Collective decoherence is possible if the distance between quantum bits is smaller than the effective wavelength of the noise held. 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 quantum bits (qubits) are enough to protect one bit quantum information, and approximately L + 1/2 ln(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. [S1050-2947(98)08410-8]. [References: 33]
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