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compared to the electronic transition energy of the atom. However, our experiments found that the interaction en-ergy was greater than any other energy sources found in the articial atoms in the circuit, and the size of the Lamb shi observed was estimated to be 87%, which was orders of magnitude greater than previously known.When the interaction eect is as large as that found in our experiments, the minimum energy state (ground state) of the light-atom coupling system deviates conspicuously from the intuitive ground state. According to conventional common knowledge, the natural ground state of a light-atom coupling system occurs when atoms are in the lowest energy state and the electromagnetic eld is in a vacuum state (i.e., the average photon number = 0) (Fig. 6 (a)). Surprisingly, we observed in our experiment that the inter-action energy was greater than the photon energy or the excitation energy of the articial atom. As a result, the average photon number in the ground state was estimated to be a nite value of 1.8, even though no external photon was injected into the system. In other words, the articial atom was surrounded by vacuum uctuations which were intensied by the large interaction energy (Fig. 6 (b)). Moreover, when the light-atom system was in the “deep strong coupling state,” selection rules for quantum transi-tion were observed due to the symmetry of the system. is indicates that light and atoms entangled*4 in all energy states, including the ground state (Fig. 6).During this study, NICT carried out experiments and analysis, NTT fabricated samples and QEERI performed theoretical interpretation.5Future prospectsAs we described above, a superconducting articial atom is an aluminum-based superconducting electric cir-cuit that behaves like a natural atom (in terms of spectro-scopic spectra, state transitions, superposition of states and entanglement). It is analogous to an atom with a lead wire attached. e quantum state of a superconducting articial atom can be controlled and measured by sending a micro-wave pulse train to it via a transmission line (lead wire). It is theoretically feasible to replicate the articial atom thousands to tens of thousands of times using semiconduc-tor microfabrication techniques. Superconducting articial atoms behave like natural atoms from the quantum me-chanical point of view. rough this research, it has been revealed that the range of energy states of light-atom coupling systems is much wider than previously known using natural atoms and suggested the possibility that many unknown energy states may be yet to be discovered.In this research, we achieved an entangled state of a FiF5Transmission spectra for the coupled systemThe horizontal and vertical axes represent the artificial atom’s bias energy level and the input microwave frequency, respectively. When the atom is in the deep strong coupling state and its bias energy level is close to zero, complicated transmission spectra are observed. This can be explained only by the existence of the newly discovered DSC ground state consisting of the artificial atom and virtual photonsStrong couplingDeep strong couplingCoupling strengthFiF6Energy ground state of the coupled system. (source : Fig. 5 of NTT Technical Review vol.13, pp.44-47, 2017. [8])(a) Typical lowest energy state (ground state)(b) New ground state: atom and light are entangled*4Entanglement refers to a state in which quantum mechanical correlations exist among multiple particles. When two photons (or electrons, qubits, etc.) are entangled, they exhibit unique properties—the state of one photon will deter-mine the state of the other, and their relationship is independent of the distance between them. Entanglement is a vital quantum state from the viewpoint of putting quantum cryptography and quantum computation into practice. Entanglement is also an important quantum system property actively used to-day.694-3 New Regime of Circuit Quantum Electro Dynamics