Demonstration of Picosecond Time Resolution in Double-Oscillator Time-to-Digital Converter Using Single-Flux-Quantum Circuits

We have been developing a time-to-digital converter (TDC) using single-flux-quantum (SFQ) circuits in order to realize an imaging system based on a superconducting strip single-photon detector (SSPD). In this system, the absorption position of a single photon in a meander-shaped SSPD is determined by measuring the delay time of the detected signal propagating through an SSPD delay line. Increasing the time resolution of the TDC is the primary concern for improving the spatial resolution of the system. Here we focused on the double-oscillator SFQ TDC, where the difference of oscillation periods between two SFQ ring oscillators determines the time resolution. In the previous study, a time resolution of about 4 ps was demonstrated by using the Nb standard Josephson integrated circuit (IC) process with a critical current density of 2.5 kA/cm
²
. In this study, we improved the time resolution by adopting the Nb advanced Josephson IC process with a critical current density of 10 kA/cm
²
and by optimizing the circuit parameters of the coincidence detector. The histogram and the linearity of the delay measurements show that the time resolution is improved to up to 2 ps.