Computational distinguishability of degradable and antidegradable channels

A channel is degradable if there exists a second channel that maps the output state of the channel to the environment state. These channels satisfy the property that the output state contains more information about the input than the environment does. A complementary class of channels is the antidegradable channels, which admit channels that map the environment state to the output state of the channel. In this paper we show that the computational problem of distinguishing two channels remains PSPACE-complete when restricted to these classes of channels. This is shown using a construction of Cubitt, Ruskai, and Smith that embeds any channel into a degradable channel, and a related construction for the case of antidegradable channels.