Search results for "Density matrix"
showing 10 items of 106 documents
Quantum kinetic theory for fermions in temporally varying backrounds
2008
We derive quantum kinetic equations for fermions in a homogeneous time-dependent background in presence of decohering collisions, by use of the Schwinger-Keldysh CTP-formalism. The quantum coherence (between particles and antiparticles) is found to arise from new spectral solutions for the dynamical 2-point correlation function in the mean field limit. The physical density matrix $\rho$ and its dynamics is shown to be necessarily dependent on the extrenous information on the system, and expressions that relate $\rho$ to fundamental coherence functions and fermionic particle and antiparticle numbers are derived. For an interacting system we demonstrate how smooth decoherence effects are indu…
Coherent quasiparticle approximation (cQPA) and nonlocal coherence
2010
We show that the dynamical Wigner functions for noninteracting fermions and bosons can have complex singularity structures with a number of new solutions accompanying the usual mass-shell dispersion relations. These new shell solutions are shown to encode the information of the quantum coherence between particles and antiparticles, left and right moving chiral states and/or between different flavour states. Analogously to the usual derivation of the Boltzmann equation, we impose this extended phase space structure on the full interacting theory. This extension of the quasiparticle approximation gives rise to a self-consistent equation of motion for a density matrix that combines the quantum…
Convergence of density-matrix expansions for nuclear interactions
2010
We extend density-matrix expansions in nuclei to higher orders in derivatives of densities and test their convergence properties. The expansions allow for converting the interaction energies characteristic to finite- and short-range nuclear effective forces into quasi-local density functionals. We also propose a new type of expansion that has excellent convergence properties when benchmarked against the binding energies obtained for the Gogny interaction.
Tau-spin correlations at the z-peak: aplanarities of the decay products
1991
8 páginas, 3 figuras, 1 tabla.-- CERN-TH-5932-90 ; FTUV-90-26.
Kinetic transport theory with quantum coherence
2008
We derive transport equations for fermions and bosons in spatially or temporally varying backgrounds with special symmetries, by use of the Schwinger-Keldysh formalism. In a noninteracting theory the coherence information is shown to be encoded in new singular shells for the 2-point function. Imposing this phase space structure to the interacting theory leads to a a self-consistent equation of motion for a physcial density matrix, including coherence and a well defined collision integral. The method is applied e.g. to demonstrate how an initially coherent out-of-equlibrium state approaches equlibrium through decoherence and thermalization.
Observation of meson central production in baryon exchange processes
1991
The central production of ρ0,f2 and ρ30 mesons is observed for the first time in processes which are originated by π+p reactions proceeding via baryon exchange mechanism. The data come from the CERN WA56 experiment designed to separate the baryon exchange reactions in π+p-collisions at 20 GeV/c. We report on the measured integral and differential cross sections and also give the density matrix elements of the meson resonances observed.
Witnessing non-Markovian effects of quantum processes through Hilbert-Schmidt speed
2020
Non-Markovian effects can speed up the dynamics of quantum systems while the limits of the evolution time can be derived by quantifiers of quantum statistical speed. We introduce a witness for characterizing the non-Markovianity of quantum evolutions through the Hilbert-Schmidt speed (HSS), which is a special type of quantum statistical speed. This witness has the advantage of not requiring diagonalization of evolved density matrix. Its sensitivity is investigated by considering several paradigmatic instances of open quantum systems, such as one qubit subject to phase-covariant noise and Pauli channel, two independent qubits locally interacting with leaky cavities, V-type and $\Lambda $-typ…
Landau-Zener problem in a three-level neutrino system with non-linear time dependence
2006
We consider the level-crossing problem in a three-level system with non-linearly time-varying Hamiltonian (time-dependence $t^{-3}$). We study the validity of the so-called independent crossing approximation in the Landau-Zener model by making comparison with results obtained numerically in density matrix approach. We also demonstrate the failure of the so-called "nearest zero" approximation of the Landau-Zener level-crossing probability integral.
Phase diagram of the quarter-filled extended Hubbard model on a two-leg ladder
2000
We investigate the ground-state phase diagram of the quarter-filled Hubbard ladder with nearest-neighbor Coulomb repulsion V using the Density Matrix Renormalization Group technique. The ground-state is homogeneous at small V, a ``checkerboard'' charge--ordered insulator at large V and not too small on-site Coulomb repulsion U, and is phase-separated for moderate or large V and small U. The zero-temperature transition between the homogeneous and the charge-ordered phase is found to be second order. In both the homogeneous and the charge-ordered phases the existence of a spin gap mainly depends on the ratio of interchain to intrachain hopping. In the second part of the paper, we construct an…
Analytical O(αs) corrections to the beam frame double-spin density matrix elements of e+e−→tt¯
2016
We provide analytical results for the $O({\ensuremath{\alpha}}_{s})$ corrections to the double-spin density matrix elements in the reaction ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}t\overline{t}$. These concern the elements $ll$, $lt$, $ln$, $tt$, $tn$, and $nn$ of the double-spin density matrix elements where $l$, $t$, $n$ stand for longitudinal, transverse and normal orientations with respect to the beam frame spanned by the electron and the top quark momentum.