1,482 research outputs found
Entangled photons from a strongly coupled quantum dot-cavity system
A quantum dot strongly coupled to a photonic crystal has been recently
proposed as a source of entangled photon pairs [R. Johne et al., Phys. Rev.
Lett. 100, 240404 (2008)]. The biexction decay via intermediate polariton
states can be used to overcome the natural splitting between the exciton states
coupled to the horizontally and vertically polarized light modes, so that high
degrees of entanglement can be expected. We investigate theoretically the
features of realistic dot-cavity systems, including the effect of the different
oscillator strength of excitons resonances coupled to the different
polarizations of light. We show that in this case, an independent adjustment of
the cavity resonances is needed in order to keep a high entanglement degree. We
also consider the case when the biexciton-exciton transition is also strongly
coupled to a cavity mode. We show that a very fast emission rate can be
achieved allowing the repetition rates in the THz range. Such fast emission
should however be paid for by a very complex tuning of the many strongly
coupled resonances involved and by a loss of quantum efficiency. Altogether a
strongly coupled dot-cavity system seems to be very promising as a source of
entangled photon pairs.Comment: 7 pages, 5 figure
Using of small-scale quantum computers in cryptography with many-qubit entangled states
We propose a new cryptographic protocol. It is suggested to encode
information in ordinary binary form into many-qubit entangled states with the
help of a quantum computer. A state of qubits (realized, e.g., with photons) is
transmitted through a quantum channel to the addressee, who applies a quantum
computer tuned to realize the inverse unitary transformation decoding of the
message. Different ways of eavesdropping are considered, and an estimate of the
time needed for determining the secret unitary transformation is given. It is
shown that using even small quantum computers can serve as a basis for very
efficient cryptographic protocols. For a suggested cryptographic protocol, the
time scale on which communication can be considered secure is exponential in
the number of qubits in the entangled states and in the number of gates used to
construct the quantum network
Quantum Cryptography Based on the Time--Energy Uncertainty Relation
A new cryptosystem based on the fundamental time--energy uncertainty relation
is proposed. Such a cryptosystem can be implemented with both correlated photon
pairs and single photon states.Comment: 5 pages, LaTex, no figure
Neutrinos in a spherical box
In the present paper we study some neutrino properties as they may appear in
the low energy neutrinos emitted in triton decay with maximum neutrino energy
of 18.6 keV. The technical challenges to this end can be achieved by building a
very large TPC capable of detecting low energy recoils, down to a a few tenths
of a keV, within the required low background constraints. More specifically We
propose the development of a spherical gaseous TPC of about 10-m in radius and
a 200 Mcurie triton source in the center of curvature. One can list a number of
exciting studies, concerning fundamental physics issues, that could be made
using a large volume TPC and low energy antineutrinos: 1) The oscillation
length involving the small angle of the neutrino mixing matrix, directly
measured in this disappearance experiment, is fully contained inside the
detector. Measuring the counting rate of neutrino-electron elastic scattering
as a function of the distance of the source will give a precise and unambiguous
measurement of the oscillation parameters free of systematic errors. In fact
first estimates show that even with a year's data taking a sensitivity of a few
percent for the measurement of the above angle will be achieved. 2) The low
energy detection threshold offers a unique sensitivity for the neutrino
magnetic moment which is about two orders of magnitude beyond the current
experimental limit. 3) Scattering at such low neutrino energies has never been
studied and any departure from the expected behavior may be an indication of
new physics beyond the standard model. In this work we mainly focus on the
various theoretical issues involved including a precise determination of the
Weinberg angle at very low momentum transfer.Comment: 16 Pages, LaTex, 7 figures, talk given at NANP 2003, Dubna, Russia,
June 23, 200
All-Versus-Nothing Proof of Einstein-Podolsky-Rosen Steering
Einstein-Podolsky-Rosen steering is a form of quantum nonlocality
intermediate between entanglement and Bell nonlocality. Although Schr\"odinger
already mooted the idea in 1935, steering still defies a complete
understanding. In analogy to "all-versus-nothing" proofs of Bell nonlocality,
here we present a proof of steering without inequalities rendering the
