  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.
