Title: LoRaQ: Optimized Low Rank Approximated Quantization Error for 4-bit Quantization
Primary area: generative models
Keywords: Post-Training Quantization, Transformers, Diffusion, Image Generation

Abstract: Post-training quantization (PTQ) is essential for deploying large diffusion-based transformers on resource-constrained hardware. However, aggressive 4-bit quantization introduces significant degradation in generative performance. While existing solutions mitigate quantization error through outlier smoothing or rotation techniques, low-rank approximation methods that add auxiliary linear branches to each quantized layer represent a promising new paradigm. Yet, these approaches suffer from computational overhead due to the data movement required by full-precision (W16A16) branches, limiting practical deployment. In addition, data-based calibration contributes to the complexity of the quantization process and involves risks such as potential accuracy degradation. We propose LoRaQ (low-rank approximated quantization), a data-free calibration approach to optimize quantization error compensation. This method can be used in composition with other PTQ models. LoRaQ further enables mixed-precision configurations by quantizing the low-rank branch itself, overcoming the limitations of prior work. While LoRaQ achieves superior quantization performance than state-of-the-art methods in their native W4A4 setting on PixArt-Sigma and SANA, it also allows for configurations such as W8A8, W6A6 and W4A8 for low-rank branch alongside a W4 main layer. This reduces data movement overhead and enables a fully quantized, hardware-efficient solution.