detection of correlations leading to a violation of steering inequalities
unnecessary. We show that, given any two-qubit entangled state, the existence
of certain projective measurement by Alice so that Bob's normalized conditional
states can be regarded as two different pure states provides a criterion for
Alice-to-Bob steerability. A steering inequality equivalent to the
all-versus-nothing proof is also obtained. Our result clearly demonstrates that
there exist many quantum states which do not violate any previously known
steering inequality but are indeed steerable. Our method offers advantages over
the existing methods for experimentally testing steerability, and sheds new
light on the asymmetric steering problem.Comment: 7 pages, 2 figures. Accepted in Sci. Re
Entanglement and the SU(2) phase states in atomic systems
We show that a system of 2n identical two-level atoms interacting with n
cavity photons manifests entanglement and that the set of entangled states
coincides with the so-called SU(2) phase states. In particular, violation of
classical realism in terms of the GHZ and GHSH conditions is proved. We discuss
a new property of entanglement expressed in terms of local measurements. We
also show that generation of entangled states in the atom-photon systems under
consideration strongly depends on the choice of initial conditions and that the
parasitic influence of cavity detuning can be compensated through the use of
Kerr medium.Comment: 10 pages, 1 figur
On the Rees-Sciama effect: maps and statistics
Small maps of the Rees-Sciama (RS) effect are simulated by using an
appropriate N-body code and a certain ray-tracing procedure. A method designed
for the statistical analysis of cosmic microwave background (CMB) maps is
applied to study the resulting simulations. These techniques, recently proposed
--by our team-- to consider lens deformations of the CMB, are adapted to deal
with the RS effect. This effect and the deviations from Gaussianity associated
to it seem to be too small to be detected in the near future. This conclusion
follows from our estimation of both the RS angular power spectrum and the RS
reduced n-direction correlation functions for n<7.Comment: 11 pages, 13 figures, to appear in MNRA
Nuclear Inelastic X-Ray Scattering of FeO to 48 GPa
The partial density of vibrational states has been measured for Fe in
compressed FeO (w\"ustite) using nuclear resonant inelastic x-ray scattering.
Substantial changes have been observed in the overall shape of the density of
states close to the magnetic transiton around 20 GPa from the paramagnetic (low
pressure) to the antiferromagnetic (high pressure) state. Our data indicate a
substantial softening of the aggregate sound velocities far below the
transition, starting between 5 and 10 GPa. This is consistent with recent
radial x-ray diffraction measurements of the elastic constants in FeO. The
results indicate that strong magnetoelastic coupling in FeO is the driving
force behind the changes in the phonon spectrum of FeO.Comment: 4 pages, 4 figure
Pitch then power: limitations to acceleration in quadrupeds
Rapid acceleration and deceleration are vital for survival in many predator and prey animals and are important attributes of animal and human athletes. Adaptations for acceleration and deceleration are therefore likely to experience strong selective pressures—both natural and artificial. Here, we explore the mechanical and physiological constraints to acceleration. We examined two elite athletes bred and trained for acceleration performance (polo ponies and racing greyhounds), when performing maximal acceleration (and deceleration for ponies) in a competitive setting. We show that maximum acceleration and deceleration ability may be accounted for by two simple limits, one mechanical and one physiological. At low speed, acceleration and deceleration may be limited by the geometric constraints of avoiding net nose-up or tail-up pitching, respectively. At higher speeds, muscle power appears to limit acceleration
Entanglement Creation Using Quantum Interrogation
We present some applications of high efficiency quantum interrogation
("interaction free measurement") for the creation of entangled states of
separate atoms and of separate photons. The quantum interrogation of a quantum
object in a superposition of object-in and object-out leaves the object and
probe in an entangled state. The probe can then be further entangled with other
objects in subsequent quantum interrogations. By then projecting out those
cases were the probe is left in a particular final state, the quantum objects
can themselves be left in various entangled states. In this way we show how to
generate two-, three-, and higher qubit entanglement between atoms and between
photons. The effect of finite efficiency for the quantum interrogation is
delineated for the various schemes.Comment: 7 pages, 13 figures, Submitted to PR
- …