3,108 research outputs found
Composable security of delegated quantum computation
Delegating difficult computations to remote large computation facilities,
with appropriate security guarantees, is a possible solution for the
ever-growing needs of personal computing power. For delegated computation
protocols to be usable in a larger context---or simply to securely run two
protocols in parallel---the security definitions need to be composable. Here,
we define composable security for delegated quantum computation. We distinguish
between protocols which provide only blindness---the computation is hidden from
the server---and those that are also verifiable---the client can check that it
has received the correct result. We show that the composable security
definition capturing both these notions can be reduced to a combination of
several distinct "trace-distance-type" criteria---which are, individually,
non-composable security definitions.
Additionally, we study the security of some known delegated quantum
computation protocols, including Broadbent, Fitzsimons and Kashefi's Universal
Blind Quantum Computation protocol. Even though these protocols were originally
proposed with insufficient security criteria, they turn out to still be secure
given the stronger composable definitions.Comment: 37+9 pages, 13 figures. v3: minor changes, new references. v2:
extended the reduction between composable and local security to include
entangled inputs, substantially rewritten the introduction to the Abstract
Cryptography (AC) framewor
Domain-wall depinning assisted by pure spin currents
We study the depinning of domain walls by pure diffusive spin currents in a
nonlocal spin valve structure based on two ferromagnetic permalloy elements
with copper as the nonmagnetic spin conduit. The injected spin current is
absorbed by the second permalloy structure with a domain wall and from the
dependence of the wall depinning field on the spin current density we find an
efficiency of 6*10^{-14}T/(A/m^2), which is more than an order of magnitude
larger than for conventional current induced domain wall motion. Theoretically
we reproduce this high efficiency, which arises from the surface torques
exerted by the absorbed spin current that lead to efficient depinning.Comment: 11 pages, 3 figures, accepted for publication in Phys. Rev. Let
Hubbard band or oxygen vacancy states in the correlated electron metal SrVO?
We study the effect of oxygen vacancies on the electronic structure of the
model strongly correlated metal SrVO. By means of angle-resolved
photoemission (ARPES) synchrotron experiments, we investigate the systematic
effect of the UV dose on the measured spectra. We observe the onset of a
spurious dose-dependent prominent peak at an energy range were the lower
Hubbard band has been previously reported in this compound, raising questions
on its previous interpretation. By a careful analysis of the dose dependent
effects we succeed in disentangling the contributions coming from the oxygen
vacancy states and from the lower Hubbard band. We obtain the intrinsic ARPES
spectrum for the zero-vacancy limit, where a clear signal of a lower Hubbard
band remains. We support our study by means of state-of-the-art ab initio
calculations that include correlation effects and the presence of oxygen
vacancies. Our results underscore the relevance of potential spurious states
affecting ARPES experiments in correlated metals, which are associated to the
ubiquitous oxygen vacancies as extensively reported in the context of a
two-dimensional electron gas (2DEG) at the surface of insulating
transition metal oxides.Comment: Manuscript + Supplemental Material, 12 pages, 9 figure
FACT -- the First Cherenkov Telescope using a G-APD Camera for TeV Gamma-ray Astronomy (HEAD 2010)
Geiger-mode Avalanche Photodiodes (G-APD) bear the potential to significantly
improve the sensitivity of Imaging Air Cherenkov Telescopes (IACT). We are
currently building the First G-APD Cherenkov Telescope (FACT) by refurbishing
an old IACT with a mirror area of 9.5 square meters and construct a new, fine
pixelized camera using novel G-APDs. The main goal is to evaluate the
performance of a complete system by observing very high energy gamma-rays from
the Crab Nebula. This is an important field test to check the feasibility of
G-APD-based cameras to replace at some time the PMT-based cameras of planned
future IACTs like AGIS and CTA. In this article, we present the basic design of
such a camera as well as some important details to be taken into account.Comment: Poster shown at HEAD 2010, Big Island, Hawaii, March 1-4, 201
On the Gold Standard for Security of Universal Steganography
While symmetric-key steganography is quite well understood both in the
information-theoretic and in the computational setting, many fundamental
questions about its public-key counterpart resist persistent attempts to solve
them. The computational model for public-key steganography was proposed by von
Ahn and Hopper in EUROCRYPT 2004. At TCC 2005, Backes and Cachin gave the first
universal public-key stegosystem - i.e. one that works on all channels -
achieving security against replayable chosen-covertext attacks (SS-RCCA) and
asked whether security against non-replayable chosen-covertext attacks (SS-CCA)
is achievable. Later, Hopper (ICALP 2005) provided such a stegosystem for every
efficiently sampleable channel, but did not achieve universality. He posed the
question whether universality and SS-CCA-security can be achieved
simultaneously. No progress on this question has been achieved since more than
a decade. In our work we solve Hopper's problem in a somehow complete manner:
As our main positive result we design an SS-CCA-secure stegosystem that works
for every memoryless channel. On the other hand, we prove that this result is
the best possible in the context of universal steganography. We provide a
family of 0-memoryless channels - where the already sent documents have only
marginal influence on the current distribution - and prove that no
SS-CCA-secure steganography for this family exists in the standard
non-look-ahead model.Comment: EUROCRYPT 2018, llncs styl
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