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which was obtained by irradiating the light on a uorescent screen placed outside the cavity. Its intensity prole is quite similar to Gaussian distribution, promising feasibility to collimate on a single ion.Figure 7 shows the intensity prole of 159-nm light as measured outside the cavity. e horizontal axis in the gure represents fundamental leakage from the cavity. e maximum output at 159 nm reached 6.4 μW against 650 mW Ti:S laser input, and the input-output relation indi-cated a good t to the 5th power curve (i.e. e intensity of output light is proportional to the 5th power of funda-mental wave intensity). is promises availability of stronger output light by enhancing the fundamental wave.e output light at 159 nm consists of as many fre-quency modes as about 1.9 × 105, but only a few of them are capable of resonating with 1S0-1P1 transition of In+. e authors evaluated the number of 159-nm uorescent pho-tons that should be observed when the resonant frequency modes were used in quantum state measurement of a single In+. Assuming ideal collimation and measuring conditions, the calculation indicated the following: 87 photons/sec/μW. In view of the currently available output power (6.4 μW), the above calculation predicts 550 photons per second, which roughly corresponds to the typical number (approx. 500/sec) that would be available if the alternative method — use of 1S0-3P1 transition — is em-ployed. Note that there is still space for improvement to perform quantum state measurement of In+ at a higher rate. Some of the possibilities include the pursuit of long-term system stability, and implementation of an optical system for ecient introduction to the ion trap.3Sympathetic cooling of indium ione ion trap is a device designed for stable accumula-tion of charged particles, but it does not feature the ability to cool them by itself. To control and/or measure quantum states of ions, they must be brought into a quiescent state by employing a cooling method. e most common method used for this purpose is laser cooling, but with limited applicability because it shows selectivity to specic ion species. In+ is an example of an ion that dees easy laser cooling because of the lack of relevant optical transi-tions. In this research, the authors targeted Ca+ (relatively easy to laser cool) as the media to sympathetically cool In+ for control and measurement.3.1Observation of sympathetically cooled indium ione motions of multiple of ions accumulated in an ion trap exhibit a collective nature under the inuence of the trapping electric eld and inter-ion coulomb force, and can be described as a collective vibration mode. Laser cooling an ion translates into cooling of a collective vibration mode in its entirety. is cooling method, called sympathetic cooling, enables cooling of ion species that are not readily cooled independently. e authors conducted experiments to sympathetically cool In+, for which Ca+ and In+ are ac-cumulated in a linear ion trap, and Ca+ was laser cooled.Figure 8 (a), (b), and (c) show resonance uorescence images from Ca+ ions captured by a feeble-light imaging device. ese images represent the array of three ions: an In+ ion is added to the two laser-cooled Ca+ ions by way of resonance photoionization. ese images were obtained by driving the trap under application of very weak voltage that only suces to interchange positions among the ions, and indicates the existence of a non-uorescent ion. e frequency at which the ion array vibrates collectively is determined by the arrangement and mass of the constituent ions. Based on this principle, the non-uorescent ion can be identied to be In+ (mass 115) using the vibration mode frequency measurement method described below.FiF7　Intensity profile of the 159-nm lightFiF6　Fluorescent image of the vacuum UV coherent light4 Quantum Node Technology 74　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)