We present a full density-matrix simulation of the quantum memory and computing performance of the distance-3 logical qubit Surface-17, following a recently proposed quantum circuit and using experimental error parameters for transmon qubits in a planar circuit QED architecture. We use this simulation to optimize components of the QEC scheme (e.g., trading off stabilizer measurement infidelity for reduced cycle time) and to investigate the benefits of feedback harnessing the fundamental asymmetry of relaxation-dominated error in the constituent transmons. A lower-order approximate calculation extends these predictions to the distance-$5$ Surface-49. These results clearly indicate error rates below the fault-tolerance threshold of surface code, and the potential for Surface-17 to perform beyond the break-even point of quantum memory. At state-of-the-art qubit relaxation times and readout speeds, Surface-49 could surpass the break-even point of computation.
Reduced and projected two-particle entanglement at finite temperatures
我们提出了一个在有限温度下介观导体中两粒子纠缠产生和检测的理论。与零温度极限相反，从发射的多体态投射出的密度矩阵的纠缠不同于约化的密度矩阵的纠缠，可以通过电流相关测量检测到。我们表明，在非常一般的条件下，减少纠缠构成了一个下界的投影纠缠。将该理论应用于最近的实验[Neder et al，Nature 448 333（2007）]，在费米Hanbury Brown Twiss两粒子干涉仪上，我们发现，尽管实验中产生了可观的纠缠，但可检测的纠缠接近于零。
Variationally optimizing infinite projected entangled-pair states at large bond dimensions: A split corner transfer matrix renormalization group approach
Quantum phase diagram of spin-$1$ $J_1-J_2$ Heisenberg model on the square lattice: an infinite projected entangled-pair state and density matrix renormalization group study
Matrix product operator algebras I: representations of weak Hopf algebras and projected entangled pair states
University of Vienna（维也纳大学）；Universidad Complutense de Madrid（马德里康普顿斯大学）；Instituto de Ciencias Matemáticas（数学科学研究所）；Max-Planck-Institute of Quantum Optics（马克斯·普朗克量子光学研究所）；Munich Center for Quantum Science and Technology（慕尼黑量子科学与技术中心）
