Shot-Noise-Driven Escape in Hysteretic Josephson Junctions

We have measured the influence of shot noise on hysteretic Josephson junctions initially in the macroscopic quantum tunneling regime. The escape threshold current into the resistive state decreases monotonically with increasing average current through the scattering conductor, which is another tunnel junction. Escape is predominantly determined by excitation due to the wideband shot noise. This process is equivalent to thermal activation (TA) over the barrier at effective temperatures up to about 4 times the critical temperature of the superconductor. The presented TA model is in excellent agreement with the experimental results.

Figure 1

(a) Measurement configuration. A split JJ in MQT regime measures the current fluctuations due to the shot noise of the scattering junction in the right arm. (b) Processes leading to escape from the zero-voltage (supercurrent) state into the resistive state. (c) Sketch of current pulses $I_{\mathrm{P}}$, with superimposed noise of $I_{\mathrm{N}}$, and (d) the associated escape characteristics. The central quantities relating to the pulses and escape histograms have been indicated in the figure.

Figure 2

Histogram positions under shot-noise injection. (a) Escape histograms of sample B when current pulses have been injected through the long injection line (open circles) and through the two noise sources (source 1, squares; source 2, solid circles), respectively. The lines are the corresponding theoretical results. (b) Experimental results (circles) on the switching threshold current $I_{0.5}$ against the average current ${\overline{I}}_{\mathrm{N}}$ through scatterer 2 in sample B. The solid line is again the result of the used theoretical model. The inset shows the $IV$ curve of the noise source. (c) Similar data as in (b) but for sample A. The rising curve shows the equivalent temperature $T^{*}$, which ranges from 2 to 5 K. Pulse length was $\Delta t=800\mu \mathrm{s}$.

Figure 3

The width of the histograms, $\Delta I$, when shot noise and thermal noise are applied. (a) The width against the average current ${\overline{I}}_{\mathrm{N}}$ through the scatterer. The dots are the experimental results and the solid line is the result of the theoretical model. (b) $\Delta I$ vs $I_{0.5}$ under two different experimental conditions. The solid circles are data with elevated shot-noise temperature $T^{*}$, whereas the open circles are with variable bath temperature $T$. The solid line is the result of the thermal activation model with a constant energy gap, and the dashed line takes into account the suppression of the BCS gap [16] in the measurement with increased $T$.