3,427 research outputs found
Five-dimensional null & time-like supersymmetric geometries
We show that there exist supersymmetric solutions of five-dimensional, pure,
Supergravity such that the norm of the supersymmetric Killing
vector, built out of the Killing spinor, is a real not-everywhere analytic
function such that all its derivatives vanish at a point where the Killing
vector field becomes null. The norm of the Killing vector field then is not an
analytic function on a neighborhood around this point. We explicitly construct
such solutions by using a multi-center Gibbons-Hawking base. Although many of
these solutions have infinite charges, we find explicit examples with finite
charges that asymptote to and discuss their physical
interpretation.Comment: 19 pages, 4 figures, open-vanishing conclusion changed due to
non-analiticity of the Killing vecto
Symmetry-Enhanced Performance of Dynamical Decoupling
We consider a system with general decoherence and a quadratic dynamical
decoupling sequence (QDD) for the coherence control of a qubit coupled to a
bath of spins. We investigate the influence of the geometry and of the initial
conditions of the bath on the performance of the sequence. The overall
performance is quantified by a distance norm . It is expected that
scales with , the total duration of the sequence, as , where and are the number of pulses of the outer
and of the inner sequence, respectively. We show both numerically and
analytically that the state of the bath can boost the performance of QDD under
certain conditions: The scaling of QDD for a given number of pulses can be
enhanced by a factor of 2 if the bath is prepared in a highly symmetric state
and if the system Hamiltonian is SU(2) invariant.Comment: 9 pages, 4 figures, published versio
Generalization of short coherent control pulses: extension to arbitrary rotations
We generalize the problem of the coherent control of small quantum systems to
the case where the quantum bit (qubit) is subject to a fully general rotation.
Following the ideas developed in Pasini et al (2008 Phys. Rev. A 77, 032315),
the systematic expansion in the shortness of the pulse is extended to the case
where the pulse acts on the qubit as a general rotation around an axis of
rotation varying in time. The leading and the next-leading corrections are
computed. For certain pulses we prove that the general rotation does not
improve on the simpler rotation with fixed axis.Comment: 6 pages, no figures; published versio
Frequency modulated pulses for quantum bits coupled to time-dependent baths
We consider the coherent control of a quantum bit by the use of short pulses
with finite duration \tau_p. By shaping the pulse, we perturbatively decouple
the dynamics of the bath from the dynamics of the quantum bit during the pulse.
Such shaped pulses provide single quantum bit gates robust against decoherence
which are useful for quantum information processing. We extend previous results
in two ways: (i) we treat frequency modulated pulses and (ii) we pass from
time-independent baths to analytically time-dependent baths. First and second
order solutions for \pi- and \pi/2-pulses are presented. They are useful in
experiments where amplitude modulation is difficult to realize.Comment: 10 pages, 6 figure
Efficient Coherent Control by Optimized Sequences of Pulses of Finite Duration
Reliable long-time storage of arbitrary quantum states is a key element for
quantum information processing. In order to dynamically decouple a spin or
quantum bit from a dephasing environment, we introduce an optimized sequence of
control pulses of finite durations \tau\pp and finite amplitudes. The
properties of this sequence of length stem from a mathematically rigorous
derivation. Corrections occur only in order and \tau\pp^3 without
mixed terms such as T^N\tau\pp or T^N\tau\pp^2. Based on existing
experiments, a concrete setup for the verification of the properties of the
advocated realistic sequence is proposed.Comment: 8 pages, 1 figur
Exact Results on Dynamical Decoupling by -Pulses in Quantum Information Processes
The aim of dynamical decoupling consists in the suppression of decoherence by
appropriate coherent control of a quantum register. Effectively, the
interaction with the environment is reduced. In particular, a sequence of
pulses is considered. Here we present exact results on the suppression of the
coupling of a quantum bit to its environment by optimized sequences of
pulses. The effect of various cutoffs of the spectral density of the
environment is investigated. As a result we show that the harder the cutoff is
the better an optimized pulse sequence can deal with it. For cutoffs which are
neither completely hard nor very soft we advocate iterated optimized sequences.Comment: 12 pages and 3 figure
DMRG Simulation of the SU(3) AFM Heisenberg Model
We analyze the antiferromagnetic Heisenberg chain by means of
the Density Matrix Renormalization Group (DMRG). The results confirm that the
model is critical and the computation of its central charge and the scaling
dimensions of the first excited states show that the underlying low energy
conformal field theory is the Wess-Zumino-Novikov-Witten
model.Comment: corrections and improvements adde
Anomalous behavior of control pulses in presence of noise with singular autocorrelation
We report on the anomalous behavior of control pulses for spins under
spin-spin relaxation and subject to classical noise with a singular
autocorrelation function. This behavior is not detected for noise with analytic
autocorrelation functions. The effect is manifest in the different scaling
behavior of the deviation of a real pulse to the ideal, instantaneous one.
While a standard pulse displays scaling , a
first-order refocusing pulse normally shows scaling . But in presence of cusps in the noise autocorrelation the
scaling occurs. Cusps in the autocorrelation
are characteristic for fast fluctuations in the noise with a spectral density
of Lorentzian shape. We prove that the anomalous exponent cannot be avoided; it
represents a fundamental limit. On the one hand, this redefines the strategies
one has to adopt to design refocusing pulses. On the other hand, the anomalous
exponent, if found in experiment, provides important information on the noise
properties.Comment: 11 pages, 8 figure
Bell inequality violation by entangled single photon states generated from a laser, a LED or a Halogen lamp
In single-particle or intraparticle entanglement, two degrees of freedom of a
single particle, e.g., momentum and polarization of a single photon, are
entangled. Single-particle entanglement (SPE) provides a source of non
classical correlations which can be exploited both in quantum communication
protocols and in experimental tests of noncontextuality based on the
Kochen-Specker theorem. Furthermore, SPE is robust under decoherence phenomena.
Here, we show that single-particle entangled states of single photons can be
produced from attenuated sources of light, even classical ones. To
experimentally certify the entanglement, we perform a Bell test, observing a
violation of the Clauser, Horne, Shimony and Holt (CHSH) inequality. On the one
hand, we show that this entanglement can be achieved even in a classical light
beam, provided that first-order coherence is maintained between the degrees of
freedom involved in the entanglement. On the other hand, we prove that filtered
and attenuated light sources provide a flux of independent SPE photons that,
from a statistical point of view, are indistinguishable from those generated by
a single photon source. This has important consequences, since it demonstrates
that cheap, compact, and low power entangled photon sources can be used for a
range of quantum technology applications
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