Entanglement And Macroscopic Resonant Tunneling Research Of Superconducting Qubit

Superconducting quantum bit(qubit)is important not only to physics research but also to applications such as building scalable quantum information processors.It is obvious that qubits would interact with microscopic two level system(TLS)when under external driving,external fields have some effect on the dynamics of a bipartite qubit-TLS system.Entanglement can exist not only in microscopic but also in macroscopic systems such as Josephson qubits.Recently,a variety of properties of external driving on entanglement have been researched in superconducting qubits,including damped entanglement oscillation,entanglement sudden death,and entanglement sudden death and revival.The thesis has studied the observation of macroscopic resonant tunneling phenomenon in radio frequency superconducting quantum interference devices and the macroscopic quantum effect of superconducting circuits.The thesis has studied the following aspects systematically:quantum dynamics and bipartite entanglement of coupled qubit-TLS system when driven by a resonant microwave field;quantum dynamics of macroscopic resonant tunneling of the macroscopic distinct flux states in an rf-SQUID system when the system is driven by a single-cycle sinusoidal signal.These results will be important for intensively understanding the macroscopic quantum phenomenon of superconducting qubits and effectively controlling the property of superconducting qubits.The main researches and innovation are as followings.First,using four-level bipartite theoretical model,we describe the quantum dynamics of a resonantly driven superconducting qubit coupled to a TLS.The model gives a clear physical picture of the bipartite quantum system's dynamics.Using the Schrodinger equation for pure states and the Lindblad master equation for mixed states,we simulate the dynamics of the coupled qubit-TLS system driven by resonant microwave.We also calculate the effect of different coupling strengths between a TLS and a microwave field on the dynamics of a qubit-TLS system when the bipartite system is subject to resonant microwave driving.The result demonstrates that the coupling strengths between TLS and microwave field have some effect on the coherence of the bipartite qubit-TLS system.These results should be helpful in analyzing realistic experimental situations,such as investigating the dynamics of a hybrid system where the microwave acts on both qubit and TLS(or a similar system)simultaneously.Second,we simulate the dynamics of bipartite entanglement of coupled qubit-TLS system when driven by a resonant microwave field with experiment measurement results.Our results demonstrate that we not only can quantify entanglement by measuring the state of one constituent but also be able to control the dynamics of the entanglement by adjusting the interaction time between the qubit and the resonant driving field.We also simulate the entanglement dynamics of the bipartite qubit-TLS system with and without decoherence.Because of the decoherence effect,the entanglement oscillation between the qubit and TLS decays with time and exhibits entanglement sudden death(ESD)and/or entanglement sudden death and revival(ESDR).It indicates that the entanglement oscillation and revival are originated from the qubit-TLS coupling while the entanglement decay and sudden death are due to the coupling to the environment.Third,using an rf-SQUID system,we experimentally demonstrate the observation of macroscopic resonant tunneling(MRT)phenomenon of the macroscopic distinct flux states under a single-cycle sinusoidal driving.The population of the qubit appears interference patterns corresponding to resonant tunneling peaks between states in the adjacent potential wells.We do the numerical simulations considering the intra-well and inter-well relaxation mechanism,which shows the dynamics of the qubit depends significantly on the amplitude,frequency and initial phase of the external driving signal.The significant dependence of the population distribution of the qubit on the parameters of the driving signal provides us a new way to manipulate the quantum states.