Single flux quantum (SFQ) circuits are superconducting digital circuits operating at tens of GHz with low power consumption and low timing jitter, which has been developed for high-end computing [1] or cryogenic post signal processor for superconducting detectors [2]. While high-speed operation of the SFQ circuits is demonstrated for various applications, coaxial cables are usually used for input or output interfaces between room-temperature electronics and cryogenic-temperature devices, which limits the bandwidth of signals and increases in heat inflow from room temperature. An optical fiber is good candidate for input or output interfaces due to wider bandwidth with less thermal conductivity.
A superconducting strip photon detector (SSPD) is suitable for the energy efficient input interface of the SFQ circuits because the SSPDs generate an electrical pulse by illumination of a single photon with an optical fiber coupled. Also, SSPDs have some features such as high detection efficiency, low timing jitter, and low power dissipation compared to semi-conductor photo diodes [3]. However, the typical repetition frequency is a few tens of MHz, which is not fast bitrate for the input interface of SFQ circuits.
In this work, we propose a superconducting optical input interface using SSPD based on pulse position modulation (PPM), which provides input bit patterns for SFQ digital circuits. Since a bit stream is generated by the input pulse timing position relative to the start timing in PPM encoding, the SFQ time-to-digital converter (TDC) connected with SSPD is employed for PPM configuration. In this configuration, trigger signals from pulsed laser oscillator for SSPDs and output signals from SSPDs correspond to start signal and stop signal for SFQ-TDC, respectively. The length of bit stream generated by PPM technique is determined by slot time (time resolution of TDC) and frame time (full scale time of TDC). The low timing jitter of SSPDs and SFQ circuits can reduce the slot time to a few tens ps, which enhance the number of bits. Thus, the proposed system can improve bit rate for input signal without reducing response time of SSPDs.
We investigated the relationship between the bitrate and slot time with frame time of proposed OE interface. Also, we designed the SFQ-TDC for demonstration of the proposed OE interface. The designed circuit is composed of a magnetically-coupled (MC-) DC/SFQ converter [4], an internal clock generator, a 4-bit counter based on toggle flip flops (TFFs), and a 4-bit shift register as a parallel-to-serial converter. The SFQ circuits was designed based on the fabrication process with critical current density (Jc) of 10 kA/cm2 for Josephson junction (JJ). In this design, the slot time corresponding a period of clock signals by internal clock generator was set to be 250 ps, which is longer than timing jitter of SSPDs. We plan to design the SFQ digital circuits using input bit stream pattern based on proposed optical input interface.
[1] M. Tanaka, Y. Yamanashi, N. Irie, H-J. Park, S. Iwasaki, K. Takagi, K. Taketomi, A. Fujimaki, N. Yoshikawa, H. Terai, and S. Yorozu, Supercond. Sci. Technol., 20, S305 (2007)
[2] S. Miyajima, M. Yabuno, S. Miki, S. Nagasawa, M. Hidaka, and H. Terai, Appl. Phys. Lett., 122, 182602 (2023)
[3] G. N. Gol’tsman, O. Okunev, G. Chulkova, A. Lipatov, A. Semenov, K. Smirnov, B. Voronov, A. Dzardanov, C. Williams, and R. Sobolewski, Appl. Phys. Lett. 79, 705 (2001).
[4] H. Terai, T. Yamashita, S. Miki, K. Makise, and Z. Wang, Opt. Express 20, 20115 (2012).
The circuits were fabricated in the Superconducting Quantum Circuit Fabrication Facility (Qufab) in National Institute of Advanced Industrial Science and Technology (AIST).
Keywords: single flux quantum circuit, superconducting strip photon detector, pulse position modulation, optical input interface