  The highest information rate at which quantum error-correction schemes work
reliably on a channel, which is called the quantum capacity, is proven to be
lower bounded by the limit of the quantity termed coherent information
maximized over the set of input density operators which are proportional to the
projections onto the code spaces of symplectic stabilizer codes. Quantum
channels to be considered are those subject to independent errors and modeled
as tensor products of copies of a completely positive linear map on a Hilbert
space of finite dimension, and the codes that are proven to have the desired
performance are symplectic stabilizer codes. On the depolarizing channel, this
work's bound is actually the highest possible rate at which symplectic
stabilizer codes work reliably.
