1,329 research outputs found
Unconditional Security of Three State Quantum Key Distribution Protocols
Quantum key distribution (QKD) protocols are cryptographic techniques with
security based only on the laws of quantum mechanics. Two prominent QKD schemes
are the BB84 and B92 protocols that use four and two quantum states,
respectively. In 2000, Phoenix et al. proposed a new family of three state
protocols that offers advantages over the previous schemes. Until now, an error
rate threshold for security of the symmetric trine spherical code QKD protocol
has only been shown for the trivial intercept/resend eavesdropping strategy. In
this paper, we prove the unconditional security of the trine spherical code QKD
protocol, demonstrating its security up to a bit error rate of 9.81%. We also
discuss on how this proof applies to a version of the trine spherical code QKD
protocol where the error rate is evaluated from the number of inconclusive
events.Comment: 4 pages, published versio
Robust Quantum Communication Using A Polarization-Entangled Photon Pair
Noise and imperfection of realistic devices are major obstacles for
implementing quantum cryptography. In particular birefringence in optical
fibers leads to decoherence of qubits encoded in polarization of photon. We
show how to overcome this problem by doing single qubit quantum communication
without a shared spatial reference frame and precise timing. Quantum
information will be encoded in pair of photons using ``tag'' operations which
corresponds to the time delay of one of the polarization modes. This method is
robust against the phase instability of the interferometers despite the use of
time-bins. Moreover synchronized clocks are not required in the ideal situation
no photon loss case as they are only necessary to label the different encoded
qubits.Comment: 4 pages, 2 figure
Defining the Field: Revisiting the ACA 1995 Definition of Communication Studies
This article deals with the problem of defining communication studies in higher education. In 1995, the Association for Communication Administration (ACA) convened a summer conference that produced a two-sentence definition of the field of communication. More than 100 conferees voted their unanimous approval of the definition, which was then disseminated nationally and used by communication scholar/teachers for a multiplicity of purposes. Given the potential utility of that definition and the expansion of communication studies since 1995, the present study surveyed ACA\u27s current members to determine whether they are aware the definition exists, how they have used it, and the extent to which they perceive it as representative of communication studies today. The results of that survey are reported in this article, which begins with a description of why and how this definition was originally developed. In a field as diverse and eclectic as communication, a need exists for some commonality of understanding about what constitutes the discipline\u27s subject matter. Such understanding, in the form of a definition, can serve two functions: it can provide a descriptor of the diversity, breadth, and depth of the field itself; and, it can be used to represent the discipline to an external audience, both inside and outside of academe, many of whom may still hold onto the notion that the field is committed only to the practice and study of speech making
Robust polarization-based quantum key distribution over collective-noise channel
We present two polarization-based protocols for quantum key distribution. The
protocols encode key bits in noiseless subspaces or subsystems, and so can
function over a quantum channel subjected to an arbitrary degree of collective
noise, as occurs, for instance, due to rotation of polarizations in an optical
fiber. These protocols can be implemented using only entangled photon-pair
sources, single-photon rotations, and single-photon detectors. Thus, our
proposals offer practical and realistic alternatives to existing schemes for
quantum key distribution over optical fibers without resorting to
interferometry or two-way quantum communication, thereby circumventing,
respectively, the need for high precision timing and the threat of Trojan horse
attacks.Comment: Minor changes, added reference
Experimental Implementation of Discrete Time Quantum Random Walk on an NMR Quantum Information Processor
We present an experimental implementation of the coined discrete time quantum
walk on a square using a three qubit liquid state nuclear magnetic resonance
(NMR) quantum information processor (QIP). Contrary to its classical
counterpart, we observe complete interference after certain steps and a
periodicity in the evolution. Complete state tomography has been performed for
each of the eight steps making a full period. The results have extremely high
fidelity with the expected states and show clearly the effects of quantum
interference in the walk. We also show and discuss the importance of choosing a
molecule with a natural Hamiltonian well suited to NMR QIP by implementing the
same algorithm on a second molecule. Finally, we show experimentally that
decoherence after each step makes the statistics of the quantum walk tend to
that of the classical random walk.Comment: revtex4, 8 pages, 6 figures, submitted to PR
Using error correction to determine the noise model
Quantum error correcting codes have been shown to have the ability of making
quantum information resilient against noise. Here we show that we can use
quantum error correcting codes as diagnostics to characterise noise. The
experiment is based on a three-bit quantum error correcting code carried out on
a three-qubit nuclear magnetic resonance (NMR) quantum information processor.
Utilizing both engineered and natural noise, the degree of correlations present
in the noise affecting a two-qubit subsystem was determined. We measured a
correlation factor of c=0.5+/-0.2 using the error correction protocol, and
c=0.3+/-0.2 using a standard NMR technique based on coherence pathway
selection. Although the error correction method demands precise control, the
results demonstrate that the required precision is achievable in the
liquid-state NMR setting.Comment: 10 pages, 3 figures. Added discussion section, improved figure
The resource theory of quantum reference frames: manipulations and monotones
Every restriction on quantum operations defines a resource theory,
determining how quantum states that cannot be prepared under the restriction
may be manipulated and used to circumvent the restriction. A superselection
rule is a restriction that arises through the lack of a classical reference
frame and the states that circumvent it (the resource) are quantum reference
frames. We consider the resource theories that arise from three types of
superselection rule, associated respectively with lacking: (i) a phase
reference, (ii) a frame for chirality, and (iii) a frame for spatial
orientation. Focussing on pure unipartite quantum states (and in some cases
restricting our attention even further to subsets of these), we explore
single-copy and asymptotic manipulations. In particular, we identify the
necessary and sufficient conditions for a deterministic transformation between
two resource states to be possible and, when these conditions are not met, the
maximum probability with which the transformation can be achieved. We also
determine when a particular transformation can be achieved reversibly in the
limit of arbitrarily many copies and find the maximum rate of conversion. A
comparison of the three resource theories demonstrates that the extent to which
resources can be interconverted decreases as the strength of the restriction
increases. Along the way, we introduce several measures of frameness and prove
that these are monotonically nonincreasing under various classes of operations
that are permitted by the superselection rule.Comment: 37 pages, 4 figures, Published Versio
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