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generated directly by laser oscillation, they were generated through two-step wavelength conversion utilizing spectra from a semiconductor laser: 922 nm for the former, and 946 nm for the latter. Figure 3 shows the schematic diagram of the optical system used to generate the 230-nm radia-tion. From the fundamental wave (922 nm, 150 mW out-put) emitted by the distributed Bragg reector diode laser (DBRDL), a second harmonic (461 nm) is produced using a periodically polarization-reversed KTP crystal (PPKTP), followed by another second harmonic generation by BBO crystal, resulting in the generation of coherent light at 230 nm. Figure 4 shows the intensity time variation of the coherent light (230 nm). Despite the use of a relatively complex optical setup for performing two-step wavelength conversion, satisfactory light intensity is maintained for a suciently long period for quantum state observation of In+. e basic conguration of this optical system was also used in the clock transition frequency measurements of In+ described later, but with some improvements and modica-tions: DBRDL was replaced with a combination of ECDL and a tapered optical amplier, and PPKTP crystal was replaced with a simpler waveguide-type PPLN crystal.2.3Vacuum UV light generation using higher-order harmonicse most commonly used identier in quantum state observations is the uorescent photon emitted from an ion. For example, to distinguish if the quantum state of In+ is either in 1S0 or 3P0 (see Fig. 2 (a)), a good method is to excite 1S0-1P1 transition (159 nm) and observe the response of the system. If a uorescent photon is observed, we can say that the system is in 1S0 state with almost 100% cer-tainty, and it is in 3P0 otherwise [4]. It is generally very dicult to generate single-frequency coherent light in this wavelength range. erefore, an alternative method is used to measure the quantum states of these ion species. In the case of In+, 1S0-3P1 transition (230 nm) is used as an alter-native choice. However, uorescent intensity from this transition is very weak — approx. 1/60 of Ca+ ion’s 2S1/2-2P1/2 transition (397 nm), one of the most common used transi-tions in quantum information processing — causing the measurement to require a longer time to obtain quantum state information. is factor places restrictions on the speed of repetitive quantum state measurements. Another transition of In+ ion, 1S0-1P1 (159 nm), also falls in the vacuum UV range, but its transition probability is 570 times as large as that of 1S0-3P1. erefore, it raises the possibility of quantum state observation at higher speed, only if a relevant excitation method becomes available. To realize a high-speed method to measure In+ quantum states, the authors conducted research and development on coher-ent light generation in the vacuum UV range by utilizing higher-order harmonics of a femtosecond mode-locked laser [8].e mode-locked laser can produce an extremely in-tense optical electric eld through coherent superposition of a multitude of frequency modes. is extremely intense optical electric eld gives rise to non-linear eect, enabling ecient generation of higher-order harmonics. In this re-search, the authors attempted to generate a 5th-order harmonic (159 nm) using the experimental conguration shown in Fig. 5: the output from a mode-locked Titanium:Sapphire (Ti:S) laser is accumulated in an exter-nal cavity with the same optical path length as the laser resonator to further enhance the intensity of the optical electric eld, and Xe gas is blown into the cavity to provide a medium for nonlinear excitation. e 159-nm light thus generated was drawn out of the cavity through a specially designed, uoride coated, output coupler. Figure 6 shows the spatial mode pattern of the vacuum UV coherent light, FiF5　Configuration of the experiment system (vacuum UV is generated via higher-order harmonics)Pulse compressor(SF10 prism pair)Vacuum chamberInput couplerRIC ~ 99.5% HR0th-order diff. to spectrometer1st-order diff. to balanced PDXe gas jetVUV-OC: ROC ~ 0.1%@NIR & ROC ~ 90%@VUVfor p-pol.734-4 Quantum State Engineering of Trapped Ions