162 research outputs found
Strong quantitative benchmarking of quantum optical devices
Quantum communication devices, such as quantum repeaters, quantum memories,
or quantum channels, are unavoidably exposed to imperfections. However, the
presence of imperfections can be tolerated, as long as we can verify such
devices retain their quantum advantages. Benchmarks based on witnessing
entanglement have proven useful for verifying the true quantum nature of these
devices. The next challenge is to characterize how strongly a device is within
the quantum domain. We present a method, based on entanglement measures and
rigorous state truncation, which allows us to characterize the degree of
quantumness of optical devices. This method serves as a quantitative extension
to a large class of previously-known quantum benchmarks, requiring no
additional information beyond what is already used for the non-quantitative
benchmarks.Comment: 11 pages, 7 figures. Comments are welcome. ver 2: Improved figures,
no changes to main tex
Study of the Distillability of Werner States Using Entanglement Witnesses and Robust Semidefinite Programs
We use Robust Semidefinite Programs and Entanglement Witnesses to study the
distillability of Werner states. We perform exact numerical calculations which
show 2-undistillability in a region of the state space which was previously
conjectured to be undistillable. We also introduce bases which yield
interesting expressions for the {\em distillability witnesses} and for a tensor
product of Werner states with arbitrary number of copies.Comment: 16 pages, 2 figure
Entanglement verification for quantum key distribution systems with an underlying bipartite qubit-mode structure
We consider entanglement detection for quantum key distribution systems that
use two signal states and continuous variable measurements. This problem can be
formulated as a separability problem in a qubit-mode system. To verify
entanglement, we introduce an object that combines the covariance matrix of the
mode with the density matrix of the qubit. We derive necessary separability
criteria for this scenario. These criteria can be readily evaluated using
semidefinite programming and we apply them to the specific quantum key
distribution protocol.Comment: 6 pages, 2 figures, v2: final versio
One-way quantum key distribution: Simple upper bound on the secret key rate
We present a simple method to obtain an upper bound on the achievable secret
key rate in quantum key distribution (QKD) protocols that use only
unidirectional classical communication during the public-discussion phase. This
method is based on a necessary precondition for one-way secret key
distillation; the legitimate users need to prove that there exists no quantum
state having a symmetric extension that is compatible with the available
measurements results. The main advantage of the obtained upper bound is that it
can be formulated as a semidefinite program, which can be efficiently solved.
We illustrate our results by analysing two well-known qubit-based QKD
protocols: the four-state protocol and the six-state protocol. Recent results
by Renner et al., Phys. Rev. A 72, 012332 (2005), also show that the given
precondition is only necessary but not sufficient for unidirectional secret key
distillation.Comment: 11 pages, 1 figur
Bell inequalities for three systems and arbitrarily many measurement outcomes
We present a family of Bell inequalities for three parties and arbitrarily
many outcomes, which can be seen as a natural generalization of the Mermin Bell
inequality. For a small number of outcomes, we verify that our inequalities
define facets of the polytope of local correlations. We investigate the quantum
violations of these inequalities, in particular with respect to the Hilbert
space dimension. We provide strong evidence that the maximal quantum violation
can only be reached using systems with local Hilbert space dimension exceeding
the number of measurement outcomes. This suggests that our inequalities can be
used as multipartite dimension witnesses.Comment: v1 6 pages, 4 tables; v2 Published version with minor typos correcte
Quantum benchmarking with realistic states of light
The goal of quantum benchmarking is to certify that imperfect quantum
communication devices (e.g., quantum channels, quantum memories, quantum key
distribution systems) can still be used for meaningful quantum communication.
However, the test states used in quantum benchmarking experiments may be
imperfect as well. Many quantum benchmarks are only valid for states which
match some ideal form, such as pure states or Gaussian states. We outline how
to perform quantum benchmarking using arbitrary states of light. We demonstrate
these results using real data taken from a continuous-variable quantum memory.Comment: 14 pages, 3 figures. Updated to more closely match the published
versio
Chaotic Observer-based Synchronization Under Information Constraints
Limit possibilities of observer-based synchronization systems under
information constraints (limited information capacity of the coupling channel)
are evaluated. We give theoretical analysis for multi-dimensional
drive-response systems represented in the Lurie form (linear part plus
nonlinearity depending only on measurable outputs). It is shown that the upper
bound of the limit synchronization error (LSE) is proportional to the upper
bound of the transmission error. As a consequence, the upper and lower bounds
of LSE are proportional to the maximum rate of the coupling signal and
inversely proportional to the information transmission rate (channel capacity).
Optimality of the binary coding for coders with one-step memory is established.
The results are applied to synchronization of two chaotic Chua systems coupled
via a channel with limited capacity.Comment: 7 pages, 6 figures, 27 reference
Truncated su(2) moment problem for spin and polarization states
We address the problem whether a given set of expectation values is
compatible with the first and second moments of the generic spin operators of a
system with total spin j. Those operators appear as the Stokes operator in
quantum optics, as well as the total angular momentum operators in the atomic
ensemble literature. We link this problem to a particular extension problem for
bipartite qubit states; this problem is closely related to the symmetric
extension problem that has recently drawn much attention in different contexts
of the quantum information literature. We are able to provide operational,
approximate solutions for every large spin numbers, and in fact the solution
becomes exact in the limiting case of infinite spin numbers. Solutions for low
spin numbers are formulated in terms of a hyperplane characterization, similar
to entanglement witnesses, that can be efficiently solved with semidefinite
programming.Comment: 18 pages, 1 figur
Algorithm Engineering in Robust Optimization
Robust optimization is a young and emerging field of research having received
a considerable increase of interest over the last decade. In this paper, we
argue that the the algorithm engineering methodology fits very well to the
field of robust optimization and yields a rewarding new perspective on both the
current state of research and open research directions.
To this end we go through the algorithm engineering cycle of design and
analysis of concepts, development and implementation of algorithms, and
theoretical and experimental evaluation. We show that many ideas of algorithm
engineering have already been applied in publications on robust optimization.
Most work on robust optimization is devoted to analysis of the concepts and the
development of algorithms, some papers deal with the evaluation of a particular
concept in case studies, and work on comparison of concepts just starts. What
is still a drawback in many papers on robustness is the missing link to include
the results of the experiments again in the design
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