Circuit QED

Circuit quantum electrodynamics (circuit QED) provides a means of studying the fundamental interaction between light and matter. As in the field of cavity quantum electrodynamics, a single photon within a single mode cavity coherently couples to a quantum object (atom). In contrast to cavity QED, the photon is stored in a one-dimensional on-chip resonator and the quantum object is no natural atom but an artificial one. These artificial atoms usually are mesoscopic devices which exhibit an atom-like energy spectrum. The field of circuit QED is a prominent example for quantum information processing and a promising candidate for future quantum computation.

The resonant devices used for circuit QED are superconducting coplanar waveguide microwave resonators, which are two-dimensional microwave analogues of the Fabry–Pérot interferometer. Coplanar waveguides consist of a signal carrying centerline flanked by two grounded planes. This planar structure is put on a dielectric substrate by a photolithographic process. Superconducting materials used are mostly aluminium (Al) or niobium (Nb). Dielectrics typically used as substrates are either surface oxidized silicon (Si) or sapphire (Al₂O₃). The line impedance is given by the geometric properties, which are chosen to match the 50 ${\displaystyle \Omega }$ of the peripheric microwave equipment to avoid partial reflection of the signal. The electric field is basically confined between the center conductor and the ground planes resulting in a very small mode volume ${\displaystyle V_{m}}$ which gives rise to very high electric fields per photon ${\displaystyle E_{0}}$ (compared to three-dimensional cavities).

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