Current sensitivity of quantum flux parametron (QFP) was evaluated by measuring gray zone width on the basis of both circuit simulation and measurements for superconducting sensing systems composed of a superconducting sensor array and superconducting read-out and signal processing circuits. Simulation results indicate the narrow gray zone width can be obtained by decreasing inductances comprising the QFP. Moreover, both high-sensitivity and low-power operation of the QFP can be utilized by

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... g optimized circuit parameters and the excitation 2 current that has long rise time. Gray zone width of approximately 0.5 A, which is smaller than that of the Josephson current comparator based on a single flux quantum circuit, was experimentally obtained by using the trapezoidal excitation current that has rise time of 50 s. These results indicate the QFP is promising for the read-out circuit in the superconducting sensing systems because of high-sensitivity and low-power operation.