Toward fault-tolerant quantum computation without concatenation

It is known that quantum error correction via concatenated codes can be clone with exponentially small failure rate if the error rate for physical qubits is below a certain accuracy threshold. Other, unconcatenated codes with their own attractive features such as improved accuracy threshold, local operations, have also been studied. By iteratively distilling a certain two-qubit entangled state it is shown how to perform an encoded Toffoli gate, important for universal computation, on codes whose C-NOT operation is bitwise. For certain codes, over a very large range of block sizes, this technique requires at most one concatenation.