34,359 research outputs found
Quantum interference-induced stability of repulsively bound pairs of excitations
We study the dynamics of two types of pairs of excitations which are bound
despite their strong repulsive interaction. One corresponds to doubly occupied
sites in one-dimensional Bose-Hubbard systems, the so-called doublons. The
other is pairs of neighboring excited spins in anisotropic Heisenberg spin-1/2
chains. We investigate the possibility of decay of the bound pairs due to
resonant scattering by a defect or due to collisions of the pairs. We find that
the amplitudes of the corresponding transitions are very small. This is a
result of destructive quantum interference and explains the stability of the
bound pairs.Comment: 12 pages, 3 figure
Revivals, classical periodicity, and zitterbewegung of electron currents in monolayer graphene
Revivals of electric current in graphene in the presence of an external
magnetic field are described. It is shown that when the electrons are prepared
in the form of wave packets assuming a Gaussian population of only positive (or
negative) energy Landau levels, the presence of the magnetic field induce
revivals of the electron currents, besides the classical cyclotron motion. When
the population comprises both positive and negative energy Landau levels,
revivals of the electric current manifest simultaneously with zitterbewegung
and the classical cyclotron motion. We relate the temporal scales of these
three effects and discuss to what extent these results hold for real graphene
samples
Antiresonance and interaction-induced localization in spin and qubit chains with defects
We study a spin chain with an anisotropic XXZ coupling in an external field.
Such a chain models several proposed types of a quantum computer. The chain
contains a defect with a different on-site energy. The interaction between
excitations is shown to lead to two-excitation states localized next to the
defect. In a resonant situation scattering of excitations on each other might
cause decay of an excitation localized on the defect. We find that destructive
quantum interference suppresses this decay. Numerical results confirm the
analytical predictions.Comment: Updated versio
Transforming triangulations on non planar-surfaces
We consider whether any two triangulations of a polygon or a point set on a
non-planar surface with a given metric can be transformed into each other by a
sequence of edge flips. The answer is negative in general with some remarkable
exceptions, such as polygons on the cylinder, and on the flat torus, and
certain configurations of points on the cylinder.Comment: 19 pages, 17 figures. This version has been accepted in the SIAM
Journal on Discrete Mathematics. Keywords: Graph of triangulations,
triangulations on surfaces, triangulations of polygons, edge fli
Manipulation of the dynamics of many-body systems via quantum control methods
We investigate how dynamical decoupling methods may be used to manipulate the
time evolution of quantum many-body systems. These methods consist of sequences
of external control operations designed to induce a desired dynamics. The
systems considered for the analysis are one-dimensional spin-1/2 models, which,
according to the parameters of the Hamiltonian, may be in the integrable or
non-integrable limits, and in the gapped or gapless phases. We show that an
appropriate control sequence may lead a chaotic chain to evolve as an
integrable chain and a system in the gapless phase to behave as a system in the
gapped phase. A key ingredient for the control schemes developed here is the
possibility to use, in the same sequence, different time intervals between
control operations.Comment: 10 pages, 3 figure
Loading of a Bose-Einstein condensate in the boson-accumulation regime
We study the optical loading of a trapped Bose-Einstein condensate by
spontaneous emission of atoms in excited electronic state in the
Boson-Accumulation Regime. We generalize the previous simplified analysis of
ref. [Phys. Rev. A 53, 2466 (1996)], to a 3D case in which more than one trap
level of the excited state trap is considered. By solving the corresponding
quantum many-body master equation, we demonstrate that also for this general
situation the photon reabsorption can help to increase the condensate fraction.
Such effect could be employed to realize a continuous atom laser, and to
overcome condensate losses.Comment: 7 pages, 5 eps figures, uses epl.st
Many-particle confinement by constructed disorder and quantum computing
Many-particle confinement (localization) is studied for a 1D system of
spinless fermions with nearest-neighbor hopping and interaction, or
equivalently, for an anisotropic Heisenberg spin-1/2 chain. This system is
frequently used to model quantum computers with perpetually coupled qubits. We
construct a bounded sequence of site energies that leads to strong
single-particle confinement of all states on individual sites. We show that
this sequence also leads to a confinement of all many-particle states in an
infinite system for a time that scales as a high power of the reciprocal
hopping integral. The confinement is achieved for strong interaction between
the particles while keeping the overall bandwidth of site energies
comparatively small. The results show viability of quantum computing with
time-independent qubit coupling.Comment: An invited paper for the topical issue of J. Opt. B on quantum
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