  Most existing cross-modal retrieval methods employ two-stream encoders with
different architectures for images and texts, \textit{e.g.}, CNN for images and
RNN/Transformer for texts. Such discrepancy in architectures may induce
different semantic distribution spaces and limit the interactions between
images and texts, and further result in inferior alignment between images and
texts. To fill this research gap, inspired by recent advances of Transformers
in vision tasks, we propose to unify the encoder architectures with
Transformers for both modalities. Specifically, we design a cross-modal
retrieval framework purely based on two-stream Transformers, dubbed
\textbf{Hierarchical Alignment Transformers (HAT)}, which consists of an image
Transformer, a text Transformer, and a hierarchical alignment module. With such
identical architectures, the encoders could produce representations with more
similar characteristics for images and texts, and make the interactions and
alignments between them much easier. Besides, to leverage the rich semantics,
we devise a hierarchical alignment scheme to explore multi-level
correspondences of different layers between images and texts. To evaluate the
effectiveness of the proposed HAT, we conduct extensive experiments on two
benchmark datasets, MSCOCO and Flickr30K. Experimental results demonstrate that
HAT outperforms SOTA baselines by a large margin. Specifically, on two key
tasks, \textit{i.e.}, image-to-text and text-to-image retrieval, HAT achieves
7.6\% and 16.7\% relative score improvement of Recall@1 on MSCOCO, and 4.4\%
and 11.6\% on Flickr30k respectively. The code is available at
\url{https://github.com/LuminosityX/HAT}.
