Transportable Al+ Optical Clock

We are establishing a portable aluminum ion quantum logic optical clock within the HITec building in Hannover. This setup will facilitate comparisons between two clocks of this type, enable side-by-side comparisons with clocks at other metrology institutes, and support demonstrations in relativistic geodesy.

Accurate leveling of the Earth's surface is crucial for environmental monitoring tasks such as observing groundwater resources or documenting the melting of polar ice. Chronometric leveling can provide precise local measurements of gravitational potential through the Einstein redshift in clock frequency. This shift, a consequence of general relativity, causes clocks in a gravitational potential to tick slower compared to a clock outside of it. Since the effect is very small, two highly accurate clocks are necessary, with a fractional frequency accuracy of 10-18 required to achieve a height resolution of approximately 1 cm.

The development of a portable aluminum clock allows us to conduct side-by-side comparisons of two identical clocks in our lab (as detailed in our Aluminum Quantum Logic Clock project), thereby verifying their accuracy. For direct comparisons with clocks at other metrology institutes, we have several options:

  1. Physically transporting the portable clock to another institute for a direct comparison.
  2. Using a highly stable fiber link, like the one between PTB and HITec, for clock comparisons. However, for accuracies at the 10-18 level, we must know the height difference between the two institutes to within 1 cm, a precision unattainable with current leveling methods. Here, an accurate portable clock can be a solution, acting as a mobile calibration standard and incidentally measuring the height differences between institutes.

The aluminum clock's design focuses on portability, compact setups, and modular integration into standard 19-inch racks. Technical improvements over the laboratory system include:

  • A smaller system footprint, utilizing a single small vacuum pump (NEG type), a direct 397 nm laser (without SHG), and a compact 854 nm DFB laser.
  • A titanium vacuum chamber with a low magnetic susceptibility of 1.8x10-4, reducing magnetic field gradients within the ion trap.
  • Increased fiberization of the laser system for potentially more stable operations and reduced need for optical realignment.
  • Assembly on separate breadboards connected via optical fibers, allowing installation in a shipping container for use as a mobile laboratory with quick setup time in the field.
  • Laser realignment to the ions using piezo-motor-driven mirror holders.
  • The clock laser comprises a highly stable cavity (fractional frequency instability of approximately 2x10-16) and a compact, phase-stabilized frequency quadrupling setup with two single-pass SHG stages.
  • The ions will be held in a segmented multi-electrode multilayer linear Paul trap, facilitating ion loading and experiments in different trap segments and the transfer of ions between segments. The precision manufacturing process ensures minimal micromotion for several ions in the trap, allowing for a fractional uncertainty of 10-18-and below.

The Team:

Benjamin Kraus, Constantin Nauk, Joost Hinrichs, Gayatri Sasidharan, Piet O. Schmidt

Former Members:

Stephan Hannig