Quadratic programming over orthogonal matrices encompasses a broad class of hard optimization problems that do not have an efficient quantum representation. Such problems are instances of the little noncommutative Grothendieck problem (LNCG), a generalization of binary quadratic programs to continuous, noncommutative variables. In this work, we establish a natural embedding for this class of LNCG problems onto a fermionic Hamiltonian, thereby enabling the study of this classical problem with the tools of quantum information. This embedding is accomplished by a new representation of orthogonal matrices as fermionic quantum states, which we achieve through the well-known double covering of the orthogonal group. Correspondingly, the embedded LNCG Hamiltonian is a two-body fermion model. Determining extremal states of this Hamiltonian provides an outer approximation to the original problem, a quantum analogue to classical semidefinite relaxations. In particular, when optimizing over the \emph{special} orthogonal group our quantum relaxation obeys additional, powerful constraints based on the convex hull of rotation matrices. The classical size of this convex-hull representation is exponential in matrix dimension, whereas our quantum representation requires only a linear number of qubits. Finally, to project the relaxed solution back into the feasible space, we propose rounding procedures which return orthogonal matrices from appropriate measurements of the quantum state. Through numerical experiments we provide evidence that this rounded quantum relaxation can produce high-quality approximations.
Nonequilibrium Effects on Quantum Correlations: Discord, Mutual Information and Entanglement of a Two-Fermionic System in Bosonic and Fermionic Environments
Fermionic Hamiltonian engineering with local control
Hamburg University of Technology（汉堡工业大学）；Institute for Quantum Inspired and Quantum Optimization（量子启发与量子优化研究所）；Zentrum für Optische Quantentechnologien（光学量子技术中心）；Institut für Quantenphysik, Universität Hamburg（汉堡大学量子物理研究所）；The Hamburg Centre for Ultrafast Imaging（汉堡超快成像中心）
Fermionic non-Gaussianity via Bell sampling: monotones and efficient quantum algorithms
Technical University of Munich（慕尼黑工业大学）；TUM School of Natural Sciences, Physics Department（自然科学与工程学院物理系）；Munich Center for Quantum Science and Technology (MCQST)（慕尼黑量子科学与技术中心）
