Deutsche Physikalische Gesellschaft e. V. (DPG)

WWW-Server für Frühjahrstagungen und E-Verhandlungen

E-Verhandlungen 2001
Programm und Abstracts der Sitzung Q 22

Nonclassical interference from coherently driven 2-level atoms

•Christoph Skornia^1,2, Joachim von Zanthier¹, Girish S. Agarwal^1,3, Ersnt Werner² und Herbert Walther^1
1Max-Planck-Institut für Quantenoptik and Sektion Physik der LMU München, D-85748 Garching, Germany
²Institut für Theoretische Physik, Universität Regensburg, D-93040 Regensburg, Germany
³Physical Research Laboratory, Navrangpura, Ahmedabad-380 009, India

We show analytically the existence of nonclassical correlations in the radiation of two 2-level atoms which are coherently driven by a continuous laser source [1]. The 2 photon correlation function of the fluorescence light exhibits a spatial interference pattern which is not present in classical radiation. An aspect of the phenomenon is that bunched and antibunched light is emitted in different spatial directions. The observed features change with the distance of the atoms. The correlations are induced by a state reduction due to the measurement process when the detection of the photons does not distinguish between the individual atoms. It is interesting to note that the observed phenomena show up even without the assumption of interatomic interaction.

[1] C. Skornia, J. von Zanthier, G. S. Agarwal, E. Werner, H. Walther, submitted to Phys. Rev. Lett.

Quanteneffekt der Messung auf Subsysteme eines zusammengesetzten Objekts

•Sascha Wallentowitz
Fachbereich Physik, Universität Rostock, Universitätsplatz 3, D-18051 Rostock

Die mit der Messung einer kollektiven Observablen assoziierte Zustandsprojektion wird für das einzelne Subsystem eines zusammengesetzten Objekts untersucht. Dabei ergibt sich wiederum eine Projektion des Subsystems, allerdings modifiziert durch eine Nichtlinearität, die vom Erwartungswert der Observablen bestimmt wird. Mit Hilfe der entwickelten theoretischen Beschreibung kann die Frage erörtert werden, wie sich z.B. der Quantenzustand des einzelnen, gemittelten Atoms in einem N-Atom System ändert, wenn an dem zugrundeliegenden Gesamtsystem eine Messung einer additiven Observablen stattfindet. Die physikalische Interpretation der erwähnten nichtlinearen Modifikation und deren Abhängigkeit von der Atomzahl N liefert eine klare Einsicht in die projektive Einzelatom Dynamik.

Efficient Raman side band generation in a coherent atomic medium

•A. F. Huss, N. Peer, R. Lammegger, E. A. Korsunsky und L. Windholz
Institut f. Experimentalphysik, Technische Universität Graz, Österreich

Excitation of a L-type atomic level system by two resonant laser frequencies w₁ and w₂ results in the creation of a superposition dark state via destructive quantum interference. Preparation of atoms in this dark state through optical pumping leads to reduced absorption and an enhanced nonlinear optical susceptibility, thus nonlinear optical processes become efficient. Atoms in this dark state act as a local oscillator at frequency w₁₂ = w₂ - w₁. A third frequency w₃ beats against the local oscillator producing sum- and difference frequencies w₃ ±nw₁₂, i.e. Raman side bands.

We experimentally demonstrate the generation of Raman side bands in sodium atomic vapor excited on D1 line. First order side bands for w₁,w₂ and up to third order side bands for w₃ have been observed. A small dark state decay rate of only a few kHz ensures high generation efficiency even at low input light power (10 mW).

Parametric Dispersion in a Coherent Population System

•Mario Müller, Rolf-Hermann Rinkleff und Karsten Danzmann
Institut für Atom-und Molekülphysik, Universität Hannover, Callinstr. 38, D-30167 Hannover, Germany

We report on recent experiments on the investigation of absorptive and dispersive properties of a coherently prepared medium, that shows electromagnetically induced transparency on a extremely narrow dark resonance. We have carried out the experimental study of a two photon excitation via quantum interference in a three level cesium atomic system. The phase shift of the coupling laser field was measured with a three beam heterodyne interferometer [1] relative to an off-resonant reference laser field in dependence of the frequency of the probe laser field acting on the second transition. The power of the coupling and the probe field was varied over several orders of magnitude. Even with coupling powers in the nW-range the interferometer shows a good signal-to-noise ratio. The obtained parametric phase shifts are in good agreement with a semiclassical model.

