27,998 research outputs found
A simple proof of the unconditional security of quantum key distribution
Quantum key distribution is the most well-known application of quantum
cryptography. Previous proposed proofs of security of quantum key distribution
contain various technical subtleties. Here, a conceptually simpler proof of
security of quantum key distribution is presented. The new insight is the
invariance of the error rate of a teleportation channel: We show that the error
rate of a teleportation channel is independent of the signals being
transmitted. This is because the non-trivial error patterns are permuted under
teleportation. This new insight is combined with the recently proposed quantum
to classical reduction theorem. Our result shows that assuming that Alice and
Bob have fault-tolerant quantum computers, quantum key distribution can be made
unconditionally secure over arbitrarily long distances even against the most
general type of eavesdropping attacks and in the presence of all types of
noises.Comment: 13 pages, extended abstract. Comments will be appreciate
Security proof of a three-state quantum key distribution protocol without rotational symmetry
Standard security proofs of quantum key distribution (QKD) protocols often
rely on symmetry arguments. In this paper, we prove the security of a
three-state protocol that does not possess rotational symmetry. The three-state
QKD protocol we consider involves three qubit states, where the first two
states, |0_z> and |1_z>, can contribute to key generation and the third state,
|+>=(|0_z>+|1_z>)/\sqrt{2}, is for channel estimation. This protocol has been
proposed and implemented experimentally in some frequency-based QKD systems
where the three states can be prepared easily. Thus, by founding on the
security of this three-state protocol, we prove that these QKD schemes are, in
fact, unconditionally secure against any attacks allowed by quantum mechanics.
The main task in our proof is to upper bound the phase error rate of the qubits
given the bit error rates observed. Unconditional security can then be proved
not only for the ideal case of a single-photon source and perfect detectors,
but also for the realistic case of a phase-randomized weak coherent light
source and imperfect threshold detectors. Our result on the phase error rate
upper bound is independent of the loss in the channel. Also, we compare the
three-state protocol with the BB84 protocol. For the single-photon source case,
our result proves that the BB84 protocol strictly tolerates a higher quantum
bit error rate than the three-state protocol; while for the coherent-source
case, the BB84 protocol achieves a higher key generation rate and secure
distance than the three-state protocol when a decoy-state method is used.Comment: 10 pages, 3 figures, 2 column
Quantum Gambling Using Three Nonorthogonal States
We provide a quantum gambling protocol using three (symmetric) nonorthogonal
states. The bias of the proposed protocol is less than that of previous ones,
making it more practical. We show that the proposed scheme is secure against
non-entanglement attacks. The security of the proposed scheme against
entanglement attacks is shown heuristically.Comment: no essential correction, 4 pages, RevTe
Practical Decoy State for Quantum Key Distribution
Decoy states have recently been proposed as a useful method for substantially
improving the performance of quantum key distribution. Here, we present a
general theory of the decoy state protocol based on only two decoy states and
one signal state. We perform optimization on the choice of intensities of the
two decoy states and the signal state. Our result shows that a decoy state
protocol with only two types of decoy states--the vacuum and a weak decoy
state--asymptotically approaches the theoretical limit of the most general type
of decoy state protocols (with an infinite number of decoy states). We also
present a one-decoy-state protocol. Moreover, we provide estimations on the
effects of statistical fluctuations and suggest that, even for long distance
(larger than 100km) QKD, our two-decoy-state protocol can be implemented with
only a few hours of experimental data. In conclusion, decoy state quantum key
distribution is highly practical.Comment: 31 pages. 6 figures. Preprint forma
Dilatonic Black Holes, Naked Singularities and Strings
We extend a previous calculation which treated Schwarschild black hole
horizons as quantum mechanical objects to the case of a charged, dilaton black
hole. We show that for a unique value of the dilaton parameter `a', which is
determined by the condition of unitarity of the S matrix, black holes transform
at the extremal limit into strings.Comment: 8 pages, REVTE
Strategy updating rules and strategy distributions in dynamical multiagent systems
In the evolutionary version of the minority game, agents update their
strategies (gene-value ) in order to improve their performance. Motivated by
recent intriguing results obtained for prize-to-fine ratios which are smaller
than unity, we explore the system's dynamics with a strategy updating rule of
the form (). We find that the strategy
distribution depends strongly on the values of the prize-to-fine ratio , the
length scale , and the type of boundary condition used. We show that
these parameters determine the amplitude and frequency of the the temporal
oscillations observed in the gene space. These regular oscillations are shown
to be the main factor which determines the strategy distribution of the
population. In addition, we find that agents characterized by
(a coin-tossing strategy) have the best chances of survival at asymptotically
long times, regardless of the value of and the boundary conditions
used.Comment: 4 pages, 7 figure
DC electrical distribution systems in bulidings
Author name used in this publication: Bryan PongVersion of RecordPublishe
Isotope Spectroscopy
The measurement of isotopic ratios provides a privileged insight both into
nucleosynthesis and into the mechanisms operating in stellar envelopes, such as
gravitational settling. In this article, we give a few examples of how isotopic
ratios can be determined from high-resolution, high-quality stellar spectra. We
consider examples of the lightest elements, H and He, for which the isotopic
shifts are very large and easily measurable, and examples of heavier elements
for which the determination of isotopic ratios is more difficult. The presence
of 6Li in the stellar atmospheres causes a subtle extra depression in the red
wing of the 7Li 670.7 nm doublet which can only be detected in spectra of the
highest quality. But even with the best spectra, the derived Li abundance
can only be as good as the synthetic spectra used for their interpretation. It
is now known that 3D non-LTE modelling of the lithium spectral line profiles is
necessary to account properly for the intrinsic line asymmetry, which is
produced by convective flows in the atmospheres of cool stars, and can mimic
the presence of 6Li. We also discuss briefly the case of the carbon isotopic
ratio in metal-poor stars, and provide a new determination of the nickel
isotopic ratios in the solar atmosphere.Comment: AIP Thinkshop 10 "High resolution optical spectroscopy", invited
talk, AN in pres
A classical analogue of entanglement
We show that quantum entanglement has a very close classical analogue, namely
secret classical correlations. The fundamental analogy stems from the behavior
of quantum entanglement under local operations and classical communication and
the behavior of secret correlations under local operations and public
communication. A large number of derived analogies follow. In particular
teleportation is analogous to the one-time-pad, the concept of ``pure state''
exists in the classical domain, entanglement concentration and dilution are
essentially classical secrecy protocols, and single copy entanglement
manipulations have such a close classical analog that the majorization results
are reproduced in the classical setting. This analogy allows one to import
questions from the quantum domain into the classical one, and vice-versa,
helping to get a better understanding of both. Also, by identifying classical
aspects of quantum entanglement it allows one to identify those aspects of
entanglement which are uniquely quantum mechanical.Comment: 13 pages, references update
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