S^2NN: Sub-bit Spiking Neural Networks - Wei, Wenjie; Zhang, Malu; Zhang, Jieyuan; Belatreche, Ammar; Wang, Shuai; Shan, Yimeng; Liu, Hanwen; Cao, Honglin; Wang, Guoqing; Yang, Yang; Li, Haizhou (Advances in Neural Information Processing Systems, 2025)