1,020 research outputs found
Numerical Studies of the two-leg Hubbard ladder
The Hubbard model on a two-leg ladder structure has been studied by a
combination of series expansions at T=0 and the density-matrix renormalization
group. We report results for the ground state energy and spin-gap
at half-filling, as well as dispersion curves for one and two-hole
excitations. For small both and show a dramatic drop near
, which becomes more gradual for larger . This
represents a crossover from a "band insulator" phase to a strongly correlated
spin liquid. The lowest-lying two-hole state rapidly becomes strongly bound as
increases, indicating the possibility that phase separation may
occur. The various features are collected in a "phase diagram" for the model.Comment: 10 figures, revte
A simple nearest-neighbor two-body Hamiltonian system for which the ground state is a universal resource for quantum computation
We present a simple quantum many-body system - a two-dimensional lattice of
qubits with a Hamiltonian composed of nearest-neighbor two-body interactions -
such that the ground state is a universal resource for quantum computation
using single-qubit measurements. This ground state approximates a cluster state
that is encoded into a larger number of physical qubits. The Hamiltonian we use
is motivated by the projected entangled pair states, which provide a
transparent mechanism to produce such approximate encoded cluster states on
square or other lattice structures (as well as a variety of other quantum
states) as the ground state. We show that the error in this approximation takes
the form of independent errors on bonds occurring with a fixed probability. The
energy gap of such a system, which in part determines its usefulness for
quantum computation, is shown to be independent of the size of the lattice. In
addition, we show that the scaling of this energy gap in terms of the coupling
constants of the Hamiltonian is directly determined by the lattice geometry. As
a result, the approximate encoded cluster state obtained on a hexagonal lattice
(a resource that is also universal for quantum computation) can be shown to
have a larger energy gap than one on a square lattice with an equivalent
Hamiltonian.Comment: 5 pages, 1 figure; v2 has a simplified lattice, an extended analysis
of errors, and some additional references; v3 published versio
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