12,488 research outputs found
Less than perfect quantum wavefunctions in momentum-space: How phi(p) senses disturbances in the force
We develop a systematic approach to determine the large |p| behavior of the
momentum-space wavefunction, phi(p), of a one-dimensional quantum system for
wich the position-space wavefunction, psi(x), has a discontinuous derivative at
any order. We find that if the k-th derivative of the potential energy function
has a discontinuity, there is a corresponding discontinuity in psi^{(k+2)}(x)
at the same point. This discontinuity leads directly to a power-law tail in the
momentum-space wavefunction proportional to 1/p^{k+3}. A number of familiar
pedagogical examples are examined in this context, leading to a general
derivation of the result.Comment: 22 pages, 2 figures. To appear in Am. J. Phy
Picard-Fuchs Equations and Special Geometry
We investigate the system of holomorphic differential identities implied by
special K\"ahlerian geometry of four-dimensional N=2 supergravity. For
superstring compactifications on \cy threefolds these identities are equivalent
to the Picard-Fuchs equations of algebraic geometry that are obeyed by the
periods of the holomorphic three-form. For one variable they reduce to linear
fourth-order equations which are characterized by classical -generators; we
find that the instanton corrections to the Yukawa couplings are directly
related to the non-vanishing of . We also show that the symplectic
structure of special geometry can be related to the fact that the Yukawa
couplings can be written as triple derivatives of some holomorphic function
. Moreover, we give the precise relationship of the Yukawa couplings of
special geometry with three-point functions in topological field theory.Comment: 43 page
A high bandwidth quantum repeater
We present a physical- and link-level design for the creation of entangled
pairs to be used in quantum repeater applications where one can control the
noise level of the initially distributed pairs. The system can tune
dynamically, trading initial fidelity for success probability, from high
fidelity pairs (F=0.98 or above) to moderate fidelity pairs. The same physical
resources that create the long-distance entanglement are used to implement the
local gates required for entanglement purification and swapping, creating a
homogeneous repeater architecture. Optimizing the noise properties of the
initially distributed pairs significantly improves the rate of generating
long-distance Bell pairs. Finally, we discuss the performance trade-off between
spatial and temporal resources.Comment: 5 page
DNA nanotweezers studied with a coarse-grained model of DNA
We introduce a coarse-grained rigid nucleotide model of DNA that reproduces
the basic thermodynamics of short strands: duplex hybridization,
single-stranded stacking and hairpin formation, and also captures the essential
structural properties of DNA: the helical pitch, persistence length and
torsional stiffness of double-stranded molecules, as well as the comparative
flexibility of unstacked single strands. We apply the model to calculate the
detailed free-energy landscape of one full cycle of DNA 'tweezers', a simple
machine driven by hybridization and strand displacement.Comment: 4 pages, 5 figure
Diffuse MeV Gamma-rays and Galactic 511 keV Line from Decaying WIMP Dark Matter
The origin of both the diffuse high-latitude MeV gamma-ray emission and the
511 keV line flux from the Galactic bulge are uncertain. Previous studies have
invoked dark matter physics to independently explain these observations, though
as yet none has been able to explain both of these emissions within the
well-motivated framework of Weakly-Interacting Massive Particles (WIMPs). Here
we use an unstable WIMP dark matter model to show that it is in fact possible
to simultaneously reconcile both of these observations, and in the process show
a remarkable coincidence: decaying dark matter with MeV mass splittings can
explain both observations if positrons and photons are produced with similar
branching fractions. We illustrate this idea with an unstable branon, which is
a standard WIMP dark matter candidate appearing in brane world models with
large extra dimensions. We show that because branons decay via three-body final
states, they are additionally unconstrained by searches for Galactic MeV
gamma-ray lines. As a result, such unstable long-lifetime dark matter particles
provide novel and distinct signatures that can be tested by future observations
of MeV gamma-rays.Comment: 19 pages, 4 figure
Hands-on Gravitational Wave Astronomy: Extracting astrophysical information from simulated signals
In this paper we introduce a hands-on activity in which introductory
astronomy students act as gravitational wave astronomers by extracting
information from simulated gravitational wave signals. The process mimics the
way true gravitational wave analysis will be handled by using plots of a pure
gravitational wave signal. The students directly measure the properties of the
simulated signal, and use these measurements to evaluate standard formulae for
astrophysical source parameters. An exercise based on the discussion in this
paper has been written and made publicly available online for use in
introductory laboratory courses.Comment: 5 pages, 4 figures; submitted to Am. J. Phy
Weak non-linearities and cluster states
We propose a scalable approach to building cluster states of matter qubits
using coherent states of light. Recent work on the subject relies on the use of
single photonic qubits in the measurement process. These schemes have a low
initial success probability and low detector efficiencies cause a serious
blowup in resources. In contrast, our approach uses continuous variables and
highly efficient measurements. We present a two-qubit scheme, with a simple
homodyne measurement system yielding an entangling operation with success
probability 1/2. Then we extend this to a three-qubit interaction, increasing
this probability to 3/4. We discuss the important issues of the overhead cost
and the time scaling, showing how these can be vastly improved with access to
this new probability range.Comment: 5 pages, to appear in Phys. Rev.
Metallization of Fluid Hydrogen
The electrical resistivity of liquid hydrogen has been measured at the high
dynamic pressures, densities and temperatures that can be achieved with a
reverberating shock wave. The resulting data are most naturally interpreted in
terms of a continuous transition from a semiconducting to a metallic, largely
diatomic fluid, the latter at 140 GPa, (ninefold compression) and 3000 K. While
the fluid at these conditions resembles common liquid metals by the scale of
its resistivity of 500 micro-ohm-cm, it differs by retaining a strong pairing
character, and the precise mechanism by which a metallic state might be
attained is still a matter of debate. Some evident possibilities include (i)
physics of a largely one-body character, such as a band-overlap transition,
(ii) physics of a strong-coupling or many-body character,such as a Mott-Hubbard
transition, and (iii) processes in which structural changes are paramount.Comment: 12 pages, RevTeX format. Figures available on request; send mail to:
[email protected] To appear: Philosophical Transaction of the Royal
Society
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