5,161 research outputs found
Completeness of the classical 2D Ising model and universal quantum computation
We prove that the 2D Ising model is complete in the sense that the partition
function of any classical q-state spin model (on an arbitrary graph) can be
expressed as a special instance of the partition function of a 2D Ising model
with complex inhomogeneous couplings and external fields. In the case where the
original model is an Ising or Potts-type model, we find that the corresponding
2D square lattice requires only polynomially more spins w.r.t the original one,
and we give a constructive method to map such models to the 2D Ising model. For
more general models the overhead in system size may be exponential. The results
are established by connecting classical spin models with measurement-based
quantum computation and invoking the universality of the 2D cluster states.Comment: 4 pages, 1 figure. Minor change
Disorder-driven electronic localization and phase separation in superconducting Fe1+yTe0.5Se0.5 single crystals
We have investigated the influence of Fe-excess on the electrical transport
and magnetism of Fe1+yTe0.5Se0.5 (y=0.04 and 0.09) single crystals. Both
compositions exhibit resistively determined superconducting transitions (Tc)
with an onset temperature of about 15 K. From the width of the superconducting
transition and the magnitude of the lower critical field Hc1, it is inferred
that excess of Fe suppresses superconductivity. The linear and non-linear
responses of the ac-susceptibility show that the superconducting state for
these compositions is inhomogeneous. A possible origin of this phase separation
is a magnetic coupling between Fe-excess occupying interstitial sites in the
chalcogen planes and those in the Fe-square lattice. The temperature derivative
of the resistivity drho/dT in the temperature range Tc < T < Ta with Ta being
the temperature of a magnetic anomaly, changes from positive to negative with
increasing Fe. A log 1/T divergence of the resistivity above Tc in the sample
with higher amount of Fe suggests a disorder driven electronic localization.Comment: 7 page
Superconducting Microwave Cavity Made of Bulk MgB2
We report the successful manufacture and characterization of a microwave
resonant cylindrical cavity made of bulk MgB2 superconductor (Tc = 38.5 K),
which has been produced by the Reactive Liquid Mg Infiltration technique. The
quality factor of the cavity for the TE011 mode, resonating at 9.79 GHz, has
been measured as a function of the temperature. At T = 4.2 K, the unloaded
quality factor is 2.2x10^5; it remains of the order of 10^5 up to T ~ 30 K. We
discuss the potential performance improvements of microwave cavities built from
bulk MgB2 materials produced by reactive liquid Mg infiltration.Comment: 7 pages, 2 embedded figures, accepted for publication in Supercond.
Sci. Techno
Coherent control of trapped ions using off-resonant lasers
In this paper we develop a unified framework to study the coherent control of
trapped ions subject to state-dependent forces. Taking different limits in our
theory, we can reproduce two different designs of a two-qubit quantum gate
--the pushing gate [1] and the fast gates based on laser pulses from Ref.
[2]--, and propose a new design based on continuous laser beams. We demonstrate
how to simulate Ising Hamiltonians in a many ions setup, and how to create
highly entangled states and induce squeezing. Finally, in a detailed analysis
we identify the physical limits of this technique and study the dependence of
errors on the temperature. [1] J.I. Cirac, P. Zoller, Nature, 404, 579, 2000.
[2] J.J. Garcia-Ripoll, P. Zoller, J.I. Cirac, PRL 67, 062318, 200
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