  Adiabatic quantum-flux-parametron (AQFP) circuits can operate with extremely
small energy dissipation (~1 zJ per junction at 5 GHz) and high sensitivity (~1
{\mu}A) owing to adiabatic switching. Thus, AQFP logic is suitable to use as
readout interfaces for cryogenic detectors, such as superconducting nanowire
single-photon detectors (SSPDs). In order to extend the application of AQFP
logic to various detectors, it is crucial to achieve even better sensitivity
because some detectors, such as WSi SSPDs and transition edge sensors (TESs),
require sub-{\mu}A sensitivity. In the present study, we propose a
high-sensitivity AQFP comparator with a current transformer (CT), which
increases the equivalent input current and thereby improves the sensitivity.
Numerical simulation shows that the proposed comparator achieves a sensitivity
approximately six times better than that of a conventional AQFP comparator.
Furthermore, we demonstrate a sensitivity of 46 nA at 4.2 K for a sampling
frequency of 500 Hz in the experiment.
