Project B03: Multi-ion spectroscopy for optical clocks

Introduction

As the systematic error budgets of ion-based frequency standards continue to improve, ever-longer measurement times are required to average over the statistical noise and obtain a measurement uncertainty that reflects the achievable accuracy. Shorter averaging times can be achieved either by reducing laser phase noise, which currently limits the spectroscopic probe time and hence the observed linewidth of many investigated atomic resonances, or by increasing the number of ions to overcome the fundamental limit on the signal-to-noise ratio of measurements on a single quantum system.

Within this project we pursue three different approaches in three different setups: linear ion chains in a precision trap will enable high-accuracy In⁺ and Yb⁺ multi-ion clocks, multiple ensembles of Yb⁺ ions in spatially separate trap zones will enable dead-time free clock readout, while we will achieve long probe times for a Al⁺/Ca⁺ quantum logic clock through pre-stabilisation of the clock laser light using an ensemble of Ca⁺ ions, realising a multi-ensemble clock. The investigated systems are therefore complementary in their goals, but synergetic in the employed quantum engineering techniques. The project contributes to all of DQ-mat’s core objectives, namely scaling quantum systems, manipulating and characterising quantum systems, composing quantum systems, enhancing quantum sensors and metrology, testing fundamental physics and exploring many-body physics.

In the third funding period we will pursue various routes to improve the statistical and systematic uncertainties in the three investigated multi-ion systems and leverage the resulting clock performance for tests of fundamental physics. Statistical uncertainties will be improved by increased ion numbers and the development of quantum-enhanced and multi-ensemble interrogation protocols. For reduced systematic uncertainties, we will improve our knowledge of atomic parameters via spectroscopic measurements, develop new cooling techniques and engineer collective states with reduced sensitivity to external perturbations. Within a first optically integrated prototype ion trap, we will demonstrate a first multi-ensemble ¹⁷³Yb⁺ clock with reduced light shifts and demonstrate scalability in clocks with long probe time. We will directly observe time dilation in engineered (non-classical) states of motion in ion Coulomb crystals. We will perform tests of fundamental physics through repeated comparisons of the clocks within this project and B02 with overall uncertainties of order 1 × 10⁻¹⁸.