Circuit QED:Light-matter Interaction

Light-matter interaction is one of the fundamental processes of nature,and a s-ingle atom coupled to a cavity mode,is the simplest fully-quantized light-matter in-teraction model.The theoretical analysis and experimental research on this model has allowed to considerably deepen our understanding of quantum mechanics.Cavity Quantum Electrodynamics(CQED),which describes an natural atom interacting with a cavity mode,has been established as a well-controlled atom-cavity system for realizing this light-matter interaction model.With the progress in the experimental study over the past decades,the CQED system has been demonstrated as an ideal experimental platform for studying quantum optics and quantum-information processing.Moreover,our deepening understanding of quantum world along with the progress in experimen-tal technology and the result of exchanging principles and techniques between different disciplines,such as low temperature physics,superconductivity,microwave engineer-ing,and condensed matter physics,have made possible to treat the quantum coher-ence as a new undeveloped resource which is available for real-world application,such as engineering the light-matter interaction for encoding,manipulating,and extracting quantum information(e.g.,quantum information processing).As a solid version of cavity QED,the circuit QED system have gained substantial interest as possible candi-dates for implementing quantum information processing and the flexibility of quantum operation along with the potential of scaling up make this system as one of the most promising platform for realizing quantum computer.With the aim of developing new quantum technologies for quantum information processing,In this thesis,the physical mechanisms that leads to the effective atom-photon interaction in circuit QED system have been presented,and thus providing new schemes for quantum state engineering.In the main text,the circuit QED system has been explored from two different perspectives,as follow,(1)the superconducting qubit is at the heart of the qubit-resonator system.Meanwhile,the resonator serves as an auxiliary system.Due to the lager anharmonicity of the qubits,the two lowest levels are naturally used as two level systems for encoding quantum information(i.e.,qubit).However,the superconducting qubit actually possesses multiple energy levels,and the higher level of the qubits may also have important impact on these processes even un-der conditions when real occupation of this level is negligible.The physics behind these processes is that these processes is mediated by the virtual process involved with higher energy levels.By exploring these virtual processes,one can observe new phys-ical process in the atom-photon system,such as a single photon exciting a two or more atoms simultaneously and coherent exchange interaction among multi-atom mediated by the resonator.(2)the resonator is the primary parts of the system,while the super-conducting qubit serves as an auxiliary system for engineering the quantum state of the resonator.The microwave resonator can be treated as an ideal quantum harmonic oscillator with infinitely large Hilbert space,which provides a promising alternative to qubit based encoding schemes for implementing quantum error-correction.Mean-while,the superconducting resonator with a high quality factor can be easily designed and fabricated with the existing technology.However,although harmonic oscillator is rather simper quantum system,compared with the superconducting qubit which can be prepared in an arbitrary quantum state using microwave drive,the harmonic oscillator can only be prepared in a coherent state with microwave drive.In general,in order to engineer the quantum state of the harmonic oscillator system,one need a nonlinear element(e.g.,superconducting qubit)embedded in the system.By coupling a super-conducting resonator off-resonantly to an ancillary three-level superconducting qubit,we can realize a Kerr resonator with two-photon driving that enables us to preparing the Schrodinger cat states via adiabatic evolution.Meanwhile,by coupling multi-resonator to a superconducting qubit,one can realize a coupled network consisting of two-photon driven Kerr resonators,which can be used for implementing quantum information pro-cessing.