Dr. Nils Huntemann, Physikalisch-Technische Bundesanstalt, Braunschweig, Germany

«Optical Clocks with Trapped Yb⁺ Ions»

The talk summarizes work on optical atomic clocks at PTB employing trapped Yb⁺ ions. The ¹⁷¹Yb⁺ ion provides two atomic transitions that are well suited as a frequency reference: the ²S_1/2-²D_3/2 electric quadrupole (E2) transition at 436 nm and the ²S_1/2-²F_7/2 electric octupole (E3) transition at 467 nm. The latter is known for its yearlong excited state lifetime and shows smaller sensitivity to external electric and magnetic fields. While we evaluate systematic uncertainties of our clocks based on the E2 transition to about 3E-17, an order of magnitude smaller uncertainty is achieved using the E3 reference transition [1]. This evaluation has recently been supported by an agreement of two clocks based on the E3 transition in our laboratory with 4E-18 total uncertainty [2].  Because of the electronic structure of the excited ²F_7/2 state, the data acquired over 6 months allowed us to improve the best limits for a violation of local Lorentz symmetry for electrons by 2 orders of magnitude. Repeated measurements of the frequency ratio of the E3 and E2 transition can be used to test local position invariance. Here, we found the most stringent limits for temporal drifts and a potential coupling to gravity for the fine structure constant a and the proton-to-electron mass ratio m [3]. To further enhance the clock performance, we are presently pursuing research on composite clock systems in which we use Sr⁺ as an ancillary ion to provide sympathetic cooling during the coherent interrogation on the E3 transition. Furthermore, Sr⁺appears to provide means to increase the accuracy of Yb⁺ clocks.

While aiming at highest accuracy with our optical clocks, we also work towards robust long-term operation for contributions to the international atomic time scale TAI. Within the German quantum technology project opticlock, in collaboration with industry partners, we have developed a demonstrator of an optical clock for applications beyond basic research. This clock is set up in two 19” racks, uses the E2 transition of Yb⁺ as the reference, and recently demonstrated 14 days of unattended operation [4].

[1] N. Huntemann et al., PRL 116, 063001 (2016).
[2] C. Sanner et al., Nature 567, 204 (2019).
[3] R. Lange et al., PRL 126, 011102 (2021).
[4] www.opticlock.de

 Zoom Conference Link: https://unibas.zoom.us/j/92393280711?pwd=a2NPMkpmOXB6aUVKclo1YldPTlhMQT09
Meeting ID: 923 9328 0711, Passcode: 176456