的光学时钟系统。由于其高精度与小SWAP以及鲁棒性，这种新颖的时钟特别适合于在移动的平台上和在环境控制低的位置中的应用。这种便携性使其特别适合国防和安全部门的应用，并作为电信和公用事业网络以及数据中心和金融市场的GNNS保持时钟，保持时间长达数月。此外，它还可以实现跨大规模电信网络的频率传播和同步的新方案。在这个项目中，我们将建立太赫兹时钟与CPI提供的设备，测试其性能和测试系统在一些应用相关的场景。CPI将在其测试设施中进行环境测试，列奥纳多将在移动的平台上测试时钟的性能，英国电信将研究下一代大型光纤网络频率传播和同步方案。

In recent decades, atomic clocks have developed from being solely research instruments to indispensable and infrastructure-critical devices. Atomic clocks are now widely used in Global Navigation Satellite Systems (GNSS), data centres, power and mobile networks, financial markets for transaction time stamping, and research and development. Presently, many applications requiring high-precision timing rely on GNSS signals. However, this makes crucial infrastructure vulnerable to GNSS tampering and failure, with significant socio-economic consequences. Therefore, local high-performance atomic clocks are needed to safeguard against this. Other applications need to function in a GNSS-denied environment such as the navigation of submarines or electronic warfare and other security situations. Clock performance beyond GNSS capability is also required for state-of-the-art scientific research and advanced timekeeping.Current portable clocks currently have limited stability and accuracy or are too large and sensitive for applications on mobile platforms. While there has been immense progress in the miniaturisation of the laser systems and spectroscopy units for high-precision optical atomic clocks there are still two main challenges to overcome: The reference laser that requires a high-finesse optical cavity and the optical-frequency comb (OFC) that is required to convert the optical reference signal to a usable electronic signal.Here we propose to employ Raman transitions to create a highly stable and accurate atomic clock. In contrast to optical atomic clocks, the atomic reference stability is not transferred to the frequency of a single laser but is encoded in the frequency difference between two Raman lasers. This significantly relaxes requirements on the OFC and the optical cavity for the clock lasers.For the realisation of a THz-clock, we propose using calcium ions trapped in an RF ion trap and the Raman transition between the D3/2-level and the D5/2-level. The frequency splitting between these two states is 1.819 THz and the expected fractional frequency accuracy of the clock is better than 10-14 (systematic accuracy better than 1e-15) with a 20-litre form factor, significantly smaller than current optical clock systems.Due to its high accuracy in conjunction with small SWAP as well as robustness, this novel clock is exceptionally fit for applications on mobile platforms and in locations with low environmental control. Such portability, makes it particularly well suited for applications in the defence and security sector and as GNNS holdover clocks for telecom and utility networks as well as data centres and financial markets with holdover times of several months. Additionally, it enables novel schemes for frequency dissemination and synchronisation across large-scale telecom networks. Within this project, we will set up the THz-clock with equipment provided by CPI, characterise its performance and test the system in some application-relevant scenarios. CPI will perform environmental testing in their test facility, Leonardo will test the clock's performance on a mobile platform, and BT will investigate next-generation schemes for frequency dissemination and synchronisation across large optical fibre networks.