@article{GelssKlusKnebeletal., author = {Gelß, Patrick and Klus, Stefan and Knebel, Sebastian and Shakibaei, Zarin and Pokutta, Sebastian}, title = {Low-Rank Tensor Decompositions of Quantum Circuits}, series = {Journal of Computational Physics}, journal = {Journal of Computational Physics}, abstract = {Quantum computing is arguably one of the most revolutionary and disruptive technologies of this century. Due to the ever-increasing number of potential applications as well as the continuing rise in complexity, the development, simulation, optimization, and physical realization of quantum circuits is of utmost importance for designing novel algorithms. We show how matrix product states (MPSs) and matrix product operators (MPOs) can be used to express certain quantum states, quantum gates, and entire quantum circuits as low-rank tensors. This enables the analysis and simulation of complex quantum circuits on classical computers and to gain insight into the underlying structure of the system. We present different examples to demonstrate the advantages of MPO formulations and show that they are more efficient than conventional techniques if the bond dimensions of the wave function representation can be kept small throughout the simulation.}, language = {en} } @article{DesignolleBesanconIommazzoetal., author = {Designolle, S{\´e}bastien and Besan{\c{c}}on, Mathieu and Iommazzo, Gabriele and Knebel, Sebastian and Gelß, Patrick and Pokutta, Sebastian}, title = {Improved Local Models and New Bell Inequalities Via Frank-Wolfe Algorithms}, series = {Physical Review Research}, volume = {5}, journal = {Physical Review Research}, doi = {10.1103/PhysRevResearch.5.043059}, pages = {043059}, abstract = {In Bell scenarios with two outcomes per party, we algorithmically consider the two sides of the membership problem for the local polytope: Constructing local models and deriving separating hyperplanes, that is, Bell inequalities. We take advantage of the recent developments in so-called Frank-Wolfe algorithms to significantly increase the convergence rate of existing methods. First, we study the threshold value for the nonlocality of two-qubit Werner states under projective measurements. Here, we improve on both the upper and lower bounds present in the literature. Importantly, our bounds are entirely analytical; moreover, they yield refined bounds on the value of the Grothendieck constant of order three: 1.4367⩽KG(3)⩽1.4546. Second, we demonstrate the efficiency of our approach in multipartite Bell scenarios, and present local models for all projective measurements with visibilities noticeably higher than the entanglement threshold. We make our entire code accessible as a julia library called BellPolytopes.jl.}, language = {en} } @article{DesignolleIommazzoBesanconetal., author = {Designolle, S{\´e}bastien and Iommazzo, Gabriele and Besan{\c{c}}on, Mathieu and Knebel, Sebastian and Gelß, Patrick and Pokutta, Sebastian}, title = {Improved local models and new Bell inequalities via Frank-Wolfe algorithms}, series = {Physical Review Research}, volume = {5}, journal = {Physical Review Research}, doi = {10.1103/PhysRevResearch.5.043059}, pages = {043059}, abstract = {In Bell scenarios with two outcomes per party, we algorithmically consider the two sides of the membership problem for the local polytope: Constructing local models and deriving separating hyperplanes, that is, Bell inequalities. We take advantage of the recent developments in so-called Frank-Wolfe algorithms to significantly increase the convergence rate of existing methods. First, we study the threshold value for the nonlocality of two-qubit Werner states under projective measurements. Here, we improve on both the upper and lower bounds present in the literature. Importantly, our bounds are entirely analytical; moreover, they yield refined bounds on the value of the Grothendieck constant of order three: 1.4367⩽KG(3)⩽1.4546. Second, we demonstrate the efficiency of our approach in multipartite Bell scenarios, and present local models for all projective measurements with visibilities noticeably higher than the entanglement threshold. We make our entire code accessible as a julia library called BellPolytopes.jl.}, language = {en} }