21,799 research outputs found
Quantum Communication and Decoherence
In this contribution we will give a brief overview on the methods used to
overcome decoherence in quantum communication protocols. We give an
introduction to quantum error correction, entanglement purification and quantum
cryptography. It is shown that entanglement purification can be used to create
``private entanglement'', which makes it a useful tool for cryptographic
protocols.Comment: 31 pages, 10 figures, LaTeX, book chapter to appear in ``Coherent
Evolution in Noisy Environments'', Lecture Notes in Physics, (Springer
Verlag, Berlin-Heidelberg-New York). Minor typos correcte
Quantum Key Distribution
This chapter describes the application of lasers, specifically diode lasers,
in the area of quantum key distribution (QKD). First, we motivate the
distribution of cryptographic keys based on quantum physical properties of
light, give a brief introduction to QKD assuming the reader has no or very
little knowledge about cryptography, and briefly present the state-of-the-art
of QKD. In the second half of the chapter we describe, as an example of a
real-world QKD system, the system deployed between the University of Calgary
and SAIT Polytechnic. We conclude the chapter with a brief discussion of
quantum networks and future steps.Comment: 20 pages, 12 figure
Assumptions in Quantum Cryptography
Quantum cryptography uses techniques and ideas from physics and computer
science. The combination of these ideas makes the security proofs of quantum
cryptography a complicated task. To prove that a quantum-cryptography protocol
is secure, assumptions are made about the protocol and its devices. If these
assumptions are not justified in an implementation then an eavesdropper may
break the security of the protocol. Therefore, security is crucially dependent
on which assumptions are made and how justified the assumptions are in an
implementation of the protocol.
This thesis is primarily a review that analyzes and clarifies the connection
between the security proofs of quantum-cryptography protocols and their
experimental implementations. In particular, we focus on quantum key
distribution: the task of distributing a secret random key between two parties.
We provide a comprehensive introduction to several concepts: quantum mechanics
using the density operator formalism, quantum cryptography, and quantum key
distribution. We define security for quantum key distribution and outline
several mathematical techniques that can either be used to prove security or
simplify security proofs. In addition, we analyze the assumptions made in
quantum cryptography and how they may or may not be justified in
implementations.
Along with the review, we propose a framework that decomposes
quantum-key-distribution protocols and their assumptions into several classes.
Protocol classes can be used to clarify which proof techniques apply to which
kinds of protocols. Assumption classes can be used to specify which assumptions
are justified in implementations and which could be exploited by an
eavesdropper. Two contributions of the author are discussed: the security
proofs of two two-way quantum-key-distribution protocols and an intuitive proof
of the data-processing inequality.Comment: PhD Thesis, 221 page
Claw Finding Algorithms Using Quantum Walk
The claw finding problem has been studied in terms of query complexity as one
of the problems closely connected to cryptography. For given two functions, f
and g, as an oracle which have domains of size N and M (N<=M), respectively,
and the same range, the goal of the problem is to find x and y such that
f(x)=g(y). This paper describes an optimal algorithm using quantum walk that
solves this problem. Our algorithm can be generalized to find a claw of k
functions for any constant integer k>1, where the domains of the functions may
have different size.Comment: 12 pages. Introduction revised. A reference added. Weak lower bound
delete
Trevisan's extractor in the presence of quantum side information
Randomness extraction involves the processing of purely classical information
and is therefore usually studied in the framework of classical probability
theory. However, such a classical treatment is generally too restrictive for
applications, where side information about the values taken by classical random
variables may be represented by the state of a quantum system. This is
particularly relevant in the context of cryptography, where an adversary may
make use of quantum devices. Here, we show that the well known construction
paradigm for extractors proposed by Trevisan is sound in the presence of
quantum side information.
We exploit the modularity of this paradigm to give several concrete extractor
constructions, which, e.g, extract all the conditional (smooth) min-entropy of
the source using a seed of length poly-logarithmic in the input, or only
require the seed to be weakly random.Comment: 20+10 pages; v2: extract more min-entropy, use weakly random seed;
v3: extended introduction, matches published version with sections somewhat
reordere
On the -deformed Heisenberg uncertainty relations and discrete time
summary:The opportunity for verifying the basic principles of quantum theory and possible -deformation appears in quantum cryptography (QC) -- a new discipline of physics and information theory.\par The author, member of the group of cryptology of Praha, presents in this paper the possibility to verify the -deformation of Heisenberg uncertainty relation -deformed QM and possible discretization on the base of a model presented in the fourth section.\par In the seven sections, the author discusses these problems. First an introduction. The second section is on fractional supersymmetry and -deformed quantum mechanics (QM). So he obtains fractional superspace. In section 3, he presents basic information on quantum cryptography (QC) used then for the verification of the -deformation of QM in the null sector. In section 4, he presents a violation of quantum channel via -deformation and in section 5 the -deformed Heisenberg uncertainty relation in QC and a m
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