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that the system was cooled to its vibrational ground state. It also implicates the applicability of quantum logic spec-troscopy to retrieving quantum state information of In+ through manipulation of phonons [11], and the possibility of reducing frequency shi of optical frequency standards (caused by relativistic time dilation) down to 10-17 or below.4Measurement of indium ion clock transition frequencye ionic quantum system described above served for further research and development in the NICT Space-Time Standards Laboratory with an aim to develop state-of-the-art control and measurement technologies to be applied in optical frequency standards. Optical frequency standards involve such techniques as stabilization of laser frequency, and feedback control of the stabilized frequency to the central frequency of atomic/ionic narrow linewidth transi-tion (clock transition) to realize universally available highly accurate optical frequency. e optical frequency is converted to the frequency in the microwave domain to establish an exact second signal, whereby a tool called an optical frequency comb is used to convert frequency with-out compromising accuracy. One of the two main methods to establish optical frequency standards, the single-ion frequency standard, was proposed by H. Dehmelt in the 1980s. Research conducted in line with this method used 27Al+ and 171Yb+ as the frequency references, and reported that it involved uncertainty on the level of 10-18 [4].In+ was also included among the candidate ions pro-posed by him as the ions species to establish single-ion frequency standards, and recent theoretical research [4] predicted uncertainty on the level of 10-18 due to small frequency shis it may involve. An additional merit inher-ent to In+ is the fact that it allows the use of relatively simple techniques to measure quantum state. Other candi-dates require complicated technologies. An alternative multi-ion optical frequency standard, which takes advan-tage of these merits and integrates the features of the currently mainstream two approaches, has also been pro-posed [4]. Up to the present, only two research projects have been published on the subject of In+ application to clock frequency measurement, and the uncertainty re-ported by these papers remained on the level of 10-13 [12][13]. It is also noted that the reported frequencies have discrep-ancies larger than about 1 kHz, which lies beyond the range ascribable to experimental errors. e authors conducted clock frequency measurements using In+ as the frequency reference, the rst-stage objective of which was to reduce uncertainty in measurements to help establish the transi-tion frequency.In past experiments, laser cooling was performed using In+ ion itself, and clock transition frequency was measured in reference to the frequency standard that had been cali-brated in other organizations. In the experiments described in this report, frequency measurements were performed against In+, sympathetically cooled using Ca+, with refer-ence to two in-house (NICT) calibrated frequency stan-dards. Fluorescent light from 1S0-3P1 transition (see Fig.2 (a)) was measured to determine the quantum states of In+: coherent light (clock laser) was rst irradiated to excite clock transition, and the response of In+ was observed using a 230-nm coherent light source. If a uorescent photon is captured in this observation, then the clock transition was not excited, and non-capture of it indicates successful excitation. Based on these data, excitation prob-ability was calculated and the spectrum of clock transition was obtained. In this experiment, the number of photons emitted from In+ was around 250 in one second. Figure 10 shows an example of spectra. Two symmetrically distrib-uted spectra (centered at the clock transition frequency ν0) were obtained by changing the initial state of In+ and by changing the polarization of the clock laser. e fre-quency ν0 was determined by averaging ν+ and ν- (center values of these peaks) of these spectra. While the spectrum was being measured, the clock laser frequency was deter-mined with reference either to NICT-generated Coordinate Universal Time (UTC (NICT)) or to a Sr optical lattice clock. e measurements were taken 36 times and a set of ν0 values was obtained from these experiments. e clock transition frequency was determined as the average of these FiF10　In+ clock transition spectrum4 Quantum Node Technology 76　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)