The tensor product representation of quantum states leads to a promising variational approach to study quantum phase and quantum phase transitions, especially topological ordered phases which are impossible to handle with conventional methods due to their long range entanglement. However, an important issue arises when we use tensor product states (TPS) as variational states to find the ground state of a Hamiltonian: can arbitrary variations in the tensors that represent ground state of a Hamiltonian be induced by local perturbations to the Hamiltonian? Starting from a tensor product state which is the exact ground state of a Hamiltonian with $\mathbb{Z}_2$ topological order, we show that, surprisingly, not all variations of the tensors correspond to the variation of the ground state caused by local perturbations of the Hamiltonian. Even in the absence of any symmetry requirement of the perturbed Hamiltonian, one necessary condition for the variations of the tensors to be physical is that they respect certain $\mathbb{Z}_2$ symmetry. We support this claim by calculating explicitly the change in topological entanglement entropy with different variations in the tensors. This finding will provide important guidance to numerical variational study of topological phase and phase transitions. It is also a crucial step in using TPS to study universal properties of a quantum phase and its topological order.
Long-range entanglement is necessary for a topological storage of quantum information
在不利用母哈密顿量的性质或拓扑量子场论的形式的情况下，导出了纠缠熵与拓扑有序态个数之间的一般不等式.给定一个量子态$\ketψ$，我们得到了与$\ketψ$局部不可区分的不同态的数目的上界.上界仅由某些局域子系统的纠缠熵决定。作为一个例子，我们证明了$\log N \leq 2γ$的一个大类的拓扑有序系统的环面，其中$N$是拓扑保护态的数量和$γ$是恒定的纠缠熵的子校正项。我们讨论的应用程序的量子多体系统，没有任何低能量拓扑量子场论的描述，以及一般的量子纠错码的权衡界限。
Evidence for orthorhombic distortions in the ordered state of ZnCr$_2$O$_4$: A magnetic resonance study
我们提出了一个详细的电子自旋共振研究的低能量动力学和磁化在有序相的磁阻挫尖晶石ZnCr $_2$ O $_4$。我们观察到几个共振模式对应于不同的结构域，并发现域的数量可以很容易地减少场冷却的样品通过过渡点。为了描述所观察到的反铁磁共振光谱，有必要考虑到正交晶格畸变除了早期报道的四角畸变，这两个出现在反铁磁相变。
Modified Tucker Decomposition for Tensor Network and Fast Linearized Tensor Renormalization Group Algorithm for Two-Dimensional Quantum Spin Lattice Systems
