Fluxon transmission measurements of engineered long Josephson junctions for efficient computing

Single-Flux Quantum (SFQ) digital logic is typically both energy efficient and fast, but the logic that uses reversibility provides the most extreme method for improving efficiency. We are studying engineered long Josephson junctions (LJJs) that are components for future ballistic logic gates within a logic family named Reversible Fluxon Logic (RFL). Therein, the bit states are represented by two possible polarities of an SFQ. Here we test engineered LJJs with component JJ critical currents of 7.5uA and a Josephson penetration depth of approximately 2.4 unit cells. In our study, the SFQ rest energy in the Long JJ is determined to be 47zJ (regardless of bit state). The LJJs were tested in two environments, at 4.2K in a helium dunk probe (DP) and 3.5K in a cryogen-free refrigerator (CFR). The on-chip circuit consists of three parts in sequence: an SFQ launcher, the LJJ under test, and a detector that uses biased 20uA JJs. Data show that SFQ detection events are synchronous with SFQ launch events in both setups, indicating possible ballistic SFQ transmission in the LJJs. The jitter of the events in the CFR setup indicates that we are limited by signal filtering in our CFR setup and by noise in the DP setup.