  Synthetic data generation has become an emerging tool to help improve the
adversarial robustness in classification tasks since robust learning requires a
significantly larger amount of training samples compared with standard
classification tasks. Among various deep generative models, the diffusion model
has been shown to produce high-quality synthetic images and has achieved good
performance in improving the adversarial robustness. However, diffusion-type
methods are typically slow in data generation as compared with other generative
models. Although different acceleration techniques have been proposed recently,
it is also of great importance to study how to improve the sample efficiency of
generated data for the downstream task. In this paper, we first analyze the
optimality condition of synthetic distribution for achieving non-trivial robust
accuracy. We show that enhancing the distinguishability among the generated
data is critical for improving adversarial robustness. Thus, we propose the
Contrastive-Guided Diffusion Process (Contrastive-DP), which adopts the
contrastive loss to guide the diffusion model in data generation. We verify our
theoretical results using simulations and demonstrate the good performance of
Contrastive-DP on image datasets.
