We demonstrate the effect of different coupling strengths between a microscopic two-level system(TLS)and a microwave field on the dynamics of a qubit—TLS system when the bipartite system is subject to resonant microwave driving.Rabi beating with a different TLSmicrowave coupling strength is demonstrated in simulations.Entanglement,quantified by the concurrence between the qubit and TLS,both for pure states and mixed states,is simulated.When decoherence is considered,entanglement of the bipartite system oscillates with damping and exhibits entanglement sudden death and/or entanglement sudden death and revival.
The critical current density behaviors across a bicrystal grain boundary(GB) inclined to the current direction with different angles in YBa_2Cu_3O_(7-δ) bicrystal junctions in magnetic fields are investigated.There are two main reasons for the difference in critical current density in junctions at different GB inclined angles in the same magnetic field:(i) the GB plane area determines the current carrying cross section;(ii) the vortex motion dynamics at the GB affects the critical current value when the vortex starts to move along the GB by Lorentz force.Furthermore,the vortex motion in a bicrystal GB is studied by investigating transverse(Hall) and longitudinal current–voltage characteristics(I–V_(xx) and I–V_(xy)).It is found that the I–V_(xx) curve diverges from linearity at a high driving current,while the I–V_(xy) curve keeps nearly linear,which indicates the vortices inside the GB break out of the GB by Lorentz force.
We experimentally demonstrate the observation of macroscopic resonant tunneling(MRT) phenomenon of the macroscopic distinct flux states in a radio frequency superconducting quantum interference device(rf-SQUID) under a singlecycle sinusoidal driving.The population of the qubit exhibits interference patterns corresponding to resonant tunneling peaks between states in the adjacent potential wells.The dynamics of the qubit depends significantly on the amplitude,frequency,and initial phase of the driving signal.We do the numerical simulations considering the intra-well and interwell relaxation mechanism,which agree well with the experimental results.This approach provides an effective way to manipulate the qubit population by adjusting the parameters of the external driving field.
