The global positioning system (GPS) is widely known for its applications in navigation, timing, and positioning. However, its accuracy can be greatly impacted by the performance of its receiver clocks, especially for a low-cost receiver equipped with lower-grade clocks like crystal oscillators. The objective of this study is to develop a model to improve the stability of a low-cost receiver. To achieve this, a machine-learning-based linear regression algorithm is proposed to predict the differences of the low-cost GPS receiver compared to the precision timing source. Experiments were conducted using low-cost receivers like Ublox and expensive receivers like Septentrio. The model was implemented and the clocks of low-cost receivers were steered. The outcomes demonstrate a notable enhancement in the stability of low-cost receivers after the corrections were applied. This improvement underscores the efficacy of the proposed model in enhancing the performance of low-cost GPS receivers. Consequently, these low-cost receivers can be cost-effectively utilized for various purposes, particularly in applications requiring the deployment of numerous GPS receivers to achieve extensive spatial coverage.

In the development of high-performance optical quantum memories, Er ³⁺ :CaWO ₄ has emerged as a compelling rare-earth ion-doped solid-state platform owing to its long optical coherence times and compatibility with the 1550nm telecommunications band. In this work, we systematically investigate how optical polarisation and temperature influence the absorption strength, central wavelength, and linewidth of the Z ₁ → Y ₁ and Z ₁ → Y ₂ transitions, with light incident along the crystal a and c axes. We identify the Z ₁ → Y ₁ transition at 1532.6 nm with the incident laser propagating along the c -axis at cryogenic temperatures (∼3 K) as particularly advantageous for memory operation. Under these conditions, the transition exhibits a stable central wavelength, narrower linewidth, polarisation-independent absorption, and an enhanced absorption cross-section, while residing within the telecommunication C-band. These combined attributes position the Z ₁ → Y ₁ line as a robust and suitable point for Er ³⁺ :CaWO ₄ based quantum memories.

We demonstrate an economical wet etching solution for optical fibers using glass etching cream. The etching performance and surface quality on optical fibers are investigated. The solution offers good controllability and repeatable outcomes.