  Quantum channel discrimination is a fundamental problem in quantum
information science. In this study, we consider general quantum channel
discrimination problems, and derive the lower bounds of the error probability.
Our lower bounds are based on the triangle inequalities of the Bures angle and
the trace distance. As a consequence of the lower bound based on the Bures
angle, we prove the optimality of Grover's search if the number of marked
elements is fixed to some integer $\ell$. This result generalizes Zalka's
result for $\ell=1$. We also present several numerical results in which our
lower bounds based on the trace distance outperform recently obtained lower
bounds.
