614 research outputs found
Polarization and frequency disentanglement of photons via stochastic polarization mode dispersion
We investigate the quantum decoherence of frequency and polarization
variables of photons via polarization mode dispersion in optical fibers. By
observing the analogy between the propagation equation of the field and the
Schr\"odinger equation, we develop a master equation under Markovian
approximation and analytically solve for the field density matrix. We identify
distinct decay behaviors for the polarization and frequency variables for
single-photon and two-photon states. For the single photon case, purity
functions indicate that complete decoherence for each variable is possible only
for infinite fiber length. For entangled two-photon states passing through
separate fibers, entanglement associated with each variable can be completely
destroyed after characteristic finite propagation distances. In particular, we
show that frequency disentanglement is independent of the initial polarization
status. For propagation of two photons in a common fiber, the evolution of a
polarization singlet state is addressed. We show that while complete
polarization disentanglement occurs at a finite propagation distance, frequency
entanglement could survive at any finite distance for gaussian states.Comment: 2 figure
Security against eavesdropping in quantum cryptography
In this article we deal with the security of the BB84 quantum cryptography
protocol over noisy channels using generalized privacy amplification. For this
we estimate the fraction of bits needed to be discarded during the privacy
amplification step. This estimate is given for two scenarios, both of which
assume the eavesdropper to access each of the signals independently and take
error correction into account. One scenario does not allow a delay of the
eavesdropper's measurement of a measurement probe until he receives additional
classical information. In this scenario we achieve a sharp bound. The other
scenario allows a measurement delay, so that the general attack of an
eavesdropper on individual signals is covered. This bound is not sharp but
allows a practical implementation of the protocol.Comment: 11 pages including 3 figures, contains new results not contained in
my Phys. Rev. A pape
Atom-photon entanglement generation and distribution
We extend an earlier model by Law {\it et al.} \cite{law} for a cavity QED
based single-photon-gun to atom-photon entanglement generation and
distribution. We illuminate the importance of a small critical atom number on
the fidelity of the proposed operation in the strong coupling limit. Our result
points to a promisingly high purity and efficiency using currently available
cavity QED parameters, and sheds new light on constructing quantum computing
and communication devices with trapped atoms and high Q optical cavities.Comment: 7 fig
Efficient quantum cryptography network without entanglement and quantum memory
An efficient quantum cryptography network protocol is proposed with
d-dimension polarized photons, without resorting to entanglement and quantum
memory. A server on the network, say Alice, provides the service for preparing
and measuring single photons whose initial state are |0>. The users code the
information on the single photons with some unitary operations. For preventing
the untrustworthy server Alice from eavesdropping the quantum lines, a
nonorthogonal-coding technique (decoy-photon technique) is used in the process
that the quantum signal is transmitted between the users. This protocol does
not require the servers and the users to store the quantum state and almost all
of the single photons can be used for carrying the information, which makes it
more convenient for application than others with present technology. We also
discuss the case with a faint laser pulse.Comment: 4 pages, 1 figures. It also presented a way for preparing decoy
photons without a sinigle-photon sourc
Test of quantum nonlocality for cavity fields
There have been studies on formation of quantum-nonlocal states in spatially
separate two cavities. We suggest a nonlocal test for the field prepared in the
two cavities. We couple classical driving fields with the cavities where a
nonlocal state is prepared. Two independent two-level atoms are then sent
through respective cavities to interact off-resonantly with the cavity fields.
The atomic states are measured after the interaction. Bell's inequality can be
tested by the joint probabilities of two-level atoms being in their excited or
ground states. We find that quantum nonlocality can also be tested using a
single atom sequentially interacting with the two cavities. Potential
experimental errors are also considered. We show that with the present
experimental condition of 5% error in the atomic velocity distribution, the
violation of Bell's inequality can be measured.Comment: 14pages, 2figures. accepted to Phys. Rev.
