From G
Thank you for the report on the new, more precise atomic clock using emissions from ytterbium atoms (31 August, p 15). Readers may be interested to know which technical advance made such a level of stability possible.
For some time now the performance of the very best atomic clocks has been limited, not by the statistics of photon emission by the atoms themselves, but by instabilities in the laser that “interrogates” the atoms, leading to “aliasing” effects in the detected signal. The recent improvement is primarily due to advances in the performance of the interrogating lasers (arxiv.org/pdf/1305.5869).
The authors propose that even higher stability can be achieved by using two atomic systems with overlapping cycles, which would virtually eliminate the aliasing (also known as the “Dick effect”). Of course, it is only with the advent of the ultra-stable ytterbium atomic system that these statistical fluctuations become of much consequence: before then they were minor compared with those of the signal being detected.
Claremont, California, US
. John Dick
