High precision frequency references are a crucial component of many applications such as modern communication systems, electronic trading, positioning (GPS) and fundamental physics. These applications benefit from portable atomic clock systems with significantly improved accuracy. Commercially available systems are usually based on hyperfine transitions in caesium, rubidium or hydrogen and achieve a fractional frequency uncertainty of typically 10-15 (for an integration time of 1000s) and about 10-13 for portable systems.
In contrast, the best atomic clocks are based on optical transitions in trapped atomic ions. Whilst these systems reach a frequency uncertainty of better than 10-17 they are large and heavy with high power consumption. Depending on the specific utilised atomic ions, these systems require large and complex laser systems as well as precisely aligned bulk optics to condition the laser light, and to deliver it to the trapped ion. Typically, such a laser system usually fills the area of a large optical table and has several 100s W of power consumption. Furthermore, in order to detect the internal state of the trapped ion, large numerical aperture lens systems are required which are space consuming and require precise alignment. Thus, even though the ion trap structure and the vacuum system are easily integrated into a volume of a few cm3, the size of the entire system usually fills an entire laboratory.
In this project we build a portable atomic clock based on the quadruple transition in calcium ions. Taking advantage of recent technological developments, we integrate optics into the trapping electrode structure to deliver the laser light required for generating, cooling and detecting ions within the trap and to probe the clock transition. Furthermore, bulk-optic collection lenses are replaced by large numerical aperture optical fibres close to the trapped ion and an all-fibre coupled laser system for trapping calcium ions is developed. With the recent developments of narrow linewidth, fibre pigtailed diode lasers, compact, rugged and vibration immune laser systems became feasible. Additionally, fibre optical components and devices such as acousto-optical modulators, fibre isolators and fibre combiners enable the implementation of laser systems without any bulk optics.