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ods are limited due to the loss in the quantum channel (e.g. loss in the
optical fiber) and in the single-photon counters of the receivers. One can
argue that the loss in the detectors cannot be changed by an eavesdropper
in order to increase the covered distance. Here we show that the
security analysis of this scenario is not as easy as is commonly assumed,
since already two-photon processes allow eavesdropping strategies that
outperform the known photon-number splitting attack. For this reason
there is, so far, no satisfactory security analysis available in the framework
of individual attacks.
[1] B. Huttner, N. Imoto, N. Gisin and T. Mor, Phys. Rev. A 51, 1863
(1995); H. Inamori, N. Lütkenhaus and D. Mayers, quant-ph/0107017.
[2] M. Curty and N. Lütkenhaus, quant-ph/0311066.
Q 7.3 Mo 17:00 HS 101
Robustness of the BB84 Quantum Key Distribution Against
Collective Attacks — •Georgios Nikolopoulos and Gernot Alber
— Institut für Angewandte Physik, Technische Universität Darmstadt,
D-64289 Darmstadt
Quantum key distribution protocols exploit quantum correlations in
order to establish a secure key between two legitimate users. Recent work
on quantum cryptography [1-3] has revealed a remarkable link between
quantum and secret correlations. So far, it is not known, however, what is
the maximum disturbance (upper robustness bound) up to which Alice
and Bob can share non-classical correlations and what an eavesdropper
can achieve by employing an appropriately optimized strategy. We
present recent results concerning these upper robustness bounds for the
BB84 protocol under the assumption of general coherent eavesdropping
strategies.
/1/ M. Curty, M. Lewenstein, and N. Lütkenhaus, quant-ph/0307151
(2003).
/2/ A. Acin and N. Gisin, quant-ph/0310054 (2003).
/3/ A. Acin, L. Masanes, and N. Gisin, Phys. Rev. Let. 91, 167901 (2003).
Q 7.4 Mo 17:15 HS 101
Multiplexed quantum cryptography with cascaded photons
from a single quantum dot — •Thomas Aichele 1 , Valéry
Zwiller 2 , Gaël Reinaudi 1 , and Oliver Benson 1 — 1 Humboldt-
Universität zu Berlin, Nano-Optik — 2 Ecole Polytechnique Fédérale de
Lausanne, Switzerland
Efficient generation of single photons is an important task for modern
quantum technology such as quantum computation and quantum
cryptography. In quantum cryptography with single photon sources the
transition lifetime limits the maximum achievable transmission rate of
quantum bits.
Here we present and demonstrate a method to further enhance the
effective transmission rate by using multiplexing similar as in classical
communication channels. This method relies on cascaded multi-photon
emission that is observed in single InP quantum dots (QDs). Cross correlations
between different photoluminescence spectral lines prove the excistence
of this cascade and also reveal their dynamics. Auto-correlation
measurements on both the individual spectral lines as on the multiplexed
signal show single photon statistics. As a proof of principle we implemented
a BB84 quantum cryptography setup using this multiplexing
technique.
Q 7.5 Mo 17:30 HS 101
A Compact Source for Polarization Entangled Photon Pairs
— •Hannes Böhm 1 , Andreas Poppe 1 , Markus Aspelmeyer 1 ,
Thomas Jennewein 1 , and Anton Zeilinger 1,2 — 1 Institut fuer Experimentalphysik,
University of Vienna, Austria — 2 Institute for Quantum
Optics and Quantum Information, Austrian Academy of Sciences
We present a compact source for polarization-entangled photon pairs
based on spontaneous parametric down-conversion (SPDC). Its small
size, low power consumption and high efficiency make it a versatile tool
for quantum communication experiments. In a recent outdoor experiment
it was used to demonstrate the distribution of entanglement between two
independent observers separated by 600m using optical free space links.
