Optical clocks based on an ensemble of trapped ions
Final Report Abstract
During this project we evaluated the usability of a self-built Nd:YAG laser system at 946 nm for clock spectroscopy in In+ ions. In a first step, we characterized the noise spectrum of the laser and resolved frequency noise from amplitude noise with applied intensity stabilization to the laser light. We developed a digital locking electronics for the Nd:YAG laser and stabilized it onto a 30 cm long resonator made of ULE glass. In a next step, our 946 nm laser was compared to an ultra-stable laser at 1542 nm using an optical frequency comb. A fractional frequency instability of 1.1 x 10^-16 in 1 s averaging time, limited by the thermal noise floor of the ULE cavity, was successfully demonstrated. This level of stability will enable clock spectroscopy of approx. 10 In+ ions in a multi-ion clock. Assuming 100 In+ contributing to the clock signal, their estimated quantum projection noise of 7 × 10^-1 /√𝜏(𝑠) would surpass the performance of the 946 nm interrogation laser. Therefore, the stability of our clock laser system was further improved by an active transfer lock onto a cryogenic silicon cavity working over a large distance of 160 m. Within the frame of this project we also addressed questions on ultimate accuracy limits for our multi-ion approach. We investigated systematic frequency shifts for In+ ions doped with Yb+ ions inside an ion Coulomb crystal. In particular, quadrupole shifts arising from the interaction of the ion’s quadrupole moments with electric field gradients, e.g. the charge of neighboring ions, have been evaluated to be on the level of 1 x 10^-19 when 10 In+ are trapped together with 3 Yb+ ions at an axial secular frequency of 205 kHz. For a chain of 10 Yb+ ions the quadrupole shift is on the order of 1 x 10^-16, but the stability of our trap environment was measured to be stable enough to control this shift. Towards the realization of a multiion clock with Yb+, we developed a novel interrogation protocol with suppressed AC Stark shifts during clock spectroscopy. This method of combined error signal is especially promising for the interrogation of the highly-forbidden octupole transition in Yb+, where typically light-induced frequency shifts on the order of 1 kHz have to be faced. With this, our work is not only a step further towards the realization of a first multi-ion optical clock with In+ ions, but also lays the foundation for a possible take up of this new approach with Yb+ clocks.
Publications
- “Combined error signal in Ramsey spectroscopy of clock transitions”, New J. Phys. 20, 123016 (2018)
V. I. Yudin, A. V. Taichenachev, M. Basalaev, T. Zanon-Willette, T. E. Mehlstäubler, J. Pollock, M. Shuker, E. A. Donley, J. Kitching
(See online at https://doi.org/10.1088/1367-2630/aaf47c) - “946-nm Nd:YAG digitallocked laser at 1.1 × 10^−16 in 1 s and transfer-locked to a cryogenic silicon cavity”, Opt. Lett. 44, 1781-1784 (2019)
A. Didier, S. Ignatovitch, E. Benkler, M. Okhapkin, T. E. Mehlstäubler
(See online at https://doi.org/10.1364/OL.44.001781)
