Quantum Information Processes Based On Cavity Quantum Electrodynamics

Cavity quantum electrodynamics(cavity QED) describes the coherent interaction between matter and an electromagnetic field confined within a resonator structure,and is providing a useful platform for developing concepts in quantum information processing.By using high quality resonators,a strong coupling regime can be reached experimentally in which atoms coherently exchange a photon with a single light-field mode many times before dissipation sets in.This has led to fundamental studies with both microwave and optical resonators.To meet the challenges posed by quantum state engineering and quantum information processing,recent experiments and theory have focused on cavity QED with superconducting circuits which is called as circuit QED. Advances in circuit QED opened new prospects in nonclassical state generation and quantum information processing in the microwave regime.In the circuit QED,superconducting circuits are made to act like artificial atoms and a one-dimensional superconducting transmission line resonator(TLR) forms a microwave cavity.Unlike natural atoms,the properties of artificial atoms made from circuits can be designed to taste,and even manipulated in-situ. Because the qubit contains many atoms,the effective dipole moment can be much larger than an ordinary alkali atom and a Rydberg atom.This allows circuits to couple much more strongly to the cavity.This large coherent coupling allows circuits to achieve strong coupling even in the presence of the larger decoherence present in the solid state environment,then one can observe the quantum interactions of matter with single photons.Hence,circuit QED can explore new regimes of cavity QED.This opens possibilities ranging from quantum information processing to a wealth of new phenomena that can be expected in the solid state systems with cavity-mediated interactions. In this thesis,I am concerned with quantum information processes based on cavity QED systems.I study cavity QED realization of quantum algorithm, generation of a large Kerr nonlinearialty and entangled states,as well as quantum decoherence in circuit QED systems.In this thesis,we mainly focus on quantum information processes based on atomic cavity QED and circuit QED.We propose a scheme to realize quantum clock synchronization(QCS) algorithm in atomic cavity QED formalism.In our method,qubits are encoded in terms of energy levels of atoms in ladder-type configurations.Operations of atomic qubits are completed through controlling interactions between atoms and classical or/and quantized cavity fields.Single-qubit Hadamard operations, single-qubit rotate operations,two-qubit CNOT gates and two-qubit controlled phase shift operations are needed for implementing QCS algorithm. We explicitly present atom-qubit realizations of three-qubit,and four-qubit QCS algorithms.It can also be expected to use this method to realize more general quantum networks of many-qubit QCS algorithm.We present a scheme to implement quantum state transfer in a hybrid circuit-QED system where a charge and a flux qubits are coupled with a TLR which serves as a data bus.We have shown that quantum states can be transferred between the two types of qubits both in the resonant and the dispersive regime.In the resonant regime,the TLR plays the role of an intermediate.The quantum states are transferred from one qubit to the TLR and then from the TLR to the other qubit.In the dispersive regime,the TLR is eliminated adiabatically and we obtain an effective swapping interaction between the two types of qubits.Quantum state transfer between them can be realized by making use of this swap interaction.We present a scheme to create a controllable cros-Kerr interaction be- tween microwave photons in a circuit-QED system.The scheme exploits an SQUID-type charge qubit to act as a two-level artificial atom,and two TLRs as two cavities ejected by microwave photons.We show that a controllable cros-Kerr interaction can be obtained in the dispersive regime of the circuitQED system,and a large cros-phase shift between two microwave fields in the two TLRs can be reached in the parameter regime of the current circuit-QED experiments.The cross-Kerr coupling strength can be controlled through adjusting the external classical flux in the SQUID.Based on this cross-Kerr interaction,we have shown how to create a macroscopic entangled state between the two TLRs.The realization of the controllable cross-Kerr interaction for on-chip microwave photons is one of the most important steps for scalable quantum computing and quantum information processing by using coupled macroscopic quantum systems.It is believed that the on-chip cros-Kerr nonlinearity for microwave photons could open a way to the on-chip nonlinear optics involving macroscopic objects.Finally,we investigate decoherence in the circuit QED system consisting of a charge qubit and two superconducting TLRs.Actully,we propose a scheme to realize decoherence-free quantum dynamics of the bipartite consisting of the charge qubit and one superconducting TLR by using another superconducting TLR as anxiliary subsystem.In this scheme one TLR and the charge qubit constitute the controlled target system while the other TLR is the auxiliary subsystem which acts as a tool to control the target system.The whole of them forms a triple-partite circuit-QED system.It is found that in the dispersive regime of the circuit QED system,decoherence-free quantum dynamics of the bipartite target system can be realized when the auxiliary TLR subsystem is initially prepared in proper number states.This implies that by controlling and manipulating the auxiliary subsystem,one can protect quantum system against decoherence.This provides fundamental insight into the control of deeoherenee in circuit QED systems.It is believed that our present scheme opens an new way to engineer deeoherence in quantum systems.