185 research outputs found
Universal measurement apparatus controlled by quantum software
We propose a quantum device that can approximate any projective measurement
on a qubit. The desired measurement basis is selected by the quantum state of a
"program register". The device is optimized with respect to maximal average
fidelity (assuming uniform distribution of measurement bases). An interesting
result is that if one uses two qubits in the same state as a program the
average fidelity is higher than if he/she takes the second program qubit in the
orthogonal state (with respect to the first one). The average information
obtainable by the proposed measurements is also calculated and it is shown that
it can get different values even if the average fidelity stays constant.
Possible experimental realization of the simplest proposed device is presented.Comment: 4 pages, 2 figures, reference adde
Experimental asymmetric phase-covariant quantum cloning of polarization qubits
We report on two optical realizations of the asymmetric
phase-covariant cloning machines for polarization states of single photons. The
experimental setups combine two-photon interference and tunable polarization
filtering that enables us to control the asymmetry of the cloners. The first
scheme involves a special unbalanced bulk beam splitter exhibiting different
splitting ratios for vertical and horizontal polarizations, respectively. The
second implemented scheme consists of a balanced fiber coupler where photon
bunching occurs, followed by a free-space part with polarization filters. With
this later approach we were able to demonstrate very high cloning fidelities
which are above the universal cloning limit.Comment: 7 pages, 8 figure
An Ensemble Model with Ranking for Social Dialogue
Open-domain social dialogue is one of the long-standing goals of Artificial
Intelligence. This year, the Amazon Alexa Prize challenge was announced for the
first time, where real customers get to rate systems developed by leading
universities worldwide. The aim of the challenge is to converse "coherently and
engagingly with humans on popular topics for 20 minutes". We describe our Alexa
Prize system (called 'Alana') consisting of an ensemble of bots, combining
rule-based and machine learning systems, and using a contextual ranking
mechanism to choose a system response. The ranker was trained on real user
feedback received during the competition, where we address the problem of how
to train on the noisy and sparse feedback obtained during the competition.Comment: NIPS 2017 Workshop on Conversational A
Quantum cryptography with finite resources: unconditional security bound for discrete-variable protocols with one-way post-processing
We derive a bound for the security of QKD with finite resources under one-way
post-processing, based on a definition of security that is composable and has
an operational meaning. While our proof relies on the assumption of collective
attacks, unconditional security follows immediately for standard protocols like
Bennett-Brassard 1984 and six-states. For single-qubit implementations of such
protocols, we find that the secret key rate becomes positive when at least
N\sim 10^5 signals are exchanged and processed. For any other discrete-variable
protocol, unconditional security can be obtained using the exponential de
Finetti theorem, but the additional overhead leads to very pessimistic
estimates
Homogeneous geodesics of non-unimodular Lorentzian Lie groups and naturally reductive Lorentzian spaces in dimension three
We determine, for all three-dimensional non-unimodular Lie groups equipped
with a Lorentzian metric, the set of homogeneous geodesics through a point.
Together with the results of [C] and [CM2], this leads to the full
classification of three-dimensional Lorentzian g.o. spaces and naturally
reductive spaces
Antiferrodistortive phase transition in EuTiO3
X-ray diffraction, dynamical mechanical analysis and infrared reflectivity
studies revealed an antiferrodistortive phase transition in EuTiO3 ceramics.
Near 300K the perovskite structure changes from cubic Pm-3m to tetragonal
I4/mcm due to antiphase tilting of oxygen octahedra along the c axis (a0a0c- in
Glazer notation). The phase transition is analogous to SrTiO3. However, some
ceramics as well as single crystals of EuTiO3 show different infrared
reflectivity spectra bringing evidence of a different crystal structure. In
such samples electron diffraction revealed an incommensurate tetragonal
structure with modulation wavevector q ~ 0.38 a*. Extra phonons in samples with
modulated structure are activated in the IR spectra due to folding of the
Brillouin zone. We propose that defects like Eu3+ and oxygen vacancies strongly
influence the temperature of the phase transition to antiferrodistortive phase
as well as the tendency to incommensurate modulation in EuTiO3.Comment: PRB, in pres
Continuous Variable Quantum Cryptography using Two-Way Quantum Communication
Quantum cryptography has been recently extended to continuous variable
systems, e.g., the bosonic modes of the electromagnetic field. In particular,
several cryptographic protocols have been proposed and experimentally
implemented using bosonic modes with Gaussian statistics. Such protocols have
shown the possibility of reaching very high secret-key rates, even in the
presence of strong losses in the quantum communication channel. Despite this
robustness to loss, their security can be affected by more general attacks
where extra Gaussian noise is introduced by the eavesdropper. In this general
scenario we show a "hardware solution" for enhancing the security thresholds of
these protocols. This is possible by extending them to a two-way quantum
communication where subsequent uses of the quantum channel are suitably
combined. In the resulting two-way schemes, one of the honest parties assists
the secret encoding of the other with the chance of a non-trivial superadditive
enhancement of the security thresholds. Such results enable the extension of
quantum cryptography to more complex quantum communications.Comment: 12 pages, 7 figures, REVTe
The Security of Practical Quantum Key Distribution
Quantum key distribution (QKD) is the first quantum information task to reach
the level of mature technology, already fit for commercialization. It aims at
the creation of a secret key between authorized partners connected by a quantum
channel and a classical authenticated channel. The security of the key can in
principle be guaranteed without putting any restriction on the eavesdropper's
power.
The first two sections provide a concise up-to-date review of QKD, biased
toward the practical side. The rest of the paper presents the essential
theoretical tools that have been developed to assess the security of the main
experimental platforms (discrete variables, continuous variables and
distributed-phase-reference protocols).Comment: Identical to the published version, up to cosmetic editorial change
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