TY - JOUR T1 - Joint product numerical range and geometry of reduced density matrices Y1 - 2016 A1 - Jianxin Chen A1 - Cheng Guo A1 - Zhengfeng Ji A1 - Yiu-Tung Poon A1 - Nengkun Yu A1 - Bei Zeng A1 - Jie Zhou AB - The reduced density matrices of a many-body quantum system form a convex set, whose three-dimensional projection Θ is convex in R3. The boundary ∂Θ of Θ may exhibit nontrivial geometry, in particular ruled surfaces. Two physical mechanisms are known for the origins of ruled surfaces: symmetry breaking and gapless. In this work, we study the emergence of ruled surfaces for systems with local Hamiltonians in infinite spatial dimension, where the reduced density matrices are known to be separable as a consequence of the quantum de Finetti's theorem. This allows us to identify the reduced density matrix geometry with joint product numerical range Π of the Hamiltonian interaction terms. We focus on the case where the interaction terms have certain structures, such that ruled surface emerge naturally when taking a convex hull of Π. We show that, a ruled surface on ∂Θ sitting in Π has a gapless origin, otherwise it has a symmetry breaking origin. As an example, we demonstrate that a famous ruled surface, known as the oloid, is a possible shape of Θ, with two boundary pieces of symmetry breaking origin separated by two gapless lines. UR - http://arxiv.org/abs/1606.07422 ER - TY - JOUR T1 - Discontinuity of Maximum Entropy Inference and Quantum Phase Transitions JF - New Journal of Physics Y1 - 2015 A1 - Jianxin Chen A1 - Zhengfeng Ji A1 - Chi-Kwong Li A1 - Yiu-Tung Poon A1 - Yi Shen A1 - Nengkun Yu A1 - Bei Zeng A1 - Duanlu Zhou AB - In this paper, we discuss the connection between two genuinely quantum phenomena --- the discontinuity of quantum maximum entropy inference and quantum phase transitions at zero temperature. It is shown that the discontinuity of the maximum entropy inference of local observable measurements signals the non-local type of transitions, where local density matrices of the ground state change smoothly at the transition point. We then propose to use the quantum conditional mutual information of the ground state as an indicator to detect the discontinuity and the non-local type of quantum phase transitions in the thermodynamic limit. VL - 17 U4 - 083019 UR - http://arxiv.org/abs/1406.5046v2 CP - 8 J1 - New J. Phys. U5 - 10.1088/1367-2630/17/8/083019 ER -