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IEEE Transactions on Applied Superconductivity

Measurement of Environmental Impedance at Plasma Frequency of Josephson Junction With Microwave Enhanced Thermal Escape

Bo Mao, Siyuan Han

Published Jun 1, 2007• 3 citations
Publisher

Abstract

We have developed a simple and accurate method to determine the environmental impedance seen by a phase qubit at the plasma frequency of the junction by comparing the measured and simulated thermal escape rate in strong microwave field at 4.2 K. The results show that for our sample 1/Re[Y(omega)] is about 2.5 kOmega, where Re[Y(omega)] is the real part of the frequency-dependent environmental admittance.

DOI
10.1109/tasc.2007.898123

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We have developed a simple and accurate method to determine the environmental impedance seen by a phase qubit at the plasma frequency of the junction by comparing the measured and simulated thermal escape rate in strong microwave field at 4.2 K. The results show that for our sample 1/Re[Y(omega)] is about 2.5 kOmega, where Re[Y(omega)] is the real part of the frequency-dependent environmental admittance.
The capacitance of a Josephson tunnel junction is an important parameter, which determines its plasma frequency ωp and thus the energy level spacing in the quantum regime. When a microwave signal at the frequency of ω is applied to a current-biased junction in its zero-voltage state, the escape rate from this state is somehow enhanced. The enhancement is expected to go through a pronounced maximum when ωp=ω. Making use of such microwave generated resonant activation in the thermal regime, we have developed a time-resolved measurement scheme for determining the junction capacitance with an uncertainty of less than 2%.
We have observed a dissipative phase diffusion branch in arrays of hysteretic high-Tc{T}_{c}Tc​ intrinsic Josephson junctions. By comparing the data with a thermal activation model we extract the impedance seen by the junction in which phase diffusion is occurring. At the plasma frequency this junction is isolated from its environment and it sees its own large (\ensuremath{\sim} k\ensuremath{\Omega}) impedance. Our results suggest that stacks of Josephson junctions may be used for isolation purposes in the development of a solid state quantum computer.
The rf impedance and the wide-band response to radiation of a resistively shunted junction (RSJ) model Josephson junction have been measured in the presence of thermal noise, using a phase-locked-loop analog of the RSJ model. In the conditions for which analytical calculations are valid there is good agreement between the theory and the analog. When the RSJ model is shunted with a capacitance, a plasma type resonance can occur in the rf impedance when the junction is biased in the supercurrent (in-lock). When thermal noise is present, this plasma resonance can also occur when a nonzero average voltage is generated across the junction. We found that in this case a resonance also occurs in the wide-band response to radiation. This resonance takes place at a frequency that can be identified as the attempt frequency, with which the junction attempts to escape the phase locked condition in which it exists for a fraction of the time even though the average voltage across the junction is nonzero. The average lifetimes of the junction in lock τin (?=0) and out of lock τout (?=?out) were also measured in the presence of thermal noise. τin is in good agreement with existing theory and τout is derived from the measurements.
We investigate, by experiments and numerical simulations, thermal activation processes of Josephson tunnel junctions in the presence of microwave radiation. When the applied signal resonates with the Josephson plasma frequency oscillations, the switching current may become multivalued in a temperature range far exceeding the classical to quantum crossover temperature. Plots of the switching currents traced as a function of the applied signal frequency show very good agreement with the functional forms expected from Josephson plasma frequency dependencies on the bias current. Throughout, numerical simulations of the corresponding thermally driven classical Josephson junction model show very good agreement with the experimental data.
We have fabricated intrinsic Josephson junction arrays using Tl2Ba2CaCu2O8thin films. The junctions have dimensions of order 1 mum2and therefore high slope resistance in the range 1 kOmega to 10 kOmega per junction on the quasiparticle branches. We have irradiated the junctions with microwaves at a frequency of 6.37 GHz. We observe both microwave-induced vortex-flow branches and strong suppression of the switching current but only when no junctions are in the voltage state. We attribute this observation to the shunting of the high-resistance junctions by the environmental impedance, and discuss the significance of this result for the application of intrinsic junctions as THz sources and qubits.