20,959 research outputs found
Quantum Levy flights and multifractality of dipolar excitations in a random system
We consider dipolar excitations propagating via dipole-induced exchange among
immobile molecules randomly spaced in a lattice. The character of the
propagation is determined by long-range hops (Levy flights). We analyze the
eigen-energy spectra and the multifractal structure of the wavefunctions. In 1D
and 2D all states are localized, although in 2D the localization length can be
extremely large leading to an effective localization-delocalization crossover
in realistic systems. In 3D all eigenstates are extended but not always
ergodic, and we identify the energy intervals of ergodic and non-ergodic
states. The reduction of the lattice filling induces an ergodic to non-ergodic
transition, and the excitations are mostly non-ergodic at low filling.Comment: 5 pages, 6 figure
Quantum phases of interacting phonons in ion traps
The vibrations of a chain of trapped ions can be considered, under suitable
experimental conditions, as an ensemble of interacting phonons, whose quantum
dynamics is governed by a Bose--Hubbard Hamiltonian. In this work we study the
quantum phases which appear in this system, and show that thermodynamical
properties, such as critical parameters and critical exponents, can be measured
in experiments with a limited number of ions. Besides that, interacting phonons
in trapped ions offer us the possibility to access regimes which are difficult
to study with ultracold bosons in optical lattices, like models with attractive
or site--dependent phonon-phonon interactions.Comment: 10 page
Disorderless quasi-localization of polar gases in one-dimensional lattices
One-dimensional polar gases in deep optical lattices present a severely
constrained dynamics due to the interplay between dipolar interactions, energy
conservation, and finite bandwidth. The appearance of dynamically-bound
nearest-neighbor dimers enhances the role of the dipolar tail,
resulting, in the absence of external disorder, in quasi-localization via dimer
clustering for very low densities and moderate dipole strengths. Furthermore,
even weak dipoles allow for the formation of self-bound superfluid lattice
droplets with a finite doping of mobile, but confined, holons. Our results,
which can be extrapolated to other power-law interactions, are directly
relevant for current and future lattice experiments with magnetic atoms and
polar molecules.Comment: 5 + 2 Page
Bosonization and entanglement spectrum for one-dimensional polar bosons on disordered lattices
The extended Bose-Hubbard model subjected to a disordered potential is
predicted to display a rich phase diagram. In the case of uniform random
disorder one finds two insulating quantum phases -- the Mott-insulator and the
Haldane insulator -- in addition to a superfluid and a Bose glass phase. In the
case of a quasiperiodic potential further phases are found, eg the
incommensurate density wave, adiabatically connected to the Haldane insulator.
For the case of weak random disorder we determine the phase boundaries using a
perturbative bosonization approach. We then calculate the entanglement spectrum
for both types of disorder, showing that it provides a good indication of the
various phases.Comment: Submitted to NJ
Universal Quantum Degeneracy Point for Superconducting Qubits
The quantum degeneracy point approach [D. Vion et al., Science 296, 886
(2002)] effectively protects superconducting qubits from low-frequency noise
that couples with the qubits as transverse noise. However, low-frequency noise
in superconducting qubits can originate from various mechanisms and can couple
with the qubits either as transverse or as longitudinal noise. Here, we present
a quantum circuit containing a universal quantum degeneracy point that protects
an encoded qubit from arbitrary low-frequency noise. We further show that
universal quantum logic gates can be performed on the encoded qubit with high
gate fidelity. The proposed scheme is robust against small parameter spreads
due to fabrication errors in the superconducting qubits.Comment: 7 pages, 4 figure
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