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1IntroductionNICT is developing ultra-high-precision atomic clocks, and NTT (Nippon Telegraph and Telephone Corporation) and NICT are jointly pursuing safe, energy-ecient com-munications. ese technologies are indispensable to modern life and are based on the fundamental science of the interaction between light and matter at the single-photon level. Absorption and emission of light by/from any device is explained based on the interaction of light and atoms. A fundamental question in atomic physics, “How strong can the coupling of light and an atom be?,” has not been answered in spite of years of research, because it is not easy to nd appropriate methods to realize very strong coupling. is situation changed with the advent of circuit quantum electrodynamics (circuit-QED) which enabled experiment of superconducting articial atoms*1 (quantum bits) instead of natural atoms.2Cavity quantum electrodynamics (cavity-QED)e simplest model of interactions between atoms and light in QED consists of single-mode (single-wavelength) light interacting with a simple two-level atom. Such a system is thought, however, to be dicult to implement, as it is impossible to conne light to a single mode in free space (space of innite extent). is led to the idea that if light is conned to a resonator comparable in size to its wavelength, then the wavelength will change from a con-tinuum of modes to discrete modes, one of which will be able to interact with the atom. is led to the emergence of cavity QED. ese systems are generally classied ac-cording to three key parameters: (1) the coupling strength (g) between the atom and light, (2) spontaneous emission rate (γ) from the atom, and (3) loss rate (κ) of light from the resonator (Fig. 1). e strong coupling regime is achieved when the coupling strength is greater than the other two parameters (g >γ, κ), while the system is said to be in the weak coupling regime when the converse holds (g < γ, κ). Even in the weak coupling regime, phenomena such as enhanced spontaneous emission from the atom due to coupling with the resonator (Purcell eect) are observed.*1A superconducting artificial atom denotes a superconducting quantum circuit with atom-like discrete energy levels. When the atom is approximately a two-level quantum system within a certain energy or temperature range, it is also called a quantum bit. In this paper, a superconducting artificial atom refers to a superconducting flux qubit shown in the red rectangle in Fig. 3. A supercon-ducting flux qubit is actually a superconducting electric circuit containing several so-called Josephson junctions composed of an extremely thin, nanome-ter thick insulator sandwiched by superconductors. The qubit is capable of controlling the magnitude of energy level splitting within several GHz range by changing the bias magnetic flux penetrating the loop. If the energy splitting of a superconducting qubit is about several GHz, the qubit need to be operated at temperatures lower than approximately 0.1 K.4-3 New Regime of Circuit Quantum Electro DynamicsKouichi SEMBA, Fumiki YOSHIHARA, Tomoko FUSE, Sahel ASHHAB, Kosuke KAKUYANAGI, and Shiro SAITOResearchers at the National Institute of Information and Communications Technology (NICT), in collaboration with researchers at the NTT Basic Research Laboratories, Nippon Telegraph and Telephone Corporation (NTT-BRL) and the Qatar Environment and Energy Research Institute (QEERI) have discovered qualitatively new lowest energy state of a superconducting artificial atom dressed with virtual photons. The discovery was made using spectroscopic measurements on an artificial atom that is very strongly coupled to the electromagnetic field inside a superconducting resonator. This result provides a new platform to investigate the interaction between light and matter at a fundamental level, helps understand quantum phase transitions and provides a route to applications of non-classical light such as Schrödinger cat states. It may contribute to the development of quantum technologies in areas such as quantum communication, quantum simulation and computation, or quantum metrology.654 Quantum Node Technology