Quantum Entanglement In The Interacting Systems With Two Superconducting Qubits And NV Centers

Quantum entanglement was first described by Einstein,Podolsky,Rosen and Schrodinger as a strange phenomenon of quantum mechanics,questioning the com-pleteness of the theory.Later,Bell recognized that entanglement leads to experi-mentally testable deviations of quantum mechanics from classical physics.Finally,with the advent of quantum information theory,entanglement was recognized as a resource,enabling tasks like quantum cryptography,quantum teleportation,quan-turn computation,and quantum metrology.Together with the rapid experimental progress on quantum control,this lead to a rapidly growing interest in entangle-ment theory and many experiments nowadays aim at the generation of entangle-ment.An entangled system is defined to be one whose quantum state cannot be factored as a product of states of its local constituents;that is to say,they are not individual particles but are an inseparable whole.In entanglement,one constituent cannot be fully described without considering the other(s).Note that the state of a composite system is always expressible as a superposition of products of states of local constituents;it is entangled if this superposition necessarily has more than one term.Quantum systems can become entangled through various types of inter-actions.For some ways in which entanglement may be achieved for experimental purposes.Entanglement is broken when the entangled particles decohere through interaction with the environment;for example,when a measurement is made.Today,physicists can manipulate quantum states in myriad media,from op-tical and microwave photons to laser-cooled ions and atoms,artificial atoms and even macroscopic mechanical oscillators.But no single medium can meet all de-mands for realizing quantum technologies such as sensors,computers,and secure communication networks and interfacing them with existing classical infrastruc-ture.Thus,a crucial endeavor is merging the strengths of different media into hybrid quantum systems.Recently,significant progress has been made in the quantum hybrid system consisting of a variety of physical systems,which combines the merits of two or more physical systems and mitigates their individual weaknesses.Especially,the hybrid quantum model including solid-state spin systems and superconducting qubit systems,provides a promising platform to study the intriguing quantum optical phenomena as well as the fundamental quantum information science.Es-pecially,spin-qubit in the solid-state system attract considerable interest because they can be used to store and transfer the quantum information.Additionally the coherence times of isolated or peculiar spins are usually long due to their weak interaction with the environment.For instance,the diamond nitrogen-vacancy(NV)centers feature long coherence times of electron(nuclear)spin in a wide temperature range.More importantly,the NV centers have the ability to coher-ently couple to various external fields simultaneously,such as both optical and microwave fields.In this theses,I investigate a hybrid quantum system that consists of two superconducting magnetic flux qubits and one NV-center spin ensemble.Enat-nglement generation and entanglement transfer are studied in detail by the use of entanglement dynamics method and quantum measurement approach.This thesis includes five chapters.The first chapter briefly introduces the relevant background information and research status of quantum entanglement.The second chapter mainly introduces a few key quantum systems,some ba-sic concepts and methods related to this thesis,such as Josephson junction,the superconducting qubit,the NV centers in a diamond,the concept of quantum entanglement,entanglement measures and basic theory of fidelity.In the third chapter,I first construct an interacting model consisting of two superconducting magnetic flux qubits and a single NV-center spin ensemble,and deduce the effective Hamiltonian of the interaction system under large detuning conditions by the use of Nakajima transformation method,obtain the analytical solution of the time evolution operator of the hybrid quantum system under the dispersion condition.Then,quantum entanglement dynamics among two super-conducting magnetic flux qubits and one NV-centers spin ensemble is studied in detail for various different initial states.It is found that a three-body W-like state can be dynamically generated.For the case that the two magnetic flux qubits are in the initial entanglement and the NV-center is in the ground state and the other case that a magnetic flux qubit has the initial entanglement with the NV center and the other magnetic flux qubit is in the ground state,two-body and three-body entanglement dynamics of the whole system is studied.It is indicated that the two-body entanglement dynamics of the system is periodic.By adjusting the coupling strength and the frequency detuning between the system,I can control the period of the two-body entanglement dynamics.Besides,it is found that the three-body residual entanglement of the total system remains zero.In the fourth chapter,I study the transfer process of two-body quantum en-tanglement under single excitation conditions.By using the dynamic regulation method and the quantum measurement approach,the quantum entanglement be-tween the NV-center and the first qubit can be transferred into the quantum en-tanglement between the NV-center and the first qubit,or can be transferred into the quantum entanglement between the first qubit and the second qubit.Thus,the quantum entanglement distribution is achieved.The three entanglement transfer processes of hybrid entanglement to hybrid entanglement,hybrid entanglement to non-hybrid entanglement,and non-hybrid entanglement to hybrid entanglement are systematically studied.It is found that by controlling of the parameters and evolution time of system can not only realize the transfer of quantum state and quantum entanglement,but also under certain conditions can realize the complete transfer of quantum state and the amplification of quantum entanglement.In addition,this chapter also studies the transfer process of two-body quantum en-tanglement under the double excitation conditions.It is shown that transfer from the non-hybrid quantum entanglement between two qubits to the hybrid quan-tum entanglement between the NV-center and qubit can be realized by the use of dynamic regulation method and quantum measurement approach.Thus,the non-hybrid quantum entanglement to the hybrid quantum entanglement distribution is achieved.It is found that the parameters and evolution time of control system can not only realize the transfer of the non-hybrid quantum entanglement to the hybrid quantum entanglement,but also produce the amplification of quantum en-tanglement.Compared with the quantum entanglement amplification under single excitation conditions,the quantum entanglement amplification under the double excitation condition has a greater amplification ability.In the last chapter,I briefly summarize this thesis and make some prospects for future work.