  Collective decoherence is possible if the departure between quantum bits is
smaller than the effective wave length of the noise field. Collectivity in the
decoherence helps us to devise more efficient quantum codes. We present a class
of optimal quantum codes for preventing collective amplitude damping to a
reservoir at zero temperature. It is shown that two qubits are enough to
protect one bit quantum information, and approximately $L+ 1/2 \log_2((\pi
L)/2)$ qubits are enough to protect $L$ qubit information when $L$ is large.
For preventing collective amplitude damping, these codes are much more
efficient than the previously-discovered quantum error correcting or avoiding
codes.