Deterministic secure direct communication using GHZ states and swapping quantum entanglement
We present a deterministic secure direct communication scheme via
entanglement swapping, where a set of ordered maximally entangled
three-particle states (GHZ states), initially shared by three spatially
separated parties, Alice, Bob and Charlie, functions as a quantum information
channel. After ensuring the safety of the quantum channel, Alice and Bob apply
a series local operations on their respective particles according to the
tripartite stipulation and the secret message they both want to send to
Charlie. By three Alice, Bob and Charlie's Bell measurement results, Charlie is
able to infer the secret messages directly. The secret messages are faithfully
transmitted from Alice and Bob to Charlie via initially shared pairs of GHZ
states without revealing any information to a potential eavesdropper. Since
there is not a transmission of the qubits carrying the secret message between
any two of them in the public channel, it is completely secure for direct
secret communication if perfect quantum channel is used.Comment: 9 pages, no figur
Social media and teacher professional learning communities
peer-reviewed.Background: An extensive and international evidence base positions professional learning communities (PLCs) as an effective continued professional development (CPD) mechanism that can impact on teachers’ practices and, in turn, students’ learning. The landscape of teacher PLCs is continuously developing; notably through teachers’ uses of social media. Yet, there is limited robust evidence identifying the characteristics of social media PLCs that impact on teachers’ learning and practice.
Purpose: This exploratory study examined the characteristics of a specific Twitter-based professional learning community – #pechat. The research questions were: (i) what is the nature of a Twitter-based professional learning community? and (ii) what characteristics of a Twitter-based professional learning community develop learning and practice?
Methods: Data were generated from 901 tweets between 100 participants; and 18 in-depth semi-structured elicitation interviews with participants and moderators of the Twitter-based professional learning community. Data were analysed through a process of deliberation, and a relativist approach informed quality.
Findings: Two themes are reported to explain the nature of the Twitter-based professional learning community and the different types of characteristics of #pechat that developed learning and practice. The first theme engagement shows how different participants of #pechat engaged with discussions and how moderators played a key role in facilitating discussions between participants. The second theme shared practices shows how discussions between participants of #pechat led to the development of new practices that some teachers were able to use to accomplish particular objectives in their physical education lessons.
Conclusion: The analysis of the data provided evidence to suggest that #pechat is a PLC and is representative of an established group of practitioners. These characteristics should be considered in the design of future online professional development experiences. Facilitator or moderator training could support the development of social media based PLCs that subsequently and positively impact on teachers’ practices.ACCEPTEDpeer-reviewe
Failure due to fatigue in fiber bundles and solids
We consider first a homogeneous fiber bundle model where all the fibers have
got the same stress threshold beyond which all fail simultaneously in absence
of noise. At finite noise, the bundle acquires a fatigue behavior due to the
noise-induced failure probability at any stress. We solve this dynamics of
failure analytically and show that the average failure time of the bundle
decreases exponentially as the stress increases. We also determine the
avalanche size distribution during such failure and find a power law decay. We
compare this fatigue behavior with that obtained phenomenologically for the
nucleation of Griffith cracks. Next we study numerically the fatigue behavior
of random fiber bundles having simple distributions of individual fiber
strengths, at stress less than the bundle's strength (beyond which it fails
instantly). The average failure time is again seen to decrease exponentially as
the stress increases and the avalanche size distribution shows similar power
law decay. These results are also in broad agreement with experimental
observations on fatigue in solids. We believe, these observations regarding the
failure time are useful for quantum breakdown phenomena in disordered systems.Comment: 13 pages, 4 figures, figures added and the text is revise
Recovering coherence from decoherence: a method of quantum state reconstruction
We present a feasible scheme for reconstructing the quantum state of a field
prepared inside a lossy cavity. Quantum coherences are normally destroyed by
dissipation, but we show that at zero temperature we are able to retrieve
enough information about the initial state, making possible to recover its
Wigner function as well as other quasiprobabilities. We provide a numerical
simulation of a Schroedinger cat state reconstruction.Comment: 8 pages, in RevTeX, 4 figures, accepted for publication in Phys. Rev.
A (november 1999
A new perturbative expansion of the time evolution operator associated with a quantum system
A novel expansion of the evolution operator associated with a -- in general,
time-dependent -- perturbed quantum Hamiltonian is presented. It is shown that
it has a wide range of possible realizations that can be fitted according to
computational convenience or to satisfy specific requirements. As a remarkable
example, the quantum Hamiltonian describing a laser-driven trapped ion is
studied in detail.Comment: 32 pages; modified version with examples of my previous paper
quant-ph/0404056; to appear on the J. of Optics B: Quantum and Semiclassical
Optics, Special Issue on 'Optics and Squeeze Transformations after Einstein
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