136 research outputs found
Maximum efficiency of a linear-optical Bell-state analyzer
In a photonic realization of qubits the implementation of quantum logic is
rather difficult due the extremely weak interaction on the few photon level. On
the other hand, in these systems interference is available to process the
quantum states. We formalize the use of interference by the definition of a
simple class of operations which include linear optical elements, auxiliary
states and conditional operations.
We investigate an important subclass of these tools, namely linear optical
elements and auxiliary modes in the vacuum state. For this tools, we are able
to extend a previous quantitative result, a no-go theorem for perfect Bell
state analyzer on two qubits in polarization entanglement, by a quantitative
statement. We show, that within this subclass it is not possible to
discriminate unambiguously four equiprobable Bell states with a probability
higher than 50 %.Comment: 6 pages, 2 fig
Quantum benchmarks for the storage or transmission of quantum light from minimal resources
We investigate several recently published benchmark criteria for storage or
transmission of continuous-variable quantum information. A comparison reveals
that criteria based on a Gaussian distribution of coherent states are most
resilient to noise. We then address the issue of experimental resources and
derive an equally strong benchmark, solely based on three coherent states and
homodyne detection. This benchmark is further simplified in the presence of
naturally occurring random phases, which remove the need for active input-state
modulation.Comment: replaced by the published version, 5 pages, 4 figure
Nonlinear entanglement witnesses
Entanglement detection typically relies on linear inequalities for mean
values of certain observables (entanglement witnesses), where violation
indicates entanglement. We provide a general method to improve any of these
inequalities for bipartite systems via nonlinear expressions. The
nonlinearities are of different orders and can be directly measured in
experiments, often without any extra effort.Comment: 4 pages, 1 figure, v3: small changes, final versio
Unconditional Security of the Bennett 1992 quantum key-distribution over lossy and noisy channel
We show that the security proof of the Bennett 1992 protocol over loss-free
channel in (K. Tamaki, M. Koashi, and N. Imoto, Phys. Rev. Lett. 90, 167904
(2003)) can be adapted to accommodate loss. We assumed that Bob's detectors
discriminate between single photon states on one hand and vacuum state or
multi-photon states on the other hand.Comment: 5 pages, 2 figures. We have presented this topic at QIPC 2003 as a
poster sessio
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