The quantum analog of the classical erasure channel provides a simple example of a channel whose asymptotic capacity for faithful transmission of intact quantum states, with and without the assistance of a two-way classical side channel, can be computed exactly. We derive the quantum and classical capacities for the quantum erasure channel and related channels, and compare them to the depolarizing channel, for which only upper and lower bounds on the capacities are known.

4 pages, RevTeX, epsf macros to include 2 eps figures. See also http://vesta.physics.ucla.edu/~smolin/ Version 2, submitted to Phys. Rev. Lett., includes several small changes and corrections