Current Sensitivity Enhancement of a Quasi-One-Junction SQUID Comparator as an Input Circuit of SFQ Readout Circuit for a Superconducting Detector

摘要

We evaluated the current sensitivity of a quasi-one-junction SQUID (QOS) comparator with an input transformer at 4.2 K. A comparator based on a QOS is promising for constructing the single flux quantum (SFQ) readout circuits of an array system of multiple superconducting detectors. The QOS comparator is made of three Nb/AlO_x/Nb Josephson junctions, senses an output signal of a superconductor detector, and generates the SFQ pulses. There are strong demands for enhancing the current sensitivity of the QOS comparator because an output current from superconducting detector typically remains at the magnitude of a few μA. We fabricated the QOS comparator with an input transformer using AIST Standard Process 2, where the critical current density of the Josephson junctions is chosen as high as 2.5 kA/cm~2. We designed the input transformer to enhance the current sensitivity under the conditions of 200 μm × 200 μm in size and 20:1 in turn ratio. Consequently, we succeeded in reducing a gray zone width of the comparator, and achieved the current sensitivity of 400 nA at 4.2 K in the low frequency range.