Building an optical clock based on the Th-229 nuclear transition is the ultimate goal of the nuClock project. We like to claim that such a nuclear clock will be less sensitive to perturbations (because the nucleus is so much smaller than the orbits of valence electrons), offer a supreme quality factor (because the transition energy is so large, and the lifetime of the isomer is so long), and outperform existing clocks in flat-out all respects (well, we become a little bit emotional sometimes). We also like to claim that the nuclear clock will be very robust in operation, ideally suited for geodesy and space applications. In terms of robustness, there is now an experiment that sets a new standard.
The PTB group of Christian Lisdat built, operated, and characterized a transportable optical clock (TOC, as compared to SOC (space optical clock) and NOC (nuclear optical clock)). Such TOCs are required to compare distant optical clocks where no suitable fiber link exists. There are two other strategies for the comparison of clocks: satellite links for inter-continental comparisons, and transportable Cs clocks for same-continent comparisons, but both of these are too weak to fully exploit the accuracy of today’s best optical clocks. Transportable clocks would also be able to measure the geoid via the gravitational redshift, and they would do so with higher spatial resolution compared to satellite missions, and potentially with higher precision.
The TOC presented by PTB is based on Sr-87 and operates very much like a standard laboratory-based Sr lattice clock. A few adaptations have been applied: the Zeeman slower is made of permanent magnets to reduce heat dissipation and power consumption by coils, and a total of eight temperature sensors have been placed near the atoms (both inside and outside the vacuum) to calculate BBR corrections. Following a general trend, the diode/TA laser system for the optical lattice has been replaced by a Ti:Sapph model to avoid ASE-related issues.
The clock reaches a systematic uncertainty of 7 x 10^-17, limited by lattice Stark shifts (which have not yet been fully characterized) and cold collisions. The clock averages down as 1.3 x 10^-13 per root-tau, reaching an agreement with a stationary Sr clock in the 10^-17 range within about two minutes. These two parameters are about two orders of magnitude better than Cs microwave standards and make this TOC the best optical clock that actually left the laboratory.
A short sidenote: it seems that the PTB team forgot to show that the clock actually is transportable… we miss that YouTube video showing the PTB trailer stuck in traffic.
The publication is already available on the arXiv preprint server and has recently been accepted by Phys. Rev. Lett. for publication.