Phase Locked Atomic Interferometers for Gravity Gradiometry

精确的重力测量可以揭示我们周围隐藏的结构的有趣和重要的细节。由于重力不能被屏蔽，一种可以被动地进行这种测量的工具有许多应用，从测量地球的大地水准面到测量地下人造结构。量子技术的发展使得使用原子作为惯性传感器成为可能，这大大提高了这些测量的准确度和精确度。这些设备如何工作的关键过程是通过投掷原子并测量它们的多普勒频移的变化来检测加速度。重力梯度法测量的是两个选定点之间的重力差，而不是单个位置的绝对值。这样做的好处是抑制了共模噪声，这是绝对和相对测量中的一个问题，对于实验室以外的应用来说也是一个严重的问题。重要的是，这两个位置由严格的基线连接，并由相同的光场寻址。目前的原子干涉仪使用单一的拉曼光束通过两个原子云来消除这种共模噪声。这些云通常在同一个真空室中。不幸的是，这限制了此类设备的几何形状和便携性，使它们不适合许多实际应用，因为两个原子云必须彼此视线，而超高真空系统体积大，耗电量大。该项目旨在通过将原子云分离到单独的真空室并将它们与稳定的光纤干涉仪连接来解决这些问题。有源光纤干涉仪将起到锁相机制的作用，以保持拉曼光束的关联，从而保持两个原子云的关联。因此，可以在两个原子云之间无视线的情况下抑制共模噪声。这遵循了原子钟中用于分配光学相位的技术，并展示了这一努力所需的稳定水平。以这种方式分离原子云使我们具有极大的灵活性和便携性，为实际应用提供了这样一种设备。

Accurate measurements of gravity can reveal intriguing and important details of the hidden structure ofour surroundings. Since gravity cannot be shielded a tool which can passively make such measurementshas many applications; from measuring the geoid of the earth to surveying subterranean man-madestructures. Developments in quantum technology have enabled the use of atoms as inertial sensors whichgreatly increases the accuracy and precision of these measurements. The key process of how thesedevices work is by dropping atoms and measuring the change in their Doppler shift to detect acceleration.Gravity gradiometry measures the difference of gravity between two chosen points rather than theabsolute value at a single position. This has the benefit of rejecting common-mode noise, a problemin absolute and relative measurements and a serious issue for applications outside of the laboratory. Itis important that the two locations are connected by a rigid baseline and are addressed by the samelight field. Current atomic interferometers use a single Raman beam passing through both atom cloudsto eliminate this common-mode noise. These clouds are typically in the same vacuum chamber.Unfortunately this limits geometry and portability of such devices, making them unsuitable for manypractical applications, as the two atom clouds must have line of sight to each other and ultra-high vacuumsystems are bulky and power hungry.This project aims to address these problems by separating the atom clouds into separate vacuumchambers and connecting them with a stabilised optical fibre interferometer. The active optical fibreinterferometer will behave as a phase-locking mechanism to maintain the correlation of the Raman beamsand hence the correlation of the two atom clouds. Thus allowing the rejection of common-mode noisewithout line-of-sight between the two atom clouds. This follows a technique used in atomic clocks fordistributing optical phase and demonstrates the required stability levels for this endeavour. Separatingthe atom clouds in this manner enables us a great deal of flexibility and portability; making such a deviceideal for practical applications.