1,058 research outputs found
Unconditional security at a low cost
By simulating four quantum key distribution (QKD) experiments and analyzing
one decoy-state QKD experiment, we compare two data post-processing schemes
based on security against individual attack by L\"{u}tkenhaus, and
unconditional security analysis by Gottesman-Lo-L\"{u}tkenhaus-Preskill. Our
results show that these two schemes yield close performances. Since the Holy
Grail of QKD is its unconditional security, we conclude that one is better off
considering unconditional security, rather than restricting to individual
attacks.Comment: Accepted by International Conference on Quantum Foundation and
Technology: Frontier and Future 2006 (ICQFT'06
Unconditionally Secure Bit Commitment
We describe a new classical bit commitment protocol based on cryptographic
constraints imposed by special relativity. The protocol is unconditionally
secure against classical or quantum attacks. It evades the no-go results of
Mayers, Lo and Chau by requiring from Alice a sequence of communications,
including a post-revelation verification, each of which is guaranteed to be
independent of its predecessor.Comment: Typos corrected. Reference details added. To appear in Phys. Rev.
Let
Deuteron Momentum Distribution in KD2HPO4
The momentum distribution in KD2PO4(DKDP) has been measured using neutron
Compton scattering above and below the weakly first order
paraelectric-ferroelectric phase transition(T=229K). There is very litte
difference between the two distributions, and no sign of the coherence over two
locations for the proton observed in the paraelectric phase, as in KH2PO4(KDP).
We conclude that the tunnel splitting must be much less than 20mev. The width
of the distribution indicates that the effective potential for DKDP is
significantly softer than that for KDP. As electronic structure calculations
indicate that the stiffness of the potential increases with the size of the
coherent region locally undergoing soft mode fluctuations, we conclude that
there is a mass dependent quantum coherence length in both systems.Comment: 6 pages 5 figure
Efficient Heralding of Photonic Qubits with Apllications to Device Independent Quantum Key Distribution
We present an efficient way of heralding photonic qubit signals using linear
optics devices. First we show that one can obtain asymptotically perfect
heralding and unit success probability with growing resources. Second, we show
that even using finite resources, we can improve qualitatively and
quantitatively over earlier heralding results. In the latte r scenario, we can
obtain perfect heralded photonic qubits while maintaining a finite success
probability. We demonstrate the advantage of our heralding scheme by predicting
key rates for device independent quantum key distribution, taking imperfections
of sources and detectors into account
Optimal ratio between phase basis and bit basis in QKD
In the original BB84 protocol, the bit basis and the phase basis are used
with equal probability. Lo et al (J. of Cryptology, 18, 133-165 (2005))
proposed to modify the ratio between the two bases by increasing the final key
generation rate. However, the optimum ratio has not been derived. In this
letter, in order to examine this problem, the ratio between the two bases is
optimized for exponential constraints given Eve's information
distinguishability and the final error probability
Postponement of dark-count effects in practical quantum key-distribution by two-way post-processing
The influence of imperfections on achievable secret-key generation rates of
quantum key distribution protocols is investigated. As examples of relevant
imperfections, we consider tagging of Alice's qubits and dark counts at Bob's
detectors, while we focus on a powerful eavesdropping strategy which takes full
advantage of tagged signals. It is demonstrated that error correction and
privacy amplification based on a combination of a two-way classical
communication protocol and asymmetric Calderbank-Shor-Steane codes may
significantly postpone the disastrous influence of dark counts. As a result,
the distances are increased considerably over which a secret key can be
distributed in optical fibres reliably. Results are presented for the
four-state, the six-state, and the decoy-state protocols.Comment: Fully revised version (12 pages and 8 figures). Improved figures and
discussion added. To appear in Eur. Phys. J.
Quantum Kolmogorov Complexity and Quantum Key Distribution
We discuss the Bennett-Brassard 1984 (BB84) quantum key distribution protocol
in the light of quantum algorithmic information. While Shannon's information
theory needs a probability to define a notion of information, algorithmic
information theory does not need it and can assign a notion of information to
an individual object. The program length necessary to describe an object,
Kolmogorov complexity, plays the most fundamental role in the theory. In the
context of algorithmic information theory, we formulate a security criterion
for the quantum key distribution by using the quantum Kolmogorov complexity
that was recently defined by Vit\'anyi. We show that a simple BB84 protocol
indeed distribute a binary sequence between Alice and Bob that looks almost
random for Eve with a probability exponentially close to 1.Comment: typos correcte
Experimental quantum tossing of a single coin
The cryptographic protocol of coin tossing consists of two parties, Alice and
Bob, that do not trust each other, but want to generate a random bit. If the
parties use a classical communication channel and have unlimited computational
resources, one of them can always cheat perfectly. Here we analyze in detail
how the performance of a quantum coin tossing experiment should be compared to
classical protocols, taking into account the inevitable experimental
imperfections. We then report an all-optical fiber experiment in which a single
coin is tossed whose randomness is higher than achievable by any classical
protocol and present some easily realisable cheating strategies by Alice and
Bob.Comment: 13 page
Security of practical private randomness generation
Measurements on entangled quantum systems necessarily yield outcomes that are
intrinsically unpredictable if they violate a Bell inequality. This property
can be used to generate certified randomness in a device-independent way, i.e.,
without making detailed assumptions about the internal working of the quantum
devices used to generate the random numbers. Furthermore these numbers are also
private, i.e., they appear random not only to the user, but also to any
adversary that might possess a perfect description of the devices. Since this
process requires a small initial random seed, one usually speaks of
device-independent randomness expansion.
The purpose of this paper is twofold. First, we point out that in most real,
practical situations, where the concept of device-independence is used as a
protection against unintentional flaws or failures of the quantum apparatuses,
it is sufficient to show that the generated string is random with respect to an
adversary that holds only classical-side information, i.e., proving randomness
against quantum-side information is not necessary. Furthermore, the initial
random seed does not need to be private with respect to the adversary, provided
that it is generated in a way that is independent from the measured systems.
The devices, though, will generate cryptographically-secure randomness that
cannot be predicted by the adversary and thus one can, given access to free
public randomness, talk about private randomness generation.
The theoretical tools to quantify the generated randomness according to these
criteria were already introduced in [S. Pironio et al, Nature 464, 1021
(2010)], but the final results were improperly formulated. The second aim of
this paper is to correct this inaccurate formulation and therefore lay out a
precise theoretical framework for practical device-independent randomness
expansion.Comment: 18 pages. v3: important changes: the present version focuses on
security against classical side-information and a discussion about the
significance of these results has been added. v4: minor changes. v5: small
typos correcte
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