Readout of phase states in Josephson parametric oscillators using adiabatic quantum flux parametron

Abstract Josephson parametric oscillators (JPOs) have potential as fundamental building blocks for large-scale quantum annealing systems. To scale up such systems, both readout and control circuits need to be implemented at cryogenic temperatures. Adiabatic quantum flux parametron (AQFP) circuits are well suited for such environments due to their ultra-low power consumption. This paper proposes a phase-state readout scheme for JPOs based on AQFP circuits, which enables real-time digitization of oscillation phases. A JPO was fabricated, and the concept was experimentally validated at 4.2 K. After confirming stable oscillations in the JPO, a digital readout method using AQFP circuits was demonstrated. Analysis of the probability of obtaining a logical ‘1’ at the output revealed a clear phase-dependent behavior. Furthermore, our simulations predicted a readout fidelity of nearly 100% at millikelvin temperatures, demonstrating the feasibility of AQFP-based phase-state readout for future large-scale quantum annealing systems.