Simple parameterization of dark-resonance line shapes

We have studied the dispersion and absorption line shapes of coherent population trapping resonances (dark resonances) in thermal cesium vapor. Outside the time-of-flight regime the line shapes can be described by a simple phenomenological model: a sum of a dispersive and an absorptive Lorentzian of identical width and position, whose relative weights depend on the detuning of the laser frequencies from the optical resonance. This functional form can be derived from a theoretical model for the full multilevel structure of an alkali atom. The analytical expressions for the parameters of the line shape model show the same behavior as the experimental data. Received: 4 October 2002 / Revised version: 30 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +41-26/300-9631, E-mail: robert.wynands@unifr.ch     

Deterministic coupling of single ions to an optical cavity

By combining the technologies of ion trapping and cavity QED, we have achieved deterministic coupling of single ions to an optical field. By Doppler cooling, the spread of the ion’s wavepacket was reduced to 42 nm, corresponding to Lamb–Dicke localization. As an application, we have measured the three-dimensional structure of cavity eigenmodes with sub-wavelength precision. The setup presented is suited for the controlled coherent processing of atomic and photonic quantum information. Examples include the triggered generation of single photons and two-ion quantum gates. Received: 20 August 2002 / Revised version: 16 December 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-89/32905-200, E-mail: keller@mpq.mpg.de     

Experimental demonstration of complementarity with single photons

We demonstrate the principle of complementarity in quantum mechanics in a single-photon interference experiment. Single photons are provided by isolated, optically pumped nitrogen-vacancy centers in diamond, which can be easily addressed by confocal microscopy. In order to observe the particle-like behavior of photons, we perform an elementary Welcher-Weg measurement, detecting photons behind a beam splitter. In contrast, if we dispense with this Welcher-Weg information, we observe interference fringes with a visibility of about 96%, revealing the wave nature of the photon. Received: 29 August 2002 / Revised version: 12 December 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax +49-89/2180-5032, E-mail: Harald.Weinfurter@physik.uni-muenchen.de     

Electromagnetically induced transparency mediated by phonons in strongly coupled exciton–phonon systems

It is shown theoretically that electromagnetically induced transparency (EIT) due to strong exciton–phonon coupling can occur in strongly coupled exciton–phonon systems such as polymers and organic semiconductors and lead to ultra-slow light effects. The results indicate that the strong coupling of excitons and phonons is important, but the exciton– exciton interaction plays a small role in the generation of the EIT. Numerical results for polydiacetylene–toluene sulfonate are also presented. This EIT in a solid-state medium might be utilized for efficient multiwave mixing and quantum nondemolition measurements, as well as for novel acousto-optical devices. Received: 21 August 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. E-mail: zhukadi@yahoo.com     

Quantum interference spectra at the difference-frequency generation in four-level atomic systems

Analytical expressions of the spectral functions have been derived for a number of induced excitations that occur near the difference-frequency generation in a four-level atomic system interacting with three laser fields. The spectral heights and subnatural line widths of the induced peaks depend on the intensities of the laser fields involved. The maximum heights of the induced peaks take positive, zero and negative values, indicating that the processes of absorption (attenuation), dark resonance and stimulated emission (amplification) are likely to occur near the difference-frequency generation, respectively. Received: 15 March 2002 / Revised version: 17 June 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +1-613-954-5242     

Phonon–phonon interactions due to non-linear effects in a linear ion trap

We examine in detail the theory of the intrinsic non-linearities in the dynamics of trapped ions due to the Coulomb interaction. In particular, the possibility of mode–mode coupling, which can be a source of decoherence in trapped ion quantum computation, or can be exploited for parametric down-conversion of phonons, is discussed and conditions under which such coupling is possible are derived. Received: 8 November 2002 / Published online: 26 March 2003 RID="*" ID="*"Permanent address: MIP, Université Pierre et Marie Curie and Département de Physique, école Normale Supérieure, 75005 Paris, France RID="**" ID="**"Corresponding author. Fax: +1-505/667-1931, E-mail: dfvj@lanl.gov     

Two-color coherent population trapping in a single Cs hyperfine transition,with application in magnetometry

We investigate the coherent population trapping effect occurring between the Zeeman sublevels of a given hyperfine state of Cs when excited by a single-mode diode laser, which is frequency modulated in the kHz–MHz range. In the presence of a dc magnetic field, simultