7,698 research outputs found
Security improvement of using modified coherent state for quantum cryptography
Weak coherent states as a photon source for quantum cryptography have limit
in secure data rate and transmission distance because of the presence of
multi-photon events and loss in transmission line. Two-photon events in a
coherent state can be taken out by a two-photon interference scheme. We
investigate the security issue of utilizing this modified coherent state in
quantum cryptography. A 4 dB improvement in secure data rate or a nearly
two-fold increase in transmission distance over the coherent state are found.
With a recently proposed and improved encoding strategy, further improvement is
possible.Comment: 5 pages, 2 figures, to appear in Physical Review
Secure Direct Communication Based on Secret Transmitting Order of Particles
We propose the schemes of quantum secure direct communication (QSDC) based on
secret transmitting order of particles. In these protocols, the secret
transmitting order of particles ensures the security of communication, and no
secret messages are leaked even if the communication is interrupted for
security. This strategy of security for communication is also generalized to
quantum dialogue. It not only ensures the unconditional security but also
improves the efficiency of communication.Comment: To appear in Phys. Rev.
Some new conjugate orthogonal Latin squares
AbstractWe present some new conjugate orthogonal Latin squares which are obtained from a direct method of construction of the starter-adder type. Combining these new constructions with earlier results of K. T. Phelps and the first author, it is shown that a (3, 2, 1)- (or (1, 3, 2)-) conjugate orthogonal Latin square of order v exists for all positive integers v ≠ 2, 6. It is also shown that a (3, 2, 1)- (or (1, 3, 2)-) conjugate orthogonal idempotent Latin square of order v exists for all positive integers v ≠ 2, 3, 6 with one possible exception v = 12, and this result can be used to enlarge the spectrum of a certain class of Mendelsohn designs and provide better results for problems on embedding
Significant foreground unrelated non-acoustic anisotropy on the one degree scale in WMAP 5-year observations
The spectral variation of the cosmic microwave background (CMB) as observed
by WMAP was tested using foreground reduced WMAP5 data, by producing
subtraction maps at the 1 angular resolution between the two
cosmological bands of V and W, for masked sky areas that avoid the Galactic
disk. The resulting map revealed a non-acoustic signal over and above the
WMAP5 pixel noise, with two main properties. Firstly, it possesses quadrupole
power at the 1 level which may be attributed to foreground
residuals. Second, it fluctuates also at all values of 2, especially
on the scale (). The behavior is {\it
random and symmetrical} about zero temperature with a r.m.s. amplitude of
7 , or 10 % of the maximum CMB anisotropy, which would require
a `cosmic conspiracy' among the foreground components if it is a consequence of
their existences. Both anomalies must be properly diagnosed and corrected if
`precision cosmology' is the claim. The second anomaly is, however, more
interesting because it opens the question on whether the CMB anisotropy
genuinely represents primordial density seeds.Comment: Final version to appear in Ap
Experimental Implementation of Remote State Preparation by Nuclear Magnetic Resonance
We have experimentally implemented remote state preparation (RSP) of a qubit
from a hydrogen to a carbon nucleus in molecules of carbon-13 labeled
chloroform CHCl over interatomic distances using liquid-state
nuclear magnetic resonance (NMR) technique. Full RSP of a special ensemble of
qubits, i.e., a qubit chosen from equatorial and polar great circles on a Bloch
sphere with Pati's scheme, was achieved with one cbit communication. Such a RSP
scheme can be generalized to prepare a large number of qubit states and may be
used in other quantum information processing and quantum computing.Comment: 10 pages,5 PS figure
Energy levels of few electron quantum dots imaged and characterized by atomic force microscopy
Strong confinement of charges in few electron systems such as in atoms,
molecules and quantum dots leads to a spectrum of discrete energy levels that
are often shared by several degenerate quantum states. Since the electronic
structure is key to understanding their chemical properties, methods that probe
these energy levels in situ are important. We show how electrostatic force
detection using atomic force microscopy reveals the electronic structure of
individual and coupled self-assembled quantum dots. An electron addition
spectrum in the Coulomb blockade regime, resulting from a change in cantilever
resonance frequency and dissipation during tunneling events, shows one by one
electron charging of a dot. The spectra show clear level degeneracies in
isolated quantum dots, supported by the first observation of predicted
temperature-dependent shifts of Coulomb blockade peaks. Further, by scanning
the surface we observe that several quantum dots may reside on what
topologically appears to be just one. These images of grouped weakly and
strongly coupled dots allow us to estimate their relative coupling strengths.Comment: 11 pages, 6 figure
Entanglement Teleportation Through 1D Heisenberg Chain
Information transmission of two qubits through two independent 1D Heisenberg
chains as a quantum channel is analyzed. It is found that the entanglement of
two spin- quantum systems is decreased during teleportation via the
thermal mixed state in 1D Heisenberg chain. The entanglement teleportation will
be realized if the minimal entanglement of the thermal mixed state is provided
in such quantum channel. High average fidelity of teleportation with values
larger than 2/3 is obtained when the temperature {\it T} is very low. The
mutual information of the quantum channel declines with the
increase of the temperature and the external magnetic field. The entanglement
quality of input signal states cannot enhance mutual information of the quantum
channel.Comment: 11 pages, 4 figure
Non-Markovian reduced dynamics and entanglement evolution of two coupled spins in a quantum spin environment
The exact quantum dynamics of the reduced density matrix of two coupled spin
qubits in a quantum Heisenberg XY spin star environment in the thermodynamic
limit at arbitrarily finite temperatures is obtained using a novel operator
technique. In this approach, the transformed Hamiltonian becomes effectively
Jaynes-Cumming like and thus the analysis is also relevant to cavity quantum
electrodynamics. This special operator technique is mathematically simple and
physically clear, and allows us to treat systems and environments that could
all be strongly coupled mutually and internally. To study their entanglement
evolution, the concurrence of the reduced density matrix of the two coupled
central spins is also obtained exactly. It is shown that the dynamics of the
entanglement depends on the initial state of the system and the coupling
strength between the two coupled central spins, the thermal temperature of the
spin environment and the interaction between the constituents of the spin
environment. We also investigate the effect of detuning which in our model can
be controlled by the strength of a locally applied external magnetic field. It
is found that the detuning has a significant effect on the entanglement
generation between the two spin qubits.Comment: 9 pages (two-coulumn), 6 figures. To appear in Phys. Rev.
Fundamental Symmetries and Conservation Laws
I discuss recent progress in low-energy tests of symmetries and conservation
laws, including parity nonconservation in atoms and nuclei, electric dipole
moment tests of time-reversal invariance, beta-decay correlation studies, and
decays violating separate (family) and total lepton number.Comment: 11 pages, 5 figures; plenary talk presented at PANIC0
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