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perconducting articial atoms arranged on a two-dimen-sional chip, we can choose from two types of resonators. One is a superconducting LC resonator consisting of lumped circuit elements, and the other is a distributed superconducting resonator consisting of a half-wave trans-mission line coplanar waveguide. We should design the superconducting articial atom to reach an inductive or a capacitive regime to match the elds produced by each resonator (Fig. 1). e most important feature of this system (circuit QED system) is that it is possible to arti-cially design both the atoms and the resonator, enabling the formation of an ultrastrong coupling regime that has not been possible to achieve in cavity QED.Experiments that paved the way to modern circuit QED were performed independently and almost simultaneously in 2004 at Del University of Technology and Yale University [2][3]. e team at Yale University used a charge-type superconducting articial atom with a coplanar superconducting transmission line resonator to realize strong coupling via an electric eld. ey observed vacuum Rabi splitting in the resonator’s transmission spectrum. e rst vacuum Rabi oscillations were observed by NTT in 2006 using a magnetic ux type articial atom coupled to a superconducting LC resonator [4]. ese experiments in the strong coupling regime were reproduced well by the Jaynes-Cummings model, but two experiments in the ul-trastrong coupling regime that did not satisfy this ap-proximation were reported in 2010 [5][6]. e spectra could not be reproduced by the Jaynes-Cummings model in the region where the coupling strength g of an articial atom satises the condition g ≳ 0.1Δ, 0.1ω0 (where Δ is the atom’s transition frequency and ω0 is the resonator’s fre-quency). In our research, we have further increased the coupling strength to produce the deep strong coupling (DSC) regime (g > Δ, ω0), and we conrmed the appearance of a new lowest energy state (ground state) [7][8]. e coupling strengths that have so far been observed in circuit QED are compared in Fig. 2.・NTT, 2006Vacuum Rabi oscillationsobserved using asuperconducting artificialatom–superconducting LCresonator systemg/ωo~ 0.05Ultra-strong couplingSuperconducting artificial atom–microwave photon coupled system・WMI, 2010g/ωo ~ 0.12 achieved using a superconducting artificial atom–superconducting waveguide resonator system・TU Delft, 2010g/ωo ~ 0.1 achieved using a system in which a superconducting artificial atom was strongly coupled to a superconducting LC resonatorDeep strong coupling1coupling strength0.010.1・NICT-NTT-QEERI, 2016Deep strong coupling (DSC)achieved using asuperconducting artificialatom–superconducting LCresonator systemg/ωo~ 1.34~g/ωo・Yale University, 2004Vacuum Rabi splittingobserved using asuperconducting artificialatom–superconductingwaveguide resonator systemg/ωo~ 0.01Strong couplingFiF2　History of coupling strength in circuit QEDThe coupling strength between photons and a superconducting artificial atom increased by more than 100 times in the past 12 years. The coupling strength recently reached the DSC regime (g/ωo > 1) for the first time in 2016 by the NICT-NTT-QEERI joint team (the red textbox on the upper right), which confirmed the existence of a qualitatively new lowest energy ground state where an artificial atom is dressed with virtual photons to form a novel type of molecule.FiF3Measurement system and superconducting quantum circuits used in experiments(Left) Microwave measurement system incorporated into a dilution refrig-erator.Superconducting quantum circuits need to be precisely measured at the single microwave photon level while preventing thermal excitation. Therefore, the samples are cooled to cryogenic temperature (approx. 10 mK) using a dilution refrigerator. A superconducting quantum circuit is highly sensitive to magnetic noise and therefore is placed in a magnetic shield reducing the strength of the external magnetic field to about 1/1000.(Top right) Measured sample chip placed in the holder(Bottom right) A superconducting artificial atom coupled to a harmonic oscillator (both are made of aluminum) in the deep strong coupling state (red rectangle)674-3 New Regime of Circuit Quantum Electro Dynamics