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that stringent requirements on indistinguishability and collection efficiency
of the generated photons are met[1]. We show that these are conflicting
requirements for the recently demonstrated quantum dot single
photon sources that are incoherently pumped via reservoirs[2].
As a model for a coherently pumped single photon source, we consider
cavity-assisted Raman transitions in a single three-level system in the
Λ-configuration[3].We demonstrate that using such a source, arbitrarily
high collection efficiency and indistinguishability of the generated photons
can be obtained simultaneously with increased cavity coupling. In
the absence of dephasing, our calculations reveal maximum collection efficiency
and indistinguishability of 99 % for a Purcell factor of 40, without
necessitating the strong coupling limit. This regime, that is readily available
in current cavity-QED experiments using atoms, can potentially be
reached with a single quantum dot embedded in a microcavity.
[1] E. Knill et al., Nature 409, 46 (2001).
[2] P. Michler et al., Science 290, 2282 (2000).
[3] A. Kuhn et al., Phys. Rev. Lett. 89, 067901 (2002).
Q 45.3 Fr 11:45 HS 101
Photon polarisation entanglement from distant dipole sources
— •Almut Beige 1,2 and Yuan Liang Lim 1 — 1 Blackett Laboratory,
Imperial College London, London, UK — 2 Centre for Quantum Information,
Department of Applied Mathematics and Theoretical Physics,
University of Cambridge, Cambridge, UK
It is commonly believed that photon polarisation entanglement can
only be obtained via pair creation within the same source or via postselective
measurements on photons that overlapped within their coherence
time inside a linear optics setup. In contrast to this, we show here that
polarisation entanglement can also be produced by distant single photon
sources and without the photons ever having to meet, if the detection
of a photon does not reveal its origin – the which way information. In
the case of two sources, the entanglement arises under the condition of
two emissions in certain spatial directions and leaves the dipoles in a
maximally entangled state [1].
[1] Y. L. Lim and A. Beige, quant-ph/0308095.
Q 45.4 Fr 12:00 HS 101
Upper bounds on success probabilities in linear optics quantum
gates — •Stefan Scheel 1 and Norbert Lütkenhaus 2 — 1 Quantum
Optics and Laser Science, Blackett Laboratory, Imperial College London,
Prince Consort Road, London SW7 2BW, UK — 2 QIT group, Institut für
Theoretische Physik I, Universität Erlangen-Nürnberg, 91058 Erlangen
We develop an abstract way of defining linear-optics networks designed
to perform certain quantum gates. We will be mainly concernced with
the nonlinear sign shift (NS) gate but it will become obvious that all
other gates can be treated in a similar manner. The abstract scheme is
extremely well suited for analytical as well as numerical investigations
since it does not depend on the number of ancilla modes at all. With
that approch we analyse numerically and partially analytically the NS
gate and show evidence that indicates that the success probability of that
gate does not exceed 1/4. This is to our knowledge the strongest bound
to date.
Q 45.5 Fr 12:15 HS 101
Experimentelle vollständige Bell-Zustandsanalyse — •Gerhard
Huber 1 , Carsten Schuck 1 , Oliver Schulz 1 , Christian
Kurtsiefer 2 und Harald Weinfurter 1,3 — 1 Ludwig-Maximilians-
Universität, München — 2 Department of Physics, National University
of Singapore — 3 Max-Planck-Institut für Quantenoptik, Garching
In vielen Anwendungen der Quanteninformation und -kommunikation
werden die vier maximal verschränkten Bell-Zustände zur Kodierung der
Information zweier Qubits verwendet. Eine zentrale Aufgabe dabei ist es,
die orthogonalen Bell-Zustände unterscheidbar zu detektieren. Wie kürzlich
gezeigt wurde [1], ist dies mit linearen optischen Elementen nicht
zu bewerkstelligen. Nur zwei der vier Zustände können mit Sicherheit
unterschieden werden, bzw. im nichtdeterministischen Fall ist die Wahrscheinlichkeit,
einen Bell-Zustand richtig zu klassifizieren, nicht größer
als 50%.
Die heute gebräuchlichste Methode zur Generierung verschränkter
Photonen ist die spontane parametrische Fluoreszenz, wobei das Photon
eines Pumpstrahls in einem nichtlinearen Medium spontan in zwei Photonen
halber Energie konvertiert. Aufgrund des Entstehungsprozesses sind
diese Photonen nicht nur in ihrer Polarisation verschränkt – in diesem
Freiheitsgrad wird die eigentliche Information kodiert – sondern auch in
Energie bzw. Zeit. Indem wir uns diesen Umstand zunutze machen ist es
uns erstmals möglich, die vollständige Bell-Zustandsanalyse mit linearoptischen
Komponenten durchzuführen, d.h. jeden der vier Zustände mit
Sicherheit von den anderen zu unterscheiden.
