Logical coherence in two-dimensional compass codes

Two-dimensional compass codes are a family of quantum error-correcting codes that contain the Bacon-Shor codes, the X-Shor and Z-Shor codes, and the rotated surface codes. Previous numerical results suggest that the surface code has a constant accuracy and coherence threshold under uniform coherent rotation. However, having analytical proof supporting a constant threshold is still an open problem. It is analytically proven that the toric code can exponentially suppress logical coherence in the code distance L. However, the current analytical lower bound on the threshold for the rotation angle θ is |sin(θ)|<1/L, which linearly vanishes in L instead of being constant. We show that this lower bound is achievable by the Z-Shor code, which does not have a threshold under stochastic noise. Compass codes provide a promising direction to improve on the previous bounds. We analytically determine thresholds for two new compass code families that provide upper and lower bounds to the rotated surface code's numerically established infidelity threshold. Furthermore, using a Majorana mode-based simulator, we use random families of compass codes to smoothly interpolate between the Z-Shor codes and the X-Shor codes.

Duke Scholars

Author Kenneth R Brown Electrical and Computer Engineering