This work was supported by SFB407 of the Deutsche Forschungsgemeinschaft.

[1] M. Müller, F. Homann, R.-H. Rinkleff, A. Wicht, K. Danzmann, Phys, Rev. A 62, 060501(R) (2000)

Coherent Population Trapping Involving Rydberg and Autoionizing States in Xenon

•Michael Stellpflug, Thomas Halfmann, Knut Böhmer und Klaas Bergmann
Fachbereich Physik, Universität Kaiserslautern, D-67663 Kaiserslautern

Spectroscopy techniques relying on the interaction of strong, coherent radiation with matter permit the generation of pronounced dark resonances with an improved signal-to-noise-ratio compared to methods involving incoherent processes. We investigated Rydberg and autoionizing states in Xenon by coherent population trapping and dark resonances. A weak two-photon coupling is induced between the ground state of Xenon and an intermediate state. The strong coupling between the intermediate and the investigated excited states, generated by a second radiation field, suppresses population transfer to the intermediate state. The effect of coherent population trapping was found to be very robust with respect to the chosen atomic levels and laser-intensity fluctuations. It could even be implemented with lasers of poor coherence properties. We demonstrated the reliability of the technique with a laser system, providing laser-pulses of transform-limited bandwidth as well as with a conventional broadband laser system. The depth of the resonances clearly exceeded the value of 50 %, expected for incoherent interaction.

Stable Single Photon Sources

•Christoph Braig¹, Patrick Zarda², Christian Kurtsiefer¹ und Harald Weinfurter^1,2
1Sektion Physik der LMU München, Schellingstr. 4/III, 80799 München
²Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching

For the security of quantum cryptography but also for fundamental experiments in quantum mechanics, single photon sources are indispensable. Recently investigated single photon source systems like quantum dots or dye molecules usually need low temperatures or suffer from photo bleaching.

Our source utilizes the fluorescence from a single Nitrogen-Vacancy (NV) center in type Ib diamond. A single NV-center is excited at room temperature in a confocal microscope set-up with a frequency-doubled Nd:YVO₄ laser at 532 nm. We analyze the fluorescence light either with a grating spectrometer to clearly identify the center or with a Hanbury-Brown-Twiss setup to demonstrate the non-classical photon statistics. The fluorescence of NV-centers clearly shows antibunching without any evidence of photo bleaching.

Recent experiments concentrate on testing diamond nanocrystals and CVD layers of diamond. These systems should allow for a higher collection efficiency, and possibly contain other fluorescing defects with spectral characteristics well suited for practical applications.

Anomalous neutron Compton scattering from entangled protons of condensed systems

•T. Abdul-Redah und C. A. C.-Dreismann
I. N. Stranski-Institut, TU Berlin, D-10623 Berlin

We present experimental neutron Compton scattering (NCS) results on (1) various metallic hydrides [1] (niobium and palladium hydrides); (2) solid polystyrene [2] and partially deuterated polystyrene; (3) liquid benzene. The NCS experiments reveal that the measured total cross-section density of the protons in these systems is reduced by about 25%. The characteristic scattering time of the NCS experiment is about 0.1 - 1.0 femtoseconds. These striking results provide direct experimental evidence for short-lived quantum entanglement between protons at T=295.

[1] E. B. Karlsson et al. Europhys. Lett. 46, 617 (1999). [2] C. A. C.-Dreismann et al., J. Chem. Phys. 113, 2784 (2000).

