  This report surveys quantum error-correcting codes. As Preskill claimed, 21st
century would be the golden age of quantum error correction. Quantum channels
behave differently from classical channels, so researchers face difficulties in
developing robust quantum codes. Fortunately, the classical error control
methods have been well developed. If we can learn many lessons from classical
coding theory, we can expedite the development of quantum codes. Scientists
have discovered that quantum error correction shares many concepts with
classical counterpart. Both quantum and classical coding schemes add redundancy
to information to protect against noises. They also have similar conditions for
error detectability and correctability.
