3,824 research outputs found

### Effects of non-local initial conditions in the Quantum Walk on the line

We report an enhancement of the decay rate of the survival probability when
non-local initial conditions in position space are considered in the Quantum
Walk on the line. It is shown how this interference effect can be understood
analytically by using previously derived results. Within a restricted position
subspace, the enhanced decay is correlated with a maximum asymptotic
entanglement level while the normal decay rate corresponds to initial relative
phases associated to a minimum entanglement level.Comment: 5 pages, 1 figure, Elsevier style, to appear in Physica

### Quantum walk on the line: entanglement and non-local initial conditions

The conditional shift in the evolution operator of a quantum walk generates
entanglement between the coin and position degrees of freedom. This
entanglement can be quantified by the von Neumann entropy of the reduced
density operator (entropy of entanglement). In the long time limit, it
converges to a well defined value which depends on the initial state. Exact
expressions for the asymptotic (long-time) entanglement are obtained for (i)
localized initial conditions and (ii) initial conditions in the position
subspace spanned by the +1 and -1 position eigenstates.Comment: A few mistakes where corrected. One of them leads to a factor of 2 in
eq. (49), the other results remain unchanged. In this version, several
figures where replaced by color version

### Pion Generalized Parton Distributions within a fully covariant constituent quark model

We extend the investigation of the Generalized Parton Distribution for a
charged pion within a fully covariant constituent quark model, in two respects:
(i) calculating the tensor distribution and (ii) adding the treatment of the
evolution, needed for achieving a meaningful comparison with both the
experimental parton distribution and the lattice evaluation of the so-called
generalized form factors. Distinct features of our phenomenological covariant
quark model are: (i) a 4D Ansatz for the pion Bethe-Salpeter amplitude, to be
used in the Mandelstam formula for matrix elements of the relevant current
operators, and (ii) only two parameters, namely a quark mass assumed to hold
$m_q=~220$ MeV and a free parameter fixed through the value of the pion decay
constant. The possibility of increasing the dynamical content of our covariant
constituent quark model is briefly discussed in the context of the Nakanishi
integral representation of the Bethe-Salpeter amplitude.Comment: Pages 20, figure 11 and table 8. Minor changes. To be published in
EPJ

### Generalized Quantum Walk in Momentum Space

We consider a new model of quantum walk on a one-dimensional momentum space
that includes both discrete jumps and continuous drift. Its time evolution has
two stages; a Markov diffusion followed by localized dynamics. As in the well
known quantum kicked rotor, this model can be mapped into a localized
one-dimensional Anderson model. For exceptional (rational) values of its scale
parameter, the system exhibits resonant behavior and reduce to the usual
discrete time quantum walk on the line.Comment: 11 pages, 5 figure

### Quantum random walk on the line as a markovian process

We analyze in detail the discrete--time quantum walk on the line by
separating the quantum evolution equation into Markovian and interference
terms. As a result of this separation, it is possible to show analytically that
the quadratic increase in the variance of the quantum walker's position with
time is a direct consequence of the coherence of the quantum evolution. If the
evolution is decoherent, as in the classical case, the variance is shown to
increase linearly with time, as expected. Furthermore we show that this system
has an evolution operator analogous to that of a resonant quantum kicked rotor.
As this rotator may be described through a quantum computational algorithm, one
may employ this algorithm to describe the time evolution of the quantum walker.Comment: few typos corrected, 13 pages, 2 figures, to appear in Physica

### Hydrogen mean force and anharmonicity in polycrystalline and amorphous ice

The hydrogen mean force from experimental neutron Compton profiles is derived
using deep inelastic neutron scattering on amorphous and polycrystalline ice.
The formalism of mean force is extended to probe its sensitivity to
anharmonicity in the hydrogen-nucleus effective potential. The shape of the
mean force for amorphous and polycrystalline ice is primarily determined by the
anisotropy of the underlying quasi-harmonic effective potential. The data from
amorphous ice show an additional curvature reflecting the more pronounced
anharmonicity of the effective potential with respect to that of ice Ih.Comment: 12 pages, 7 figures, original researc

### Decoherence in the quantum walk on the line

We investigate the quantum walk on the line when decoherences are introduced
either through simultaneous measurements of the chirality and particle
position, or as a result of broken links. Both mechanisms drive the system to a
classical diffusive behavior. In the case of measurements, we show that the
diffusion coefficient is proportional to the variance of the initially
localized quantum random walker just before the first measurement. When links
between neighboring sites are randomly broken with probability $p$ per unit
time, the evolution becomes decoherent after a characteristic time that scales
as $1/p$. The fact that the quadratic increase of the variance is eventually
lost even for very small frequencies of disrupting events, suggests that the
implementation of a quantum walk on a real physical system may be severely
limited by thermal noise and lattice imperfections.Comment: Elsevier style, 18 pages. New enhanced version with more material:
new title, a new section was added and the discussion was updated; references
added; submitted to Physica

### Driving the resonant quantum kicked rotor via extended initial conditions

We study the resonances of the quantum kicked rotor subjected to an extended
initial distribution. For the primary resonances we obtain the dispersion
relation for the map of this system. We find an analytical dependence of the
statistical moments on the shape of the initial distribution. For the secondary
resonances we obtain numerically a similar dependence. This allows us to devise
an extended initial condition which produces an average angular momentum
pointing in a preset direction which increases with time with a preset ratio.Comment: 6 pages, 5 figures, send to EPJ

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