SFQ Multiplexed Signal Processing Circuits for STJ Photon Detector Arrays

We have been studying single flux quantum (SFQ) digital signal processing (DSP) circuits for superconducting tunnel junction (STJ) photon detector arrays. The proposed DSP circuit consists of a binary tree of multiplexers, a time counter, a binary counter, and a readout memory. Each multiplexer was designed such that the first arriving SFQ pulse train connects to the binary counter until the time counter reaches a set value. The time counter is set to a sufficiently long time compared with the response time of STJ photon detector. After processing the multiplexer, SFQ pulses are integrated by the binary counter. The number of integrated SFQ pulses is proportional to the number of quasi-particles generated by photon irradiation in an STJ. After the time counter reaches the set value, the time counter generates a set SFQ pulse to set the multiplexer in the ready state, the integrated value is sent to the readout memory and then the binary counter is reset. The binary tree of multiplexers could be extended to have address bits for photon imaging using an STJ array. One key component of the proposed DSP circuit was fabricated and its operation using an on-chip clock generator was successfully demonstrated.