2,701 research outputs found
Decoherence times of universal two-qubit gates in the presence of broad-band noise
The controlled generation of entangled states of two quantum bits is a
fundamental step toward the implementation of a quantum information processor.
In nano-devices this operation is counteracted by the solid-state environment,
characterized by a broadband and non-monotonic power spectrum, often 1/f at low
frequencies. For single-qubit gates, incoherent processes due to fluctuations
acting on different time scales result in peculiar short- and long-time
behavior. Markovian noise gives rise to exponential decay with relaxation and
decoherence times, T1 and T2, simply related to the symmetry of the
qubit-environment coupling Hamiltonian. Noise with the 1/f power spectrum at
low frequencies is instead responsible for defocusing processes and algebraic
short-time behavior. In this paper, we identify the relevant decoherence times
of an entangling operation due to the different decoherence channels
originating from solid-state noise. Entanglement is quantified by concurrence,
which we evaluate in an analytic form employing a multi-stage approach. The
'optimal' operating conditions of reduced sensitivity to noise sources are
identified. We apply this analysis to a superconducting \sqrt{i-SWAP} gate for
experimental noise spectra.Comment: 35 pages, 11 figure
Dark-bright gap solitons in coupled-mode one-dimensional saturable waveguide arrays
In the present work, we consider the dynamics of dark solitons as one mode of
a defocusing photorefractive lattice coupled with bright solitons as a second
mode of the lattice. Our investigation is motivated by an experiment which
illustrates that such coupled states can exist with both components in the
first gap of the linear band spectrum. This finding is further extended by the
examination of different possibilities from a theoretical perspective, such as
symbiotic ones where the bright component is supported by states of the dark
component in the first or second gap, or non-symbiotic ones where the bright
soliton is also a first-gap state coupled to a first or second gap state of the
dark component. While the obtained states are generally unstable, these
instabilities typically bear fairly small growth rates which enable their
observation for experimentally relevant propagation distances
Geometric stabilization of extended S=2 vortices in two-dimensional photonic lattices: theoretical analysis, numerical computation and experimental results
In this work, we focus our studies on the subject of nonlinear discrete
self-trapping of S=2 (doubly-charged) vortices in two-dimensional photonic
lattices, including theoretical analysis, numerical computation and
experimental demonstration. We revisit earlier findings about S=2 vortices with
a discrete model, and find that S=2 vortices extended over eight lattice sites
can indeed be stable (or only weakly unstable) under certain conditions, not
only for the cubic nonlinearity previously used, but also for a saturable
nonlinearity more relevant to our experiment with a biased photorefractive
nonlinear crystal. We then use the discrete analysis as a guide towards
numerically identifying stable (and unstable) vortex solutions in a more
realistic continuum model with a periodic potential. Finally, we present our
experimental observation of such geometrically extended S=2 vortex solitons in
optically induced lattices under both self-focusing and self-defocusing
nonlinearities, and show clearly that the S=2 vortex singularities are
preserved during nonlinear propagation
Modulational instability, solitons and beam propagation in spatially nonlocal nonlinear media
We present an overview of recent advances in the understanding of optical
beams in nonlinear media with a spatially nonlocal nonlinear response. We
discuss the impact of nonlocality on the modulational instability of plane
waves, the collapse of finite-size beams, and the formation and interaction of
spatial solitons.Comment: Review article, will be published in Journal of Optics B, special
issue on Optical Solitons, 6 figure
Feshbach Resonance Management of Bose-Einstein Condensates in Optical Lattices
We analyze gap solitons in trapped Bose-Einstein condensates (BECs) in
optical lattice potentials under Feshbach resonance management. Starting with
an averaged Gross-Pitaevsky (GP) equation with a periodic potential, we employ
an envelope wave approximation to derive coupled-mode equations describing the
slow BEC dynamics in the first spectral gap of the optical lattice. We
construct exact analytical formulas describing gap soliton solutions and
examine their spectral stability using the Chebyshev interpolation method. We
show that these gap solitons are unstable far from the threshold of local
bifurcation and that the instability results in the distortion of their shape.
We also predict the threshold of the power of gap solitons near the local
bifurcation limit.Comment: 8 pages, 4 figures (1 with six parts, 3 with two parts
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