|PLACE||R49223, Second Physics Building , NCKU|
|FIELD||Quantum Information Science|
|SPEAKER||Dr. Saxon Liou(劉少勳) - 中央研究院物理所|
|TITLE||Investigation of Josephson junction devices for quantum information application|
|ABSTRACT||Radio-frequency single electron transistor (rf-SET) and one-dimensional Josephson junction array (1D JJA) have experimental studied. In the first part, rf-SET has been a well know candidate of readout of quantum bit before 2004 for its highly sensitivity of charge. We investigated charge sensitivities of rf-SET by using amplitude (AD) and phase-shift detection (PSD) of the reflected rf signals. It was found that AD is most sensitive at the resonant frequency while PSD is most sensitive at a frequency slightly off the resonance. The maximal change in reflection amplitude and phase-shift were found proportional to the SET conductance change, and the experimental findings were confirmed by the calculation based on an equivalent circuit model.
Second, I’ll introduce the study of DC response of 1D JJA under rf irradiation. The zero-bias resistance as a function of RF amplitude in the low-frequency regime shows an oscillatory behavior, and can be explained using the phase fluctuation model for Cooper pair tunneling. It follows that an effective classical detector theory is applicable and the oscillation period can be compared with the superconducting gap. The results suggest a primary and direct rf detection scheme using 1D JJA.
In the last part, I’ll show that the rf light velocity in the 1D JJA can be modulated by an applied magnetic field, which causes a change in the junction inductance. The 1D JJA predicted as a quantum metamaterial is placed in a coplanar waveguide structure and is coupled strongly to the external transmission lines via insulating layers at the two ends. The rf velocity was inferred from measured shifts in the phase in respect to the zero magnetic field value and could be reduced continuously by 3 times, to a value of 2×10-3c, c is the light velocity in vacuum.