A Security-Aware Nonlinearity Study of FPGA-Based Time-to-Digital Converters for Quantum Key Distribution Systems

• URL: http://arxiv.org/abs/2604.00229v1
• Date: Tue, 31 Mar 2026 20:52:40 GMT
• Title: A Security-Aware Nonlinearity Study of FPGA-Based Time-to-Digital Converters for Quantum Key Distribution Systems
• Abstract summary: In quantum key distribution (QKD), raw delay-line nonuniformity can affect coincidence timing.<n>This paper analyzes how measured FPGA-TDC nonlinearity propagates to QKD timing metrics.
• Score: 1.503974529275767
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Intrinsic nonlinearity in FPGA-based time-to-digital converters (TDCs) is often treated as a calibration issue and evaluated mainly through post-correction metrics. In quantum key distribution (QKD), however, raw delay-line nonuniformity can affect coincidence timing and thereby influence accidental-coincidence rate and Quantum Bit Error Rate (QBER). This paper analyzes how measured FPGA-TDC nonlinearity propagates to QKD timing metrics using a conservative system-level model that combines random timing uncertainty and deterministic nonlinearity. We also propose fabric-level mitigation strategies based on LUT-assisted delay shaping and placement constraints to reduce severe bin-width irregularities without statistical calibrations. The method is evaluated by reproducing two open-source TDCs implemented on a low-cost Zynq-7000 FPGA. We observe reductions of 14\%-21\% in integral nonlinearity (INL) compared with the non-optimized design, leading to a reduced QBER contribution and an improvement by 3.7\%-14.2\% in the estimated secret fraction. These results suggest that raw FPGA-TDC nonlinearity deserves explicit consideration in timing-sensitive QKD implementations.

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