Challenge: Experimental results show that LaPraDoR is state-of-the-art compared with supervised dense retrieval models.
Approach: They propose a pretrained dual-tower dense retriever that does not require supervised data for training.
Outcome: The proposed method achieves state-of-the-art performance on 18 datasets of 9 zero-shot text retrieval tasks.

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Retrieval-Augmented Retrieval: Large Language Models are Strong Zero-Shot Retriever (2024.findings-acl)

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Challenge: Large-scale retrieval is indispensable in information-seeking tasks such as open-domain question answering and knowledgegrounded dialogue.
Approach: They propose to use a large language model (LLM) to augment a query with its potential answers by prompting LLMs with a composition of the query and the query’s in-domain candidates.
Outcome: The proposed method breaks brute-force combinations of retrievers with LLMs and lifts the performance of zero-shot retrieval to be very competitive on benchmark datasets.
Unsupervised Dense Retrieval with Relevance-Aware Contrastive Pre-Training (2023.findings-acl)

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Challenge: Dense retrievers have impressive performance, but their demand for abundant training data limits their application scenarios.
Approach: They propose a method which uses unlabeled data to construct pseudo-positive examples from unlabelled data and then contrastively weighs the contrastive loss of different pairs according to the estimated relevance.
Outcome: The proposed method beats the SOTA unsupervised Contriever model on BEIR and open-domain QA retrieval benchmarks and is a good few-shot learner.
ReGen: Zero-Shot Text Classification via Training Data Generation with Progressive Dense Retrieval (2023.findings-acl)

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Challenge: Recent studies show that large pretrained language models can generate training data with no task-specific or cross-task data.
Approach: They propose a retrieval-enhanced framework to create training data from a general-domain unlabeled corpus.
Outcome: The proposed framework achieves 4.3% gain over baselines and saves 70% of time compared with baselines using large language models.
Augmenting Zero-Shot Dense Retrievers with Plug-in Mixture-of-Memories (2023.emnlp-main)

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Challenge: Using mixture-of-memory augmenting to augment language models improves model generalization but with diminishing return.
Approach: They develop a mechanism that augments language models with mixture-of-memory Augmentation (MoMA) they augment strong T5-based retrievers with the option to "plug in" unseen memory at inference time.
Outcome: The proposed model outperforms methods with larger model sizes on the BEIR benchmark and achieves comparable or even better performance than methods relying on target-specific pretraining.
Unsupervised Corpus Aware Language Model Pre-training for Dense Passage Retrieval (2022.acl-long)

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Challenge: Recent research shows that fine-tuning dense retrievers to realize their capacity requires carefully designed fine-cuning techniques.
Approach: They propose a pre-training architecture that learns to condense information into the dense vector through LM pre-training and a coCondenser architecture which adds an unsupervised corpus-level contrastive loss to warm up the passage embedding space.
Outcome: The proposed architecture reduces the need for heavy data engineering and large batch training.
Precise Zero-Shot Dense Retrieval without Relevance Labels (2023.acl-long)

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Challenge: Existing dense retrieval systems that use semantic embedding similarities can be effective across tasks and languages.
Approach: They propose to pivot through Hypothetical Document Embeddings (HyDE) given a query, HyDE first zero-shot prompts an instruction-following language model to generate a hypothetical document.
Outcome: The proposed method significantly outperforms the state-of-the-art unsupervised dense retriever Contriever and shows strong performance comparable to fine-tuned retrievers across tasks and languages.
NER Retriever: Zero-Shot Named Entity Retrieval with Type-Aware Embeddings (2025.findings-emnlp)

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Challenge: NER Retriever uses a user-defined type description to retrieve documents mentioning entities of that type.
Approach: They propose a zero-shot retrieval framework for ad-hoc Named Entity Recognition . a user-defined type description is used to retrieve documents mentioning entities of that type .
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A Representation Sharpening Framework for Zero Shot Dense Retrieval (2026.eacl-long)

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Challenge: Zero-shot dense retrieval requires generic, pretrained DRs, which struggle to represent semantic differences between similar documents.
Approach: They propose a training-free representation sharpening framework that augments a document’s representation with information that helps differentiate it from similar documents in the corpus.
Outcome: The proposed framework is compatible with prior approaches to zero-shot dense retrieval and consistently improves their performance.
How to Train Your Dragon: Diverse Augmentation Towards Generalizable Dense Retrieval (2023.findings-emnlp)

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Challenge: Existing techniques to improve dense retrieval suffer from effectiveness tradeoffs between supervised and zero-shot retrieval, some argue due to the limited model capacity.
Approach: They propose to use diverse queries and sources of supervision to train a generalizable DR to achieve high accuracy in both supervised and zero-shot retrieval.
Outcome: The proposed DR can achieve state-of-the-art in supervised and zero-shot evaluations without increasing model size.
Less is More: Pretrain a Strong Siamese Encoder for Dense Text Retrieval Using a Weak Decoder (2021.emnlp-main)

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Challenge: Dense retrieval requires high-quality text sequence embeddings to support effective search in the representation space.
Approach: They propose a self-learning method that pre-trains the autoencoder using a weak decoder to push the encoder to provide better sequence representations.
Outcome: The proposed model significantly boosts the effectiveness and few-shot ability of dense retrieval models on web search, news recommendation, and open domain question answering.

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