Updated absolute frequency of the 115In+ optical clock transition

The indium ion (115In+) is one of the candidates for the single-ion optical clock with fractional frequency uncertainties in the 10−18 level proposed by Hans Dehmelt in 1980s. One of the first attempts for the realization has been started by the research group by Herbert Walther in Garching already in the late of 1980s. The first frequency measurement was published in 2001 [1], followed by two reports in 2007 [2, 3]. Unfortunately, there have been no updates since then, and the discrepancy of about 1 kHz in the reported frequencies is left unresolved. In the meanwhile the implementation with 27Al+ demonstrated the predicted fractional uncertainties in the 10−18 level. Recent theoretical calculations suggest that similar frequency uncertainties should be achieved with In+ and that enhanced frequency stability could be implemented by use of multiple In+ ions [4]. As the first step towards these goals we report on a new frequency measurement of the clock transition of In+ [5].