Subsystem fault tolerance with the Bacon-Shor code

We discuss how the presence of gauge subsystems in the Bacon-Shor code leads to remarkably simple and efficient methods for fault-tolerant error correction (FTEC). Most notably, FTEC does not require entangled ancillary states, and it can be implemented with nearest-neighbor two-qubit measurements. By using these methods, we prove a lower bound on the quantum accuracy threshold, 1.94×10-4 for adversarial stochastic noise, that improves previous lower bounds by nearly an order of magnitude. © 2007 The American Physical Society.