Diagrammatik der schwachen Lokalisierung von Licht durch kalte Atome

•Cord Axel Müller^1,2, Christian Miniatura¹ und Dominique Delande^3
1Institut Non-Linéaire de Nice, F-06560 Valbonne
²MPI für Physik komplexer Systeme, D-01189 Dresden und Sektion Physik, Ludwig-Maximilians-Universität München
³Laboratoire Kastler Brossel, F-75005 Paris

Die mittlere Amplitude vielfach gestreuter Wellen in einem ungeordneten Medium wird durch die Dyson-Gleichung, die mittlere Intensität durch die Bethe-Salpeter-Gleichung bestimmt. Wir zeigen, wie die Streuung von polarisiertem Licht an entarteten atomaren Dipolübergängen analytisch beschrieben werden kann [1,2]. Die systematische Anwendung irreduzibler Tensoren gestattet es, die Dyson- und Bethe-Salpeter-Gleichung im Regime unabhängiger Streuung mit eleganten diagrammatischen Methoden zu lösen. Insbesondere gelingt die exakte analytische Summation der Leiter- und Kreuz-Diagramme und somit die vollständige Beschreibung der schwachen Lokalisierung von Licht durch lasergekühlte Atome.

[1] T. Jonckheere, C.A. Müller, R. Kaiser, Ch. Miniatura and D. Delande, Phys. Rev. Lett. 85, 4269 (2000)

[2] C.A. Müller, T. Jonckheere, Ch. Miniatura and D. Delande, in Vorbereitung

Nonlocal evanescent modes in near field optics

•Alfons Stahlhofen
Inst. f. Physik, Rheinau 1, D-56075 Koblenz

Near field optics is governed by the physics of evanescent waves. The systematic exploration of evanescent modes started blooming with the advent of photon scanning tunneling microscopy. Despite the comprehensive numerical studies of evanescent modes done in near field optics, some basic physical facts resulting from the nonlocal character of evanescent modes, made out of virtual photons, apparently escaped notice so far. The nonlocal character of evanescent modes is illustrated and some consequences for prominent systems of near field optics are elucidated

Universal Relationship of Tunnelling Time and Frequency

•Günter Nimtz und Astrid Haibel
Universität zu Köln, II. Physikalisches Institut, Zülpicher Str. 77, 50937 Köln

Various photonic tunnelling time experiments have been carried out using electromagnetic radiation both at microwave and optical frequencies. We have carefully analyzed our own experimental results and those of others and have found that in all such experiments the tunnelling transit time is approximately equal to the reciprocal (1/f) of the corresponding frequency of radiation independent of the type or shape of the actual barrier. As a result of the mathematical equivalence between photonic and wave mechanic tunnelling we conjecture that this universal relationship holds for all tunnelling processes.

Tunnelling Time Measurements of a Double-Prism

•Astrid Haibel und Günter Nimtz
Universität zu Köln, II. Physikalisches Institut, Zülpicher Str. 77, 50937 Köln

A smart system to study tunnelling time presents the double prism in measuring the frustrated total internal reflection (FTIR). The barrier transmission time of the double-prism, or what we call here the tunnelling time can be split into two components t_tunnel = t_|| + t_^, one along the surface of the first prism due to the Goos-Hänchen shift, and another part perpendicular to the surface. The first component is related to a non-evanescent wave characterized by the real wavenumber while the second one is related to the evanescent mode traversing the gap between the two prisms. The path of a reflected as well as a transmitted beam differs now exactly by the gap distance. Our measurements revealed that both beams the reflected and the transmitted one leave the prisms at the same time. This interesting result states that the transmitted beam did not spend any time in the gap, i.e inside the photonic barrier.

Nonlocality of atomic wave packets in a linear potential

•Florian Haug, Marc Bienert, Matthias Freyberger und Wolfgang P. Schleich
Abteilung für Quantenphysik, Universität Ulm

We demonstrate that an atomic wave packet falling through a quantized cavity field splits up into spatially separated packets. The corresponding atom-field interaction entangles all quantum degrees of freedom, i.e. the quantized field, the internal state of the atom and the quantized atomic motion.

The distance between the resulting wave packets as well as the distance between the wave packets and the cavity is mesoscopic. Therefore and due to the fact that wave packets and cavity are entangled we have created a mesoscopic Schrödinger-Cat-state.

By analyzing appropriate observables and their correlations we show that this model leads to a violation of the Clauser-Horne-Shimony-Holt-inequality. This fact shows clearly that there is no chance matching such correlations with a local realistic theory.

[HOME]

[Archiv]

[E-Verhandlungen 2001]

[Berlin]

[Fachverband Q]

[Sitzung Q 22]

Zuletzt geändert am 05.06.2001

Fragen, Kommentare, Anregungen und Kritik