Portable Trapped-Ion Optical Frequency Standard
We are setting up a second, portable aluminum ion quantum logic optical clock to enable comparisons between two clocks of this type, direct comparisons to clocks at other metrology institutes, and demonstrations of relativistic geodesy.
Accurate levelling of the earth´s surface is a fundamental requirement for environmental monitoring applications like groundwater resource observation or documentation of polar ice decay. Currently, there exist two basic types of levelling methods: satellite-based levelling making use of differential gravitational forces on test masses  and optical levelling on the earth´s surface. While the first method covers essentially the entire planet, its spatial resolution is on the order of one hundred kilometers. The second method offers high spatial resolution, but is prone to errors and ambiguities resulting from the integration of measurements over intercontinental distances. Moreover, optical levelling cannot bridge levelling grids between landmasses which are not in direct line of sight.
Chronometric levelling  can overcome many of these drawbacks by making use of the Einstein shift of the clock frequency in the local gravitational potential. This shift is a measureable result of general relativity: an observer sees two clocks at different gravitational potentials “ticking” at different speeds. This requires two highly accurate clocks, namely, a fractional frequency accuracy of 10-18 is required for a height resolution of about 1 cm.
Building a portable aluminum clock enables us to compare two clocks of the same type in our lab (see Aluminium Quantum Logic Clock) side by side and therefore check their combined accuracy. To directly compare our clock(s) to clocks at other metrology institutes, there are several possibilities:
We can take the portable clock to the other institute and carry out a side-by-side comparison.
If there is a highly stable fiber link between the institutes, as there is between PTB and SYRTE , we could in principle use that for a clock comparison. However, comparing the clock accuracies on 10-18 level, this requires knowledge of the clock height difference between the two institutes accurate to 1 cm, which cannot be provided by existing levelling methods. An accurate portable clock can help solve this problem, serving as a mobile calibration standard (and measuring the height differences between institutes as a helpful side effect).
The key driving forces for the design of the second aluminum clock are portability and technical improvements compared to mark 1. Namely:
The second system is smaller than the first one, making use of only one small vacuum pump (NEG type), a direct 397 nm laser (no SHG) and a compact 854 nm DFB laser, for example.
The vacuum chamber is made of titanium featuring a very low magnetic susceptibility of 1.8x10-4, which helps to minimize the magnetic field gradient within the ion trap.
Compared to mark 1, more parts of the laser system are designed to be fiberized, potentially providing more stable operation with less need for realignment of optics.
The system is set up on distinct breadboards connected via optical fibers. Therefore, it can be installed in a shipping container, which can serve as a mobile laboratory for measurements in the field with limited set-up time.
The ions will be confined in a segmented multi-electrode multilayer linear Paul trap (as described here).
This trap features new experimental opportunities. Since the trap is segmented, the ions can be loaded in one segment and then shuttled to another “spectroscopy section”. This spatial separation of loading and measuring provides the opportunity to use ablation loading without generating patch charges that would influence the ions´ motion during the measurement via fluctuating electrical fields. Due to the high precision manufacturing process, stray RF fields are small over a large region of the trap, so that even with several ions in the trap micromotion is small enough for measurements with a fractional uncertainty of 10-18.
Confining several ions in one trap is advantageous in several respects, for instance:
The stability of any atomic frequency standard scales proportional to n-1/2, with n the number of interrogated particles. This means that interrogating multiple ions reduces the time required to reach a given statistical uncertainty.
Trapping multiple aluminum and calcium ions together in one trap enables us to investigate novel Quantum Logic Spectroscopy readout schemes. For instance, the electronic state of the clock ions might be mapped to the logic ions by encoding their state in a binary format. Then the number of logic ions would scale logarithmically with the number of clock ions .
Building a portable aluminum clock also requires finding new solutions to technical challenges. As one example, multiple robust frequency doubling cavities are required, which withstand transportation and operation in a non-lab environment. Therefore, a mechanically monolithic hermetically sealed frequency-doubling cavity for generation of the UV light has been developed, set up and tested successfully.
 Bjerhammar, Bull. Géodesique (1985)
 C. Lisdat et al., A clock network for geodesy and fundamental science, arXiv:arXiv:1511.07735 [physics], November 2015
 M. Schulte et al., Quantum Algorithmic Readout in Multi-Ion Clocks, Phys. Rev. Lett. 116, 013002 (2016)
Stephan Hannig, Nils Scharnhorst, Johannes Kramer, Lennart Pelzer, Nicholas Spethmann, Piet O. Schmidt