2008/01/14 by D. B. Uskov, D. Uskov, A. Rau +1 · 29 citations
Computer Science · Mathematics · Physics and Astronomy · #Bloch sphere #Combinatorics #Hamiltonian (control theory) #Laser-Matter Interactions and Applications #Manifold (fluid mechanics) #Mathematical physics #Mathematics #Physics #Quantum #Quantum Information and Cryptography #Quantum computer #Quantum mechanics #Quantum optics and atomic interactions #Qubit #Spins #Unit sphere #quant-ph
paper · pdf · doi:10.1103/physreva.78.022331
published in Physical Review A 78(2) (American Physical Society) · 9 pages
arxiv created 2008/01/14 · openalex publication_date 2008/08/21 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
A two-sphere (``Bloch'' or ``Poincare'') is familiar for describing the dynamics of a spin-1∕2 particle or light polarization. Analogous objects are derived for unitary groups larger than SU(2) through an iterative procedure that constructs evolution operators for higher-dimensional SU(N) in terms of lower-dimensional ones. We focus, in particular, on the SU(4) of two qubits which describes all possible logic gates in quantum computation and entangled states in quantum-information sciences. For a general Hamiltonian of SU(4) with 15 parameters, and for Hamiltonians of its various subgroups so that fewer parameters suffice, we derive Bloch-like rotation of unit vectors analogous to the one familiar for a single spin in a magnetic field. The unitary evolution of a quantal spin pair is thereby expressed as rotations of real, many-dimensional vectors. Correspondingly, the manifolds involved are Bloch two-spheres along with higher dimensional manifolds such as a four-sphere for the SO(5) subgroup and an eight-dimensional Grassmannian manifold for the general SU(4). The latter may also be viewed as two, mutually orthogonal, real six-dimensional unit vectors moving on a five-sphere with an additional phase constraint. This geometrical picture for two spins provides the extension and generalization of the Bloch sphere that has proved invaluable for the understanding of the dynamics of a single spin.