A single flux quantum (SFQ) circuit to measure quantized energy levels in a Josephson junction has been proposed and implemented, which allows us to perform high-speed and low-noise measurements of the escape rate in Josephson junctions. By using this technique, the measurement of escape rate has been carried out at 4.2 K, the temperature three times higher than the crossover temperature. The test results show periodic dependence of the escape rate on the junction bias current, which is caused by the energy quantization effect in the Josephson junction.
Single-flux quantum (SFQ) digital logic is typically energy efficient and fast, and logic that uses ballistic and reversible principles will provide a new platform for efficient computing. We are studying long Josephson junctions (LJJs), SFQs within them, and an SFQ detector, all intended for future ballistic logic gate experiments. Specifically, we launch low-energy SFQ into discrete LJJs, made from an array of 80 Josephson junctions (JJs) and connecting inductors. The component JJs have critical currents (
Ic) of only 7.5
μA. Measurements of the discrete inductors from our LJJs indicate that the LJJ operates in the continuous regime of the Josephson transmission line, giving dynamics similar to continuous LJJs. The Josephson penetration depth is approximately 2.4 unit cells, and the rest energy of the SFQ in the LJJ is$\approx
47zJ.WhiletheSFQlauncherisfairlystandard,theSFQdetectorusesJJcriticalcurrentsofonly\approx
16\mu \text{A}$. To understand the effects of noise in the circuit, we measured it in two systems: 1) at 4.2 K using a helium dunk probe; and 2) at 3.5 K in a cryogen-free refrigerator. Data show that SFQ detection events are synchronous with SFQ launch events in both systems. The additional jitter in one system over the other is predominantly attributed to noise in the detector. The traversal of the SFQ is consistent with ballistic propagation. This study shows the creation of low-energy SFQs in LJJs composed of discrete components and a method to detect these SFQs, as a step toward achieving ballistic reversible gates.