A Time-to-Digital Converter With Steady Calibration Through Single-Photon Detection

Time-to-digital converters (TDCs) are a crucial tool in a wide array of fields, in particular for quantum communication, where time taggers performance can severely affect the quality of the entire application. Nowadays, FPGA-based TDCs present a viable alternative to ASIC ones, once the nonlinear behavior due to the intrinsic nature of the device is properly mitigated. To compensate for said nonlinearities, a calibration procedure is required, which should be maintained throughout its runtime. Here, we present the design and the demonstration of a TDC that is FPGA-based showing a residual FWHM jitter of 27 ps and scalable for multichannel operation. The target application in quantum key distribution (QKD) is discussed with a calibration method based on the exploitation of single-photon detection that does not require stopping the data acquisition or using any estimation methods, thus increasing accuracy and removing data loss. The calibration was tested in a relevant environment, investigating the behavior of the device between $5~^{\circ }$ C and $80~^{\circ }$ C. Moreover, our design is capable of continuously streaming up to 12 Mevents/s for up to ~1 week without the TDC overflowing, making it ready for a real-life scenario deployment.