Fast fault-tolerant filtering of quantum codewords

Abstract

The stabilization of a quantum computer by repeated error correction can be reduced almost entirely to repeated preparation of blocks of qubits in quantum codeword states. These are multi-particle entangled states with a high degree of symmetry. The required accuracy can be achieved by measuring parity checks, using imperfect apparatus, and rejecting states which fail them. This filtering process is considered for t-error-correcting codes with t>1. It is shown how to exploit the structure of the codeword and the check matrix, so that the filter is reduced to a minimal form where each parity check need only be measured once, not > t times by the (noisy) verification apparatus. This both raises the noise threshold and also reduces the physical size of the computer. A method based on latin rectangles is proposed, which enables the most parallel version of a logic gate network to be found, for a class of networks including those used in verification. These insights allowed the noise threshold to be increased by an order of magnitude.