26,680 research outputs found
A size of ~1 AU for the radio source Sgr A* at the centre of the Milky Way
Although it is widely accepted that most galaxies have supermassive black
holes (SMBHs) at their centers^{1-3}, concrete proof has proved elusive.
Sagittarius A* (Sgr A*)^4, an extremely compact radio source at the center of
our Galaxy, is the best candidate for proof^{5-7}, because it is the closest.
Previous Very Long Baseline Interferometry (VLBI) observations (at 7mm) have
detected that Sgr A* is ~2 astronomical unit (AU) in size^8, but this is still
larger than the "shadow" (a remarkably dim inner region encircled by a bright
ring) arising from general relativistic effects near the event horizon^9.
Moreover, the measured size is wavelength dependent^{10}. Here we report a
radio image of Sgr A* at a wavelength of 3.5mm, demonstrating that its size is
\~1 AU. When combined with the lower limit on its mass^{11}, the lower limit on
the mass density is 6.5x10^{21} Msun pc^{-3}, which provides the most stringent
evidence to date that Sgr A* is an SMBH. The power-law relationship between
wavelength and intrinsic size (The size is proportional to wavelength^{1.09}),
explicitly rules out explanations other than those emission models with
stratified structure, which predict a smaller emitting region observed at a
shorter radio wavelength.Comment: 18 pages, 4 figure
Violations of local realism by two entangled quNits
Results obtained in two recent papers, \cite{Kaszlikowski} and \cite{Durt},
seem to indicate that the nonlocal character of the correlations between the
outcomes of measurements performed on entangled systems separated in space is
not robust in the presence of noise. This is surprising, since entanglement
itself is robust. Here we revisit this problem and argue that the class of
gedanken-experiments considered in \cite{Kaszlikowski} and \cite{Durt} is too
restrictive. By considering a more general class, involving sequences of
measurements, we prove that the nonlocal correlations are in fact robust.Comment: Reference added, 3 pages, accepted for publication in J. Phys. A:
Math. and Genera
Higher Security Thresholds for Quantum Key Distribution by Improved Analysis of Dark Counts
We discuss the potential of quantum key distribution (QKD) for long distance
communication by proposing a new analysis of the errors caused by dark counts.
We give sufficient conditions for a considerable improvement of the key
generation rates and the security thresholds of well-known QKD protocols such
as Bennett-Brassard 1984, Phoenix-Barnett-Chefles 2000, and the six-state
protocol. This analysis is applicable to other QKD protocols like Bennett 1992.
We examine two scenarios: a sender using a perfect single-photon source and a
sender using a Poissonian source.Comment: 6 pages, 2 figures, v2: We obtained better results by using reverse
reconciliation as suggested by Nicolas Gisi
WARNING: Physics Envy May Be Hazardous To Your Wealth!
The quantitative aspirations of economists and financial analysts have for
many years been based on the belief that it should be possible to build models
of economic systems - and financial markets in particular - that are as
predictive as those in physics. While this perspective has led to a number of
important breakthroughs in economics, "physics envy" has also created a false
sense of mathematical precision in some cases. We speculate on the origins of
physics envy, and then describe an alternate perspective of economic behavior
based on a new taxonomy of uncertainty. We illustrate the relevance of this
taxonomy with two concrete examples: the classical harmonic oscillator with
some new twists that make physics look more like economics, and a quantitative
equity market-neutral strategy. We conclude by offering a new interpretation of
tail events, proposing an "uncertainty checklist" with which our taxonomy can
be implemented, and considering the role that quants played in the current
financial crisis.Comment: v3 adds 2 reference
Security Analysis of an Untrusted Source for Quantum Key Distribution: Passive Approach
We present a passive approach to the security analysis of quantum key
distribution (QKD) with an untrusted source. A complete proof of its
unconditional security is also presented. This scheme has significant
advantages in real-life implementations as it does not require fast optical
switching or a quantum random number generator. The essential idea is to use a
beam splitter to split each input pulse. We show that we can characterize the
source using a cross-estimate technique without active routing of each pulse.
We have derived analytical expressions for the passive estimation scheme.
Moreover, using simulations, we have considered four real-life imperfections:
Additional loss introduced by the "plug & play" structure, inefficiency of the
intensity monitor, noise of the intensity monitor, and statistical fluctuation
introduced by finite data size. Our simulation results show that the passive
estimate of an untrusted source remains useful in practice, despite these four
imperfections. Also, we have performed preliminary experiments, confirming the
utility of our proposal in real-life applications. Our proposal makes it
possible to implement the "plug & play" QKD with the security guaranteed, while
keeping the implementation practical.Comment: 35 pages, 19 figures. Published Versio
Superconducting Analogues of Quantum Optical Phenomena: Macroscopic Quantum Superpositions and Squeezing in a SQUID Ring
In this paper we explore the quantum behaviour of a SQUID ring which has a
significant Josephson coupling energy. We show that that the eigenfunctions of
the Hamiltonian for the ring can be used to create macroscopic quantum
superposition states of the ring. We also show that the ring potential may be
utilised to squeeze coherent states. With the SQUID ring as a strong contender
as a device for manipulating quantum information, such properties may be of
great utility in the future. However, as with all candidate systems for quantum
technologies, decoherence is a fundamental problem. In this paper we apply an
open systems approach to model the effect of coupling a quantum mechanical
SQUID ring to a thermal bath. We use this model to demonstrate the manner in
which decoherence affects the quantum states of the ring.Comment: 9 pages, 10 figures, To be submitted to Phys. Rev. A. (changes for
referee's and editior's comments - replaced to try to get PDF working
Energy Down Conversion between Classical Electromagnetic Fields via a Quantum Mechanical SQUID Ring
We consider the interaction of a quantum mechanical SQUID ring with a
classical resonator (a parallel tank circuit). In our model we assume that
the evolution of the ring maintains its quantum mechanical nature, even though
the circuit to which it is coupled is treated classically. We show that when
the SQUID ring is driven by a classical monochromatic microwave source, energy
can be transferred between this input and the tank circuit, even when the
frequency ratio between them is very large. Essentially, these calculations
deal with the coupling between a single macroscopic quantum object (the SQUID
ring) and a classical circuit measurement device where due account is taken of
the non-perturbative behaviour of the ring and the concomitant non-linear
interaction of the ring with this device.Comment: 7 pages, 6 figure
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