31,739 research outputs found
A proposal for founding mistrustful quantum cryptography on coin tossing
A significant branch of classical cryptography deals with the problems which
arise when mistrustful parties need to generate, process or exchange
information. As Kilian showed a while ago, mistrustful classical cryptography
can be founded on a single protocol, oblivious transfer, from which general
secure multi-party computations can be built.
The scope of mistrustful quantum cryptography is limited by no-go theorems,
which rule out, inter alia, unconditionally secure quantum protocols for
oblivious transfer or general secure two-party computations. These theorems
apply even to protocols which take relativistic signalling constraints into
account. The best that can be hoped for, in general, are quantum protocols
computationally secure against quantum attack. I describe here a method for
building a classically certified bit commitment, and hence every other
mistrustful cryptographic task, from a secure coin tossing protocol. No
security proof is attempted, but I sketch reasons why these protocols might
resist quantum computational attack.Comment: Title altered in deference to Physical Review's fear of question
marks. Published version; references update
Beating the PNS attack in practical quantum cryptography
In practical quantum key distribution, weak coherent state is often used and
the channel transmittance can be very small therefore the protocol could be
totally insecure under the photon-number-splitting attack. We propose an
efficient method to verify the upper bound of the fraction of counts caused by
multi-photon pluses transmitted from Alice to Bob, given whatever type of Eve's
action. The protocol simply uses two coherent states for the signal pulses and
vacuum for decoy pulse. Our verified upper bound is sufficiently tight for QKD
with very lossy channel, in both asymptotic case and non-asymptotic case. The
coherent states with mean photon number from 0.2 to 0.5 can be used in
practical quantum cryptography. We show that so far our protocol is the
decoy-state protocol that really works for currently existing set-ups.Comment: So far this is the unique decoy-state protocol which really works
efficiently in practice. Prior art results are commented in both main context
and the Appendi
Entanglement of 2xK quantum systems
We derive an analytical expression for the lower bound of the concurrence of
mixed quantum states of composite 2xK systems. In contrast to other, implicitly
defined entanglement measures, the numerical evaluation of our bound is
straightforward. We explicitly evaluate its tightness for general mixed states
of 2x3 systems, and identify a large class of states where our expression gives
the exact value of the concurrence.Comment: 7 pages, 1 figure, to be published in Europhysics Lette
Coin Tossing is Strictly Weaker Than Bit Commitment
We define cryptographic assumptions applicable to two mistrustful parties who
each control two or more separate secure sites between which special relativity
guarantees a time lapse in communication. We show that, under these
assumptions, unconditionally secure coin tossing can be carried out by
exchanges of classical information. We show also, following Mayers, Lo and
Chau, that unconditionally secure bit commitment cannot be carried out by
finitely many exchanges of classical or quantum information. Finally we show
that, under standard cryptographic assumptions, coin tossing is strictly weaker
than bit commitment. That is, no secure classical or quantum bit commitment
protocol can be built from a finite number of invocations of a secure coin
tossing black box together with finitely many additional information exchanges.Comment: Final version; to appear in Phys. Rev. Let
Template-based Gravitational-Wave Echoes Search Using Bayesian Model Selection
The ringdown of the gravitational-wave signal from a merger of two black
holes has been suggested as a probe of the structure of the remnant compact
object, which may be more exotic than a black hole. It has been pointed out
that there will be a train of echoes in the late-time ringdown stage for
different types of exotic compact objects. In this paper, we present a
template-based search methodology using Bayesian statistics to search for
echoes of gravitational waves. Evidence for the presence or absence of echoes
in gravitational-wave events can be established by performing Bayesian model
selection. The Occam factor in Bayesian model selection will automatically
penalize the more complicated model that echoes are present in
gravitational-wave strain data because of its higher degree of freedom to fit
the data. We find that the search methodology was able to identify
gravitational-wave echoes with Abedi et al.'s echoes waveform model about 82.3%
of the time in simulated Gaussian noise in the Advanced LIGO and Virgo network
and about 61.1% of the time in real noise in the first observing run of
Advanced LIGO with significance. Analyses using this method are
performed on the data of Advanced LIGO's first observing run, and we find no
statistical significant evidence for the detection of gravitational-wave
echoes. In particular, we find combined evidence of the three events
in Advanced LIGO's first observing run. The analysis technique developed in
this paper is independent of the waveform model used, and can be used with
different parametrized echoes waveform models to provide more realistic
evidence of the existence of echoes from exotic compact objects.Comment: 16 pages, 6 figure
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
On the communication cost of entanglement transformations
We study the amount of communication needed for two parties to transform some
given joint pure state into another one, either exactly or with some fidelity.
Specifically, we present a method to lower bound this communication cost even
when the amount of entanglement does not increase. Moreover, the bound applies
even if the initial state is supplemented with unlimited entanglement in the
form of EPR pairs, and the communication is allowed to be quantum mechanical.
We then apply the method to the determination of the communication cost of
asymptotic entanglement concentration and dilution. While concentration is
known to require no communication whatsoever, the best known protocol for
dilution, discovered by Lo and Popescu [Phys. Rev. Lett. 83(7):1459--1462,
1999], requires a number of bits to be exchanged which is of the order of the
square root of the number of EPR pairs. Here we prove a matching lower bound of
the same asymptotic order, demonstrating the optimality of the Lo-Popescu
protocol up to a constant factor and establishing the existence of a
fundamental asymmetry between the concentration and dilution tasks.
We also discuss states for which the minimal communication cost is
proportional to their entanglement, such as the states recently introduced in
the context of ``embezzling entanglement'' [W. van Dam and P. Hayden,
quant-ph/0201041].Comment: 9 pages, 1 figure. Added a reference and some further explanations.
In v3 some arguments are given in more detai
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