The capacity of a noisy quantum channel for quantum communication is the fundamental limit for quantum error correction. Here we show that two quantum channels, each of whose capacity is zero, can have a nonzero capacity when used together. This uniquely quantum mechanical effect unveils a rich structure in the theory of quantum communications and points to the existence of incomparable types of quantum information, implying that the quantum capacity of a channel does not uniquely specify its ability for transmitting quantum information.