Q 7.6 Mo 17:45 HS 101
Measurements for quantum communication using linear
optics — •Philippe Raynal, Peter van Loock, and Norbert
Lütkenhaus — Institut für Theoretische Physik I, Universität
Erlangen-Nürnberg, Staudtstrasse, 7/B1 91058 Erlangen, Deutschland
82
There is a set of simple criteria to decide whether a given projection
measurement can be, in principle, exactly implemented solely by means
of linear optics utilizing photon counting or homodyne detections [1].
The linear-optics toolbox includes auxiliary photons and conditional dynamics,
i.e., the adjustment of subsequent linear-optics transformations
of the conditional states after measuring a first mode. Here we add more
tools such as phase-space displacements and squeezing and incorporate
them into the criteria. This enables one to treat important measurements
for quantum communication in the continuous-variable regime where the
signal states have an unfixed and unbounded photon number. We further
discuss the extension of our approach to generalized measurements
(POVM’s).
[1] P. van Loock, N. Lütkenhaus, PRA (in press), quant-ph/0304057.
Q 7.7 Mo 18:00 HS 101
Experimental quantum communication complexity —
•Pavel Trojek 1,2 , Christian Schmid 1,2 , Mohamed Bourennane
1,2 , Marek ˇ Zukowski 3 , and Harald Weinfurter 1,2 —
1 Ludwig-Maximilians-Universität, D-80797 München, Germany —
2 Max-Planck-Institut für Quantenoptik, D-85748 Garching, — 3 Instytut
Fizyki Teoretycznej i Astrofizyki Uniwersytet Gdański, PL-80-952
Gdańsk, Poland
Quantum communication complexity is one of the promising applications
in quantum information processing. It tackles the problem of
communication reduction during distributed computation tasks by the
utilization of quantum effects.
We report on the experimental realization of one-qubit communication
complexity protocol, in which five parties must determine in common
the correct value of a specific Boolean function with the highest possible
probability of success. This function depends on personal random data
distributed initially to each party. To accomplish the task the parties can
communicate sequentially only one qubit.
The protocol is implemented using single photon passing through all
parties and the last one performs an analysis of a photon polarization
state. The personal information of each party is encoded via a phase
transformation to the photon state. For a fair comparison with classical
one-bit protocol, no experimental runs are discarded, even if the detection
of photon fails. The obtained results enabled us to demonstrate for
the first time the superiority of quantum communication over its classical
counterpart for a broad class of distributed computation tasks.
Q 7.8 Mo 18:15 HS 101
Breaking entanglement in noisy channels — •Thomas Konrad 1 ,
Jürgen Audretsch 1 , and Lajos Diosi 2 — 1 Fachbereich Physik
Universität Konstanz — 2 Research Institute for Particle and Nuclear
Physics, Budapest
How noisy is a quantum communication channel allowed to be before
it completely destroys the entanglement between two quantum systems,
one of which traverses the channel? We address this question by relating
a rather abstract criterion [1] for entanglement breaking maps to a quantity
which characterizes the channel noise . On the way some properties
of entanglement breaking maps are reviewed.
[1] M. Horodecki, P.W. Shor and M.B.Ruskai, ”Entanglement Breaking
Channels” quant-ph/0302031
Q 7.9 Mo 18:30 HS 101
Quantum Cellular Automata and the Capacity of Quantum
Channels with Memory — •Dennis Kretschmann, Dirk
Schlingemann, and Reinhard F. Werner — Institut für Mathematische
Physik, Technische Universität Braunschweig, Mendelssohnstr.
3, 38106 Braunschweig, Germany
Any processing of quantum information, be it storage or transfer, can
be represented by a quantum channel. Quantum channel capacity expresses
quantitatively how much quantum information can be stored in
a physical device, or sent down a transmission line: it is the maximal
number of qubit transmissions per use of the channel, taken in the limit
of long messages.
Up to now most of the work on quantum channels has focused on
memoryless channels, which are characterized by the requirement that
successive channel inputs are acted on independently. In many real-world
settings this assumption cannot be justified. In this talk we show how to
describe and characterize quantum channels with memory, emphasizing
and exploiting connections with the theory of quantum cellular automata
and the physics of spin chains.