Threshold Estimate for Fault Tolerant Quantum Computation

I make a rough estimate of the accuracy threshold for fault tolerant quantum computing with concatenated codes. First I consider only gate errors and use the depolarizing channel error model. I will follow P.Shor (quant-ph/9505011) for fault tolerant error correction (FTEC) and the fault tolerant implementation of elementary operations on states encoded by the 7-qubit code. A simple computer simulation suggests a threshold for gate errors of the order \epsilon \approx 10^{-3} or better. I also give a simple argument that the threshold for memory errors is about 10 times smaller, thus \epsilon \approx 10^{-4}.