Dense retrieval (DR) converts queries and documents into dense embeddings and measures the similarity between queries and documents in vector space. One of the challenges in DR is the lack of domain-specific training data. While DR models can learn from large-scale public datasets like MS MARCO through transfer learning, evidence shows that not all DR models and domains can benefit from transfer learning equally. Recently, some researchers have resorted to large language models (LLMs) to improve the zero-shot and few-shot DR models. However, the hard prompts or human-written prompts utilized in these works cannot guarantee the good quality of generated weak queries. To tackle this, we propose soft prompt tuning for augmenting DR (SPTAR): For each task, we leverage soft prompt-tuning to optimize a task-specific soft prompt on limited ground truth data and then prompt the LLMs to tag unlabeled documents with weak queries, yielding enough weak document-query pairs to train task-specific dense retrievers. We design a filter to select high-quality example document-query pairs in the prompt to further improve the quality of weak tagged queries. To the best of our knowledge, there is no prior work utilizing soft prompt tuning to augment DR models. The experiments demonstrate that SPTAR outperforms the unsupervised baselines BM25 and the recently proposed LLMs-based augmentation method for DR.

翻译：密集检索（DR）将查询和文档转换为密集嵌入，并在向量空间中衡量查询与文档之间的相似性。DR面临的一大挑战是缺乏领域特定的训练数据。尽管DR模型可以通过迁移学习从MS MARCO等大规模公开数据集中学习，但证据表明，并非所有DR模型和领域都能同等受益于迁移学习。近期，部分研究者借助大型语言模型（LLMs）来改进零样本和少样本DR模型。然而，这些工作中使用的硬提示或人工编写的提示无法保证生成的弱查询质量。为解决这一问题，我们提出软提示调优增强密集检索（SPTAR）：针对每个任务，我们在有限真实数据上利用软提示调优优化任务特定软提示，随后引导LLMs为未标注文档生成弱查询，从而获得足够的弱文档-查询对以训练任务特定密集检索器。我们设计了一个过滤器，用于选择提示中的高质量示例文档-查询对，以进一步提升弱标注查询的质量。据我们所知，目前尚无将软提示调优用于增强DR模型的相关工作。实验表明，SPTAR在性能上优于无监督基线BM25以及近期提出的基于LLMs的DR增强方法。