  In this article, we estimate the quasi-local energy with reference to the
Minkowski spacetime [16,17], the anti-de Sitter spacetime [4], or the
Schwarzschild spacetime [3]. In each case, the reference spacetime admits a
conformal Killing-Yano 2-form which facilitates the application of the
Minkowski formula in [15] to estimate the quasi-local energy. As a consequence
of the positive mass theorems in [9,13] and the above estimate, we obtain
rigidity theorems which characterize the Minkowski spacetime and the hyperbolic
space.
