All-optical frequency standard based on a single trapped radium ion

  Ultra-narrow optical transitions in specific ions are promising candidates for a new frequency standard (“clock”). We propose to design and mount a conclusive experiment, with fully-analyzed systematics, on one single radium ion, laser-cooled to microkelvin temperature in a radiofrequency trap.

Introduction

Atomic clocks are the absolute peak of precision; we can measure nothing as accurately as time. The second is currently defined as 9,192,631,770 periods of the oscillations of a cesium atom. However, the cesium fountain clock at NIST, where these periods are counted, is not the last word on the subject. We can be much more accurate: this is the topic of our research. It can mean the redefinition of the second.

Accurate timekeeping is of great importance of navigation (GPS) and synchronization, e.g. of financial transactions or of optical computing networks. Improving timekeeping will mean the improvement of the above, and maybe much more. After all, the GPS was only invented after the atomic clock needed for the satellites. Better timekeeping will also lead to more precise experiments in fundamental physics. For instance, atomic clocks can be used to verify the constancy of constants and answer the question: Is the fine structure constant changing in time? Also, tests of general relativity are based on atomic clocks.

Experiment & Theory

Single ions, when trapped and laser-cooled to the zero point of motion, provide an excellent basis for an atomic clock. In such a system, there is a minimum of external perturbations and an absence of Doppler broadening. There are several candidates for this kind of clock, where much progress was made with Hg+. However, Ra+ has the distinct advantage that its transitions are easily probed with readily available laser diodes and that certain systematic effects –which limit the Hg+ clock- are absent. Furthermore, radium is very sensitive to change of the fine structure constant.

The single ion clock uses a very narrow optical transition as a absolute reference of a sub-Hz stabilized laser. This laser serves as the oscillator for our clock. This laser can then be tuned on a femtosecond laser comb, which serves as the clockwork. To lock the laser on the transition we use the so-called shelving method. With this method, one can detect quantum jumps, effectively amplifying the signal a million fold. This single ion clock will mean a thousand fold improvement to the state-of-the-art cesium fountain clock NIST-F1. In the end, this will improve the stability (or accuracy) by a factor of 1000, reaching accuracies of 10-18s, i.e. loosing less than half a second over a time equal to the age of the universe!