On the efficiency of nondegenerate quantum error correction codes for   Pauli channels
  We examine the efficiency of pure, nondegenerate quantum-error correction-codes for Pauli channels. Specifically, we investigate if correction of multiple errors in a block is more efficient than using a code that only corrects one error per block. Block coding with multiple-error correction cannot increase the efficiency when the qubit error-probability is below a certain value and the code size fixed. More surprisingly, existing multiple-error correction codes with a code length equal or less than 256 qubits have lower efficiency than the optimal single-error correcting codes for any value of the qubit error-probability. We also investigate how efficient various proposed nondegenerate single-error correcting codes are compared to the limit set by the code redundancy and by the necessary conditions for hypothetically existing nondegenerate codes. We find that existing codes are close to optimal. 
Gunnar Bjork, Jonas Almlof, and Isabel Sainz