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2Coherent light sources for controlling ionic quantum systemA variety of operations involved in measurement and control of ionic quantum systems — ion generation, laser cooling, quantum state initialization and measurement — make use of a single-mode coherent light. Because an ion has its characteristic resonance frequency, a coherent light source with the same wavelength must be provided. In addition, its linewidth must be narrow enough in com-parison with that of the targeted transition. Major transi-tion wavelengths and the linewidth of the two species, Ca+ and In+, are shown in Fig. 2. Transition wavelengths of Ca+ all fall in the region where they are excitable using a semiconductor laser, making it relatively easy to construct a coherent light source system. In contrast, transitions of In+ fall in the ultraviolet region, requiring one or more auxiliary techniques, such as wavelength conversion of the laser, to construct a coherent light source. is section outlines the research and development undertaken to real-ize coherent light sources for In+ and Ca+.2.1Linewidth reduction by means of optical feedbackIn the study described in this report, Ca+ is directly laser-cooled and is used in turn for sympathetic cooling of In+. For this approach to become feasible, a coherent light source that excites the 2S1/2-2P1/2 transition (397 nm) in Ca+ (see Fig. 2 (b)) is required. In view of this objective, the linewidth (around 1 MHz) produced by a diraction grat-ing controlled ECDL (Extended-Cavity Diode Laser) should be narrowed down for the implementation of laser cooling in optimum conditions. We devised an optical feedback approach [5] and conrmed that the linewidth can be reduced to 7 kHz (at measurement time 1 second) [6]. In this method, all the output light is introduced into a 3-mirror traveling wave lter cavity and a portion of the output light is fed back to the ECDL. is lter cavity approach provided an additional benet of reducing strength in the broad background of the spontaneous ra-diation by more than 30 dB [5]. is eect helps remove a known problem associated with ECDL: the spontaneous radiation background excites the 2P3/2 level (shown in Fig.2 (b)) of Ca+ inducing transition to metastable 2D5/2 state leading to inhibition of eective laser cooling. In addition to the lter cavity approach used in the ionic quantum system reported here, the authors experimented with an-other approach — i.e. an integration oriented planar opti-cal circuit that uses an optical ber cavity — and proved its operational feasibility [7].2.2UV light generation through two-step wavelength conversionTwo transitions in Fig. 2 (a) are used in In+ application: 1S0-3P1 transition (230 nm) for the observation of the quantum state of In+, and 1S0-3P0 transition (237 nm) for clock transition excitation. As these wavelengths cannot be FiF2Relevant energy levels, transition wavelengths and linewidth of In+(a) and Ca+(b)2P1/22S1/22D5/23P13P01S0(a)115In+(b)40Ca+237nm0.8Hz729nm0.2Hz397nm22MHz230nm360kHz159nm204MHz1P12P3/22D3/2866nm854nmFiF3Configuration of 230 nm coherent light source DBRDL: Distributed Bragg reflector diode laser, PID: PID controller, LIA: Lock-in amplifier, HC sig: Hänsch-Couillaud signal detectorBBOPPKTPDBRDLAOMLLCLHWHCsigPID3HCsigPID1PID2LIAHW922nm461nm230nmDriverPZTPZTPZTfrequencycontrolfrequencymonitorFiF4Time variation of the generated 230 nm coherent light01000200030004000time [s]02468230nm power [mW]4 Quantum Node Technology 72　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)