4,265 research outputs found
and systems at threshold: II. The effect of D waves
Using the two-body interactions obtained from a chiral constituent quark
model we study all and states with I=0,1,2 and
J=1/2,3/2 at threshold, taking into account all three-body configurations with
S and D wave components. We constrain further the limits for the
spin-triplet scattering length a_{1/2,1}. Using the hypertriton binding energy
we find a narrow interval for the possible values of the
spin-singlet scattering length a_{1/2,0}. We found that the system
has a quasibound state in the (I,J) = (1,1/2) channel very near threshold with
a width of about 2.1 MeV.Comment: 19 pages, 4 tables, 6 figures. Accepted for publication in Phys. Rev.
Iterative procedure for computing accessible information in quantum communication
We present an iterative algorithm that finds the optimal measurement for
extracting the accessible information in any quantum communication scenario.
The maximization is achieved by a steepest-ascent approach toward the extremal
point, following the gradient uphill in sufficiently small steps. We apply it
to a simple ad-hoc example, as well as to a problem with a bearing on the
security of a tomographic protocol for quantum key distribution.Comment: REVTeX, 4 pages, 1 figure, 1 tabl
Long-lived qubit from three spin-1/2 atoms
A system of three spin-1/2 atoms allows the construction of a
reference-frame-free (RFF) qubit in the subspace with total angular momentum
. The RFF qubit stays coherent perfectly as long as the spins of the
three atoms are affected homogeneously. The inhomogeneous evolution of the
atoms causes decoherence, but this decoherence can be suppressed efficiently by
applying a bias magnetic field of modest strength perpendicular to the plane of
the atoms. The resulting lifetime of the RFF qubit can be many days, making RFF
qubits of this kind promising candidates for quantum information storage units.
Specifically, we examine the situation of three atoms trapped
in a -laser-generated optical lattice and find that, with
conservatively estimated parameters, a stored qubit maintains a fidelity of
0.9999 for two hours.Comment: 15 pages, 9 figures; version 2 reports a much improved analysis;
version 3 contains more details about the four-atom cas
J004457+4123 (Sharov 21): not a remarkable nova in M31 but a background quasar with a spectacular UV flare
We announce the discovery of a quasar behind the disk of M31, which was
previously classified as a remarkable nova in our neighbour galaxy. The paper
is primarily aimed at the outburst of J004457+4123 (Sharov 21), with the first
part focussed on the optical spectroscopy and the improvement in the
photometric database. Both the optical spectrum and the broad band spectral
energy distribution of Sharov 21 are shown to be very similar to that of
normal, radio-quiet type 1 quasars. We present photometric data covering more
than a century and resulting in a long-term light curve that is densely sampled
over the past five decades. The variability of the quasar is characterized by a
ground state with typical fluctuation amplitudes of ~0.2 mag around B~20.5,
superimposed by a singular flare of ~2 yr duration (observer frame) with the
maximum at 1992.81 where the UV flux has increased by a factor of ~20. The
total energy in the flare is at least three orders of magnitudes higher than
the radiated energy of the most luminous supernovae, provided that it comes
from an intrinsic process and the energy is radiated isotropically. The profile
of the flare light curve appears to be in agreement with the standard
predictions for a stellar tidal disruption event where a ~10 M_sun giant star
was shredded in the tidal field of a ~2...5 10^8 M_sun black hole. The short
fallback time derived from the light curve requires an ultra-close encounter
where the pericentre of the stellar orbit is deep within the tidal disruption
radius. Gravitational microlensing provides an alternative explanation, though
the probability of such a high amplification event is very low.Comment: Accepted for publication in Astronomy and Astrophysics, 14 pages, 11
figure
Review of Human Vision Facts
Work reported herein was conducted at the Artificial Intelligence Laboratory, a Massachusetts Institute of Technology research program supported in part by the Advanced Research Projects Agency of the Department of Defense and monitored by the Office of Naval Research under Contract Number N00014-70-A-0362-0005.
Vision Flashes are informal papers intended for internal use.This note is a collection of well known interesting facts about human vision. All parameters are approximate. Some may be wrong. There are sections on retina physiology, eye optics, light adaptation, psychological curios, color and eyeball movement.MIT Artificial Intelligence Laboratory Robotics Section
Department of Defense Advanced Research Projects Agenc
Minimal qubit tomography
We present, and analyze thoroughly, a highly symmetric and efficient scheme
for the determination of a single-qubit state, such as the polarization
properties of photons emitted by a single-photon source. In our scheme there
are only four measured probabilities, just enough for the determination of the
three parameters that specify the qubit state, whereas the standard procedure
would measure six probabilities.Comment: 14 pages, 10 figures; final versio
Secure communication with a publicly known key
We present a scheme for direct and confidential communication between Alice and Bob, where there is no need for establishing a shared secret key first, and where the key used by Alice even will become known publicly. The communication is based on the exchange of single photons and each and every photon transmits one bit of Alice's message without revealing any information to a potential eavesdropper
and systems at threshold
We calculate the hypertriton binding energy and the and scattering lengths using baryon-baryon interactions obtained from a chiral
constituent quark model. We study consistently the and
systems analyzing the effect of the
conversion. Our interactions correctly predict the hypertriton binding energy.
The channel is also attractive and it might have a
bound state. From the condition of nonexistence of a (0,3/2) bound
state, an upper limit for the spin-triplet scattering length is
obtained. We also present results for the elastic and inelastic and
cross sections. The consistent description of the
scattering cross sections imposes a lower limit for the corresponding
spin-triplet scattering lengths. In the system the only attractive
channels are and , the state being the most
attractive one.Comment: 17 pages, 6 tables, 13 figures. Accepted for publication in Phys.
Rev.
Periodic and discrete Zak bases
Weyl's displacement operators for position and momentum commute if the
product of the elementary displacements equals Planck's constant. Then, their
common eigenstates constitute the Zak basis, each state specified by two phase
parameters. Upon enforcing a periodic dependence on the phases, one gets a
one-to-one mapping of the Hilbert space on the line onto the Hilbert space on
the torus. The Fourier coefficients of the periodic Zak bases make up the
discrete Zak bases. The two bases are mutually unbiased. We study these bases
in detail, including a brief discussion of their relation to Aharonov's modular
operators, and mention how they can be used to associate with the single degree
of freedom of the line a pair of genuine qubits.Comment: 15 pages, 3 figures; displayed abstract is shortened, see the paper
for the complete abstrac
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