Observations of quantum entangled states and quantum critical behavior by using a precise probing method of microwave near-field

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Quantum entangled states become essentially important in both regions of condensed matter physics and quantum information technology. Toward a complete understanding of such states, we study the development of a microwave microscope to probe the quantum critical behavior in the neighborhood of quantum critical point, and the design of a superconducting cavity resonator to investigate the quantum entangled states generated by using a quantum electrodynamics theory. We found that a spatial resolution of the microwave microscope is strongly enhanced by utilizing a microwave near-field, which is realized by the insertion of a metallic needle into a cavity resonator. We also found that such an insertion of the metallic needle is useful for a precise control of the coupling constant between the Josephson quantum bits and the microwave photons at low temperatures, which is required to create the quantum entangled states.