[1] J. Calsamiglia, N. Lütkenhaus, Appl. Phys. B 72, 67 (2001)
Q 45.6 Fr 12:30 HS 101
Continuous-variable experiments with a nonlocal single photon
— •Sergei Babitchev, Jürgen Appel, and Alexander I.
Lvovsky — Fachbereich Physik M559, Universität Konstanz
A dual-mode optical qubit, created by overlapping a single photon from
a parametric down-conversion source and a vacuum mode on a beam
splitter, is characterized by means of homodyne tomography. The reconstructed
four-dimensional density matrix extends over the entire Hilbert
space and thus reveals, for the first time, complete information about
the qubit as a state of the electromagnetic state. A nonlocal character
of the reconstructed ensemble is demonstrated by a violation of the Bell
inequality for the quadrature data converted to a dichotomic format.
A remote preparation scheme of an arbitrary single-mode qubit has
been implemented: a quadrature measurement, performed on one of the
spatial modes of the nonlocal single-photon state, collapses it into a superposition
of the single photon and the vacuum state in the other mode.
Experimental results show that the preparation efficiency of the created
qubit can be higher than that of the initial single photon.
Q 45.7 Fr 12:45 HS 101
Continuous variable quantum erasing — •Ulrik Andersen
1 , Olivier Glöckl 1 , Stefan Lorenz 1 , Radim Filip 2 , and
Gerd Leuchs 1 — 1 Institute of Optics, Information and Photonics,
Max-Planck Research Group, University of Erlangen-Nuernberg,
Staudtstr. 7/B2, D-91058 Erlangen,Germany — 2 Department of
Optics,Research Center for Optics,Palacky University,17. listopadu
50,77200 Olomouc,Czech Republic
Due to quantum complementarity the perfect measurement of two conjugate
variables is not possible. In a double slit experiment, this means
that the measurement of the which-way information will destroy the interference
information. However, if one manages to erase the which-way
information, the interference can be revived [1]. In the continuous variable
quantum erasing scheme [2] we consider a Gaussian light state which
can be characterized by two conjugate quadrature variables: the amplitude
and the phase quadrature. Information retrieval can be obtained by
implementing a quantum nondemolition measurement. Performing such
a measurement on the amplitude quadrature gives rise to excess noise
in the phase quadrature according to Heisenbergs uncertainty relation.
However, if the amplitude quadrature is erased, it is possible to revive
the phase information by correlating the outputs. The experimental realization
of such a scheme is presented. [1] M.O. Scully et al., Nature
351,111 (1991) [2] R. Filip, Phys. Rev. A 67, 42111 (2003)
Q 46 Laser in der Medizin und Umweltmesstechnik
Zeit: Freitag 11:00–13:15 Raum: HS 204
Q 46.1 Fr 11:00 HS 204
Laserosteotomie mit gepulsten CO2-Lasern — •Martin Werner
1,2 , Said Afilal 1,2 , Manfred Klasing 1 , Mikhail Ivanenko 1 ,
Peter Hering 1,2 und Antje Rätzer-Scheibe 1 — 1 Lasertechnologie
Gruppe, Forschungszentrum caesar, Bonn (www.caesar.de) — 2 Institut
für Lasermedizin, Universität Düsseldorf (www.ilm.uni-duesseldorf.de)
Die schonende Bearbeitung von Knochengewebe mit Lasern ist eine
Herausforderung, da Knochengewebe auf der einen Seite aus sehr tempe-
130
raturbeständigem Hydroxylapatit besteht, welches einen Schmelzpunkt
von über 1200 ◦ C hat, auf der anderen Seite aber lebende Zellen beherbergt,
die schon bei leichten Temperaturerhöhungen geschädigt werden.
Eine biologisch sehr gut verträgliche Möglichkeit Knochen abzutragen
bieten gepulste CO2-Laser, die zusammen mit einem feinem Luftwasserspray
verwendet werden. Das Modell des Ablationsprozesses basiert darauf,
dass das Wasser im Knochen schnell verdampft wird und bei seiner
Ausdehnung den Knochen lokal in Mikroexplosionen zerreißt.