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Environmental effects on superconducting quantum bits
Environmental effects on superconducting quantum bits
We have experimentally studied Coulomb oscillation in a superconducting single-electron transistor, where electromagnetic-environment impedance was controlled using a one-dimensional array of superconducting quantum interference devices. By decreasing environment impedance, the Coulomb oscillation exhibited a crossover from 2e to 1e periodicity. In addition, the 1e component was enhanced when additional noise was introduced by connecting a potential meter to the central island of the transistor. We propose a theoretical analysis for this parity effect that includes the effect of fluctuations arising from the finite noise temperature of the electromagnetic environment.( Physical Review B 74, 184522 (2006)(SCI))
In another work, the Cooper-pair tunneling in superconducting charge boxes in tunable electromagnetic environments is experimentally studied using a single-electron-transistor electrometer and via current-voltage characteristics measurement with small excitations. In the low-impedance regime, the environmental effect is quantified with an effective Josephson coupling energy, which is found to decrease as the environment impedance increases. In the high-impedance regime, the Cooper-pair tunneling is blockaded. For impedance above 250M , Coulomb oscillation of the zero-bias conductance suggests that Cooper-pair tunneling is completely suppressed and subgap quasiparticle tunneling becomes dominant. (Physical Review B 75, 014517(2007))
1D array of Josephson junction as a tunable detector
1D array of Josephson junction as a tunable detector
This study experimentally demonstrates a superconducting detector that can be switched for charge signal and superconducting phase signal detection by tuning a magnetic field. In zero magnetic field, the one-dimensional array of superconducting quantum interference devices exhibits superconducting coherency and is phase sensitive. The array may become insulating and charge sensitive when a small perpendicular magnetic field is applied. (Journal of Applied Physics 101, 053903(2007))
Shapiro Steps Observed in a Superconducting Single Electron Transistor
Shapiro Steps Observed in a Superconducting Single Electron Transistor
The dc current-voltage (IV) characteristics of a superconducting single electron transistor irradiated with microwaves up to 18 GHz are experimentally studied. The switching current as a function of gate voltage demonstrates clear phase-charge duality in a Josephson junction. At higher microwave power levels, Shapiro steps in IV characteristics are observed. The step height in IV can be analyzed using the model an ac-voltage source applied to a single Josephson junction. (Chinese Journal of Physics 45, 230(2007))
On the 1D Josephson junction array under RF driving
On the 1D Josephson junction array under RF driving
In this study, we experimentally study the DC response of the 1D array of ultra-small (area on the order of 100nm×100nm) Josephson junctions under RF and microwave driving. We found that the conductance of the 1D array gradually decreases and oscillates as the RF amplitude increases. This phenomenon is closely related to the oscillation of dynamical conductance as a function of DC voltage bias in absence of RF excitation. The oscillatory behavior clearly seen in current-voltage characteristic is due to the quasi-particle tunneling which is prohibited in a low bias voltage smaller than the supeconducting gap (Δ). As such the oscillation period in DC voltage can be identified as 2Δ. The effect of AC driving at a low frequency is similar to that from a high DC voltage bias. From an analysis based on P(E) theory of charge tunneling, we conclude a simple formula describing the dynamical conductance of the 1D array under the AC driving. This implies that the conductance oscillation due to RF driving is similar to that due to DC bias, having the period of AC voltage equals 2Δ. This work has been published in New Journal of Physics 10 073025(2008)
We studied the effect of electromagnetic environment on the dynamics of charge and phase particles by analyzing the quasi-reentrant behavior. By modeling the environment as an ensemble of harmonic oscillators, we calculated a finite-temperature crossover phase diagram, which agrees quantitatively with the experimental results. This study provides a leap toward understanding the effect of the electromagnetic environment on the phase-charge duality. This work has been published in EPL 96, 47004 (2011).
We also used a one-dimensional(1D) Josephson junction array as a direct detector for ac electric field inside the array from 10MHz to 20GHz. By comparing the microwave amplitude we delivered and the amplitude an array received, we deduced the dielectric constant of a 1D array. It turns out that the dielectric constant in the superconducting state is larger than that in the insulating state, suggesting pronounced charge screening effect due to mobile charge. This work has been published in EPL(2014).
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