  What is the ultimate performance for discriminating two arbitrary quantum
channels acting on a finite-dimensional Hilbert space? Here we address this
basic question by deriving a general and fundamental lower bound. More
precisely, we investigate the symmetric discrimination of two arbitrary qudit
channels by means of the most general protocols based on adaptive
(feedback-assisted) quantum operations. In this general scenario, we first show
how port-based teleportation can be used to simplify these adaptive protocols
into a much simpler non-adaptive form, designing a new type of teleportation
stretching. Then, we prove that the minimum error probability affecting the
channel discrimination cannot beat a bound determined by the Choi matrices of
the channels, establishing a general, yet computable formula for quantum
hypothesis testing. As a consequence of this bound, we derive ultimate limits
and no-go theorems for adaptive quantum illumination and single-photon quantum
optical resolution. Finally, we show how the methodology can also be applied to
other tasks, such as quantum metrology, quantum communication and secret key
generation.
