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In general, interactions between the atom and light are weak, and it is not easy to achieve strong coupling even when using a resonator. A group led by Serge Haroche at the École Normale Supérieure in Paris prepared Rydberg atoms with electrons excited to very large orbitals with principal quantum numbers of n = 50 and 51. Because of the large size of the electron orbitals, the Rydberg atoms had dipole moments some 1,250 times greater than that of a single atom, and they were able to couple strongly to the electric eld of light. e researchers also reduced the spontaneous emission rate γ from the atoms by using cir-cular electron orbitals with good symmetry and suppressed the loss rate of light κ from the resonator by using a Fabry-Pérot cavity with spherical mirrors made of superconduct-ing niobium with low dissipation (the resonator Q value as large as 108). As a result, they entered the strong coupling regime, where photons spontaneously emitted from an atom remain in the cavity and are once again absorbed by the atom. is repeated emission and absorption resulted in a phenomenon called vacuum Rabi oscillation [1]. Recent technological advancement enabled the resonator Q value to increase to its maximum limit (approximately 1010) and photon relaxation time to extend. Consequently, pho-tons in a resonator were quantied non-destructively using Rydberg atoms, and quantum feedback to stabilize the photon state in resonators was achieved using these atoms. is showed that it is possible for quantum information in the atoms to be transferred to photons, and vice versa. is property attracted attention as a fundamental technology of quantum information processing. e two pioneers in the quantum technology eld —David J. Wineland (NIST-Boulder) and Serge Haroche (Collège de France, ENS-Paris)— won the 2012 Nobel Prize in physics.3Circuit quantum electrodynamics (circuit-QED)e basic element of a superconducting quantum circuit is an LC resonator consisting of an inductor of inductance L and a capacitor of capacitance C. e resonator has equally spaced energy levels, and if its temperature is suf-ciently low compared with the level spacing, it is able to exhibit quantized level eects. Since the levels are equally spaced, it is not possible to form qubits or articial atoms using two specic levels. However, by introducing a Josephson junction into the circuit (where it acts as a nonlinear inductance), we can produce a superconducting articial atom. A Josephson junction has both an induc-tance component and a capacitance component, and the properties of the articial atom vary according to the rela-tionship between these components.Magnetic ux is a good quantum number in a junction with greater inductive energy and produces an articial atom that is more sensitive to magnetic elds, whereas electric charge is a good quantum number in a junction with greater capacitive energy and produces an articial atom that is more sensitive to electric elds. e level spacing of superconducting articial atoms produced in this way covers the microwave band from a few gigahertz to several tens of gigahertz. To enter the strong coupling regime between microwaves and superconducting articial atoms, the microwaves must be conned inside a super-conducting resonator with a strong eld (magnetic or electric). To develop a resonator that works well with su-FiF1　Three coupling regimes of the resonator (circuit) quantum electrodynamics and the model Hamiltonian.cavity QEDAtom ⇔high-Q cavitycircuit-QED (circuit-OED)Qubit ⇔Microwave OscillatorWeak coupling Strong coupling Deep strong couplingCoupling energyQuantum two-level systemSpontaneous emission rate (γ) from the two-level systemLoss rate (κ) of light fromthe harmonic oscillatorQuantum two-level Coupling Harmonic oscillator systemHarmonic oscillator （electromagnetic field）4 Quantum Node Technology66　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)