23,566 research outputs found
Field-induced structure transformation in electrorheological solids
We have computed the local electric field in a body-centered tetragonal (BCT)
lattice of point dipoles via the Ewald-Kornfeld formulation, in an attempt to
examine the effects of a structure transformation on the local field strength.
For the ground state of an electrorheological solid of hard spheres, we
identified a novel structure transformation from the BCT to the face-centered
cubic (FCC) lattices by changing the uniaxial lattice constant c under the hard
sphere constraint. In contrast to the previous results, the local field
exhibits a non-monotonic transition from BCT to FCC. As c increases from the
BCT ground state, the local field initially decreases rapidly towards the
isotropic value at the body-centered cubic lattice, decreases further, reaching
a minimum value and increases, passing through the isotropic value again at an
intermediate lattice, reaches a maximum value and finally decreases to the FCC
value. An experimental realization of the structure transformation is
suggested. Moreover, the change in the local field can lead to a generalized
Clausius-Mossotti equation for the BCT lattices.Comment: Submitted to Phys. Rev.
Nonlinear ac response of anisotropic composites
When a suspension consisting of dielectric particles having nonlinear
characteristics is subjected to a sinusoidal (ac) field, the electrical
response will in general consist of ac fields at frequencies of the
higher-order harmonics. These ac responses will also be anisotropic. In this
work, a self-consistent formalism has been employed to compute the induced
dipole moment for suspensions in which the suspended particles have nonlinear
characteristics, in an attempt to investigate the anisotropy in the ac
response. The results showed that the harmonics of the induced dipole moment
and the local electric field are both increased as the anisotropy increases for
the longitudinal field case, while the harmonics are decreased as the
anisotropy increases for the transverse field case. These results are
qualitatively understood with the spectral representation. Thus, by measuring
the ac responses both parallel and perpendicular to the uniaxial anisotropic
axis of the field-induced structures, it is possible to perform a real-time
monitoring of the field-induced aggregation process.Comment: 14 pages and 4 eps figure
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
Effects of geometric anisotropy on local field distribution: Ewald-Kornfeld formulation
We have applied the Ewald-Kornfeld formulation to a tetragonal lattice of
point dipoles, in an attempt to examine the effects of geometric anisotropy on
the local field distribution. The various problems encountered in the
computation of the conditionally convergent summation of the near field are
addressed and the methods of overcoming them are discussed. The results show
that the geometric anisotropy has a significant impact on the local field
distribution. The change in the local field can lead to a generalized
Clausius-Mossotti equation for the anisotropic case.Comment: Accepted for publications, Journal of Physics: Condensed Matte
Insecurity of Quantum Secure Computations
It had been widely claimed that quantum mechanics can protect private
information during public decision in for example the so-called two-party
secure computation. If this were the case, quantum smart-cards could prevent
fake teller machines from learning the PIN (Personal Identification Number)
from the customers' input. Although such optimism has been challenged by the
recent surprising discovery of the insecurity of the so-called quantum bit
commitment, the security of quantum two-party computation itself remains
unaddressed. Here I answer this question directly by showing that all
``one-sided'' two-party computations (which allow only one of the two parties
to learn the result) are necessarily insecure. As corollaries to my results,
quantum one-way oblivious password identification and the so-called quantum
one-out-of-two oblivious transfer are impossible. I also construct a class of
functions that cannot be computed securely in any ``two-sided'' two-party
computation. Nevertheless, quantum cryptography remains useful in key
distribution and can still provide partial security in ``quantum money''
proposed by Wiesner.Comment: The discussion on the insecurity of even non-ideal protocols has been
greatly extended. Other technical points are also clarified. Version accepted
for publication in Phys. Rev.
Multi-Prover Commitments Against Non-Signaling Attacks
We reconsider the concept of multi-prover commitments, as introduced in the
late eighties in the seminal work by Ben-Or et al. As was recently shown by
Cr\'{e}peau et al., the security of known two-prover commitment schemes not
only relies on the explicit assumption that the provers cannot communicate, but
also depends on their information processing capabilities. For instance, there
exist schemes that are secure against classical provers but insecure if the
provers have quantum information processing capabilities, and there are schemes
that resist such quantum attacks but become insecure when considering general
so-called non-signaling provers, which are restricted solely by the requirement
that no communication takes place.
This poses the natural question whether there exists a two-prover commitment
scheme that is secure under the sole assumption that no communication takes
place; no such scheme is known.
In this work, we give strong evidence for a negative answer: we show that any
single-round two-prover commitment scheme can be broken by a non-signaling
attack. Our negative result is as bad as it can get: for any candidate scheme
that is (almost) perfectly hiding, there exists a strategy that allows the
dishonest provers to open a commitment to an arbitrary bit (almost) as
successfully as the honest provers can open an honestly prepared commitment,
i.e., with probability (almost) 1 in case of a perfectly sound scheme. In the
case of multi-round schemes, our impossibility result is restricted to
perfectly hiding schemes.
On the positive side, we show that the impossibility result can be
circumvented by considering three provers instead: there exists a three-prover
commitment scheme that is secure against arbitrary non-signaling attacks
Is Quantum Bit Commitment Really Possible?
We show that all proposed quantum bit commitment schemes are insecure because
the sender, Alice, can almost always cheat successfully by using an
Einstein-Podolsky-Rosen type of attack and delaying her measurement until she
opens her commitment.Comment: Major revisions to include a more extensive introduction and an
example of bit commitment. Overlap with independent work by Mayers
acknowledged. More recent works by Mayers, by Lo and Chau and by Lo are also
noted. Accepted for publication in Phys. Rev. Let
Spin-one ferromagnets with single-ion anisotropy in a perpendicular external field
In this paper, the conventional Holstein-Primakoff method is generalized with
the help of the characteristic angle transformation [Lei Zhou and Ruibao Tao,
J. Phys. A {\bf 27} 5599 (1994)] for the spin-one magnetic systems with
single-ion anisotropies. We find that the weakness of the conventional method
for such systems can be overcome by the new approach. Two models will be
discussed to illuminate the main idea, which are the ``easy-plane" and the
``easy-axis" spin-one ferromagnet, respectively. Comparisons show that the
current approach can give reasonable ground state properties for the magnetic
system with ``easy-plane" anisotropy though the conventional method never can,
and can give a better representation than the conventional one for the magnetic
system with ``easy-axis" anisotropy though the latter is usually believed to be
a good approximation in such case. Study of the easy-plane model shows that
there is a phase transition induced by the external field, and the
low-temperature specific heat may have a peak as the field reaches the critical
value.Comment: Using LaTex. To be published in the September 1 issue of Physical
Review B (1996). Email address: [email protected]
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