  Fully Homomorphic Encryption~(FHE) is a key technology enabling
privacy-preserving computing. However, the fundamental challenge of FHE is its
inefficiency, due primarily to the underlying polynomial computations with high
computation complexity and extremely time-consuming ciphertext maintenance
operations. To tackle this challenge, various FHE accelerators have recently
been proposed by both research and industrial communities. This paper takes the
first initiative to conduct a systematic study on the 14 FHE accelerators --
cuHE/cuFHE, nuFHE, HEAT, HEAX, HEXL, HEXL-FPGA, 100$\times$, F1, CraterLake,
BTS, ARK, Poseidon, FAB and TensorFHE. We first make our observations on the
evolution trajectory of these existing FHE accelerators to establish a
qualitative connection between them. Then, we perform testbed evaluations of
representative open-source FHE accelerators to provide a quantitative
comparison on them. Finally, with the insights learned from both qualitative
and quantitative studies, we discuss potential directions to inform the future
design and implementation for FHE accelerators.
