  Entity retrieval--retrieving information about entity mentions in a query--is
a key step in open-domain tasks, such as question answering or fact checking.
However, state-of-the-art entity retrievers struggle to retrieve rare entities
for ambiguous mentions due to biases towards popular entities. Incorporating
knowledge graph types during training could help overcome popularity biases,
but there are several challenges: (1) existing type-based retrieval methods
require mention boundaries as input, but open-domain tasks run on unstructured
text, (2) type-based methods should not compromise overall performance, and (3)
type-based methods should be robust to noisy and missing types. In this work,
we introduce TABi, a method to jointly train bi-encoders on knowledge graph
types and unstructured text for entity retrieval for open-domain tasks. TABi
leverages a type-enforced contrastive loss to encourage entities and queries of
similar types to be close in the embedding space. TABi improves retrieval of
rare entities on the Ambiguous Entity Retrieval (AmbER) sets, while maintaining
strong overall retrieval performance on open-domain tasks in the KILT benchmark
compared to state-of-the-art retrievers. TABi is also robust to incomplete type
systems, improving rare entity retrieval over baselines with only 5% type
coverage of the training dataset. We make our code publicly available at
https://github.com/HazyResearch/tabi.
