2,652 research outputs found
Towards Communication-Efficient Quantum Oblivious Key Distribution
Oblivious Transfer, a fundamental problem in the field of secure multi-party
computation is defined as follows: A database DB of N bits held by Bob is
queried by a user Alice who is interested in the bit DB_b in such a way that
(1) Alice learns DB_b and only DB_b and (2) Bob does not learn anything about
Alice's choice b. While solutions to this problem in the classical domain rely
largely on unproven computational complexity theoretic assumptions, it is also
known that perfect solutions that guarantee both database and user privacy are
impossible in the quantum domain. Jakobi et al. [Phys. Rev. A, 83(2), 022301,
Feb 2011] proposed a protocol for Oblivious Transfer using well known QKD
techniques to establish an Oblivious Key to solve this problem. Their solution
provided a good degree of database and user privacy (using physical principles
like impossibility of perfectly distinguishing non-orthogonal quantum states
and the impossibility of superluminal communication) while being loss-resistant
and implementable with commercial QKD devices (due to the use of SARG04).
However, their Quantum Oblivious Key Distribution (QOKD) protocol requires a
communication complexity of O(N log N). Since modern databases can be extremely
large, it is important to reduce this communication as much as possible. In
this paper, we first suggest a modification of their protocol wherein the
number of qubits that need to be exchanged is reduced to O(N). A subsequent
generalization reduces the quantum communication complexity even further in
such a way that only a few hundred qubits are needed to be transferred even for
very large databases.Comment: 7 page
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
Unconditionally secure quantum bit commitment is impossible
The claim of quantum cryptography has always been that it can provide
protocols that are unconditionally secure, that is, for which the security does
not depend on any restriction on the time, space or technology available to the
cheaters. We show that this claim does not hold for any quantum bit commitment
protocol. Since many cryptographic tasks use bit commitment as a basic
primitive, this result implies a severe setback for quantum cryptography. The
model used encompasses all reasonable implementations of quantum bit commitment
protocols in which the participants have not met before, including those that
make use of the theory of special relativity.Comment: 4 pages, revtex. Journal version replacing the version published in
the proceedings of PhysComp96. This is a significantly improved version which
emphasis the generality of the resul
Theory of anomalous magnon softening in ferromagnetic manganites
In metallic manganites with low Curie temperatures, a peculiar softening of
the magnon spectrum close to the magnetic zone boundary has experimentally been
observed. Here we present a theory of the renormalization of the magnetic
excitation spectrum in colossal magnetoresistance compounds. The theory is
based on the modulation of magnetic exchange bonds by the orbital degree of
freedom of double-degenerate e_g electrons. The model considered is an
orbitally degenerate double-exchange system coupled to Jahn-Teller active
phonons which we treat in the limit of strong onsite repulsions. Charge and
coupled orbital-lattice fluctuations are identified as the main origin of the
unusual softening of the magnetic spectrum
Chaos in computer performance
Modern computer microprocessors are composed of hundreds of millions of
transistors that interact through intricate protocols. Their performance during
program execution may be highly variable and present aperiodic oscillations. In
this paper, we apply current nonlinear time series analysis techniques to the
performances of modern microprocessors during the execution of prototypical
programs. Our results present pieces of evidence strongly supporting that the
high variability of the performance dynamics during the execution of several
programs display low-dimensional deterministic chaos, with sensitivity to
initial conditions comparable to textbook models. Taken together, these results
show that the instantaneous performances of modern microprocessors constitute a
complex (or at least complicated) system and would benefit from analysis with
modern tools of nonlinear and complexity science
Quantum Bit Commitment with a Composite Evidence
Entanglement-based attacks, which are subtle and powerful, are usually
believed to render quantum bit commitment insecure. We point out that the no-go
argument leading to this view implicitly assumes the evidence-of-commitment to
be a monolithic quantum system. We argue that more general evidence structures,
allowing for a composite, hybrid (classical-quantum) evidence, conduce to
improved security. In particular, we present and prove the security of the
following protocol: Bob sends Alice an anonymous state. She inscribes her
commitment by measuring part of it in the + (for ) or (for
) basis. She then communicates to him the (classical) measurement outcome
and the part-measured anonymous state interpolated into other, randomly
prepared qubits as her evidence-of-commitment.Comment: 6 pages, minor changes, journal reference adde
Odd C-P contributions to diffractive processes
We investigate contributions to diffractive scattering, which are odd under
C- and P-parity. Comparison of p- and p-p scattering indicates that
these odderon contributions are very small and we show how a diquark clustering
in the proton can explain this effect. A good probe for the odderon exchange is
the photo- and electroproduction of pseudo-scalar mesons. We concentrate on the
pi^0 and show that the quasi elastic pi^0-production is again strongly
suppressed for a diquark structure of the proton whereas the cross sections for
diffractive proton dissociation are larger by orders of magnitude and rather
independent of the proton structure.Comment: 18 pages, LaTex2e, graphicx package, 14 eps figures include
Stagnation and Infall of Dense Clumps in the Stellar Wind of tau Scorpii
Observations of the B0.2V star tau Scorpii have revealed unusual stellar wind
characteristics: red-shifted absorption in the far-ultraviolet O VI resonance
doublet up to +250 km/s, and extremely hard X-ray emission implying gas at
temperatures in excess of 10^7 K. We describe a phenomenological model to
explain these properties. We assume the wind of tau Sco consists of two
components: ambient gas in which denser clumps are embedded. The clumps are
optically thick in the UV resonance lines primarily responsible for
accelerating the ambient wind. The reduced acceleration causes the clumps to
slow and even infall, all the while being confined by the ram pressure of the
outflowing ambient wind. We calculate detailed trajectories of the clumps in
the ambient stellar wind, accounting for a line radiation driving force and the
momentum deposited by the ambient wind in the form of drag. We show these
clumps will fall back towards the star with velocities of several hundred
km/sec for a broad range of initial conditions. The infalling clumps produce
X-ray emitting plasmas with temperatures in excess of (1-6)x10^7 K in bow
shocks at their leading edge. The infalling material explains the peculiar
red-shifted absorption wings seen in the O VI doublet. The required mass loss
in clumps is 3% - 30% ofthe total mass loss rate. The model developed here can
be generally applied to line-driven outflows with clumps or density
irregularities. (Abstract Abridged)Comment: To appear in the ApJ (1 May 2000). 24 pages, including 6 embedded
figure
Is Quantum Bit Commitment Really Possible?
We show that all proposed quantum bit commitment schemes are insecure because
the sender, Alice, can almost always cheat successfully by using an
Einstein-Podolsky-Rosen type of attack and delaying her measurement until she
opens her commitment.Comment: Major revisions to include a more extensive introduction and an
example of bit commitment. Overlap with independent work by Mayers
acknowledged. More recent works by Mayers, by Lo and Chau and by Lo are also
noted. Accepted for publication in Phys. Rev. Let
Strong and Electromagnetic Decays of Two New Baryons
Two recently discovered excited charm baryons are studied within the
framework of Heavy Hadron Chiral Perturbation Theory. We interpret these new
baryons which lie 308 \MeV and 340 \MeV above the as
members of a P-wave spin doublet. Differential and total decay rates for their
double pion transitions down to the ground state are calculated.
Estimates for their radiative decay rates are also discussed. We find that the
experimentally determined characteristics of the baryons may be
simply understood in the effective theory.Comment: 16 pages with 4 figures not included but available upon request,
CALT-68-191
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