5,404 research outputs found
Cooperative emission of a pulse train in an optically thick scattering medium
An optically thick cold atomic cloud emits a coherent flash of light in the
forward direction when the phase of an incident probe field is abruptly
changed. Because of cooperativity, the duration of this phenomena can be much
shorter than the excited lifetime of a single atom. Repeating periodically the
abrupt phase jump, we generate a train of pulses with short repetition time,
high intensity contrast and high efficiency. In this regime, the emission is
fully governed by cooperativity even if the cloud is dilute.Comment: 5 pages, 3 figure
Experimental quantum key distribution with active phase randomization
Phase randomization is an important assumption made in many security proofs
of practical quantum key distribution (QKD) systems. Here, we present the first
experimental demonstration of QKD with reliable active phase randomization. One
key contribution is a polarization-insensitive phase modulator, which we added
to a commercial phase-coding QKD system to randomize the global phase of each
bit. We also proposed a simple but useful method to verify experimentally that
the phase is indeed randomized. Our result shows very low QBER (<1%). We expect
this active phase randomization process to be a standard part in future QKD
set-ups due to its significance and feasibility.Comment: 3 pages, 3 figures, RevTE
Unconditionally secure key distillation from multi-photons
In this paper, we prove that the unconditionally secure key can be
surprisingly extracted from {\it multi}-photon emission part in the photon
polarization-based QKD. One example is shown by explicitly proving that one can
indeed generate an unconditionally secure key from Alice's two-photon emission
part in ``Quantum cryptography protocols robust against photon number splitting
attacks for weak laser pulses implementations'' proposed by V. Scarani {\it et
al.,} in Phys. Rev. Lett. {\bf 92}, 057901 (2004), which is called SARG04. This
protocol uses the same four states as in BB84 and differs only in the classical
post-processing protocol. It is, thus, interesting to see how the classical
post-processing of quantum key distribution might qualitatively change its
security. We also show that one can generate an unconditionally secure key from
the single to the four-photon part in a generalized SARG04 that uses six
states. Finally, we also compare the bit error rate threshold of these
protocols with the one in BB84 and the original six-state protocol assuming a
depolarizing channel.Comment: The title has changed again. We considerably improved our
presentation, and furthermore we proposed & analyzed a security of a modified
SARG04 protocol, which uses six state
Relations between the SNO and the Super Kamiokande solar neutrino rates
By comparing the neutrino spectra measured by SNO and Super Kamiokande, we
obtain inequalities between the ratios of observed rate to SSM rate for the two
experiments. These inequalities apply to a possibly energy-dependent reduction
of the SSM flux and to the case of neutrino oscillations. We use them to
examine the relationship between the two experiments expected for the MSW and
``Just-So" oscillation scenarios.Comment: 8 pages + 3 figures, REVTeX, uuencoded file created on a VMS syste
On the performance of two protocols: SARG04 and BB84
We compare the performance of BB84 and SARG04, the later of which was
proposed by V. Scarani et al., in Phys. Rev. Lett. 92, 057901 (2004).
Specifically, in this paper, we investigate SARG04 with two-way classical
communications and SARG04 with decoy states. In the first part of the paper, we
show that SARG04 with two-way communications can tolerate a higher bit error
rate (19.4% for a one-photon source and 6.56% for a two-photon source) than
SARG04 with one-way communications (10.95% for a one-photon source and 2.71%
for a two-photon source). Also, the upper bounds on the bit error rate for
SARG04 with two-way communications are computed in a closed form by considering
an individual attack based on a general measurement. In the second part of the
paper, we propose employing the idea of decoy states in SARG04 to obtain
unconditional security even when realistic devices are used. We compare the
performance of SARG04 with decoy states and BB84 with decoy states. We find
that the optimal mean-photon number for SARG04 is higher than that of BB84 when
the bit error rate is small. Also, we observe that SARG04 does not achieve a
longer secure distance and a higher key generation rate than BB84, assuming a
typical experimental parameter set.Comment: 48 pages, 10 figures, 1 column, changed Figs. 7 and
Cooperative Emission of a Coherent Superflash of Light
We investigate the transient coherent transmission of light through an
optically thick cold stron-tium gas. We observe a coherent superflash just
after an abrupt probe extinction, with peak intensity more than three times the
incident one. We show that this coherent superflash is a direct signature of
the cooperative forward emission of the atoms. By engineering fast transient
phenomena on the incident field, we give a clear and simple picture of the
physical mechanisms at play.Comment: 4 Fig., 5 page
What could be learnt from Positronium for Quarkonium?
In order to fulfill Low's theorem requirements, a new lowest order basis for
bound state decay computations is proposed, in which the binding energy is
treated non-perturbatively. The properties of the method are sketched by
reviewing standard positronium decay processes. Then, it is shown how applying
the method to quarkonia sheds new light on some longstanding puzzles.Comment: 12 pages, 10 figures. Talk given at the ETH Workshop on Positronium
Physics, May 30-31, 2003, Zurich, Switzerlan
Statistical interaction modeling of bovine herd behaviors
While there has been interest in modeling the group behavior of herds or flocks, much of this work has focused on simulating their collective spatial motion patterns which have not accounted for individuality in the herd and instead assume a homogenized role for all members or sub-groups of the herd. Animal behavior experts have noted that domestic animals exhibit behaviors that are indicative of social hierarchy: leader/follower type behaviors are present as well as dominance and subordination, aggression and rank order, and specific social affiliations may also exist. Both wild and domestic cattle are social species, and group behaviors are likely to be influenced by the expression of specific social interactions. In this paper, Global Positioning System coordinate fixes gathered from a herd of beef cows tracked in open fields over several days at a time are utilized to learn a model that focuses on the interactions within the herd as well as its overall movement. Using these data in this way explores the validity of existing group behavior models against actual herding behaviors. Domain knowledge, location geography and human observations, are utilized to explain the causes of these deviations from this idealized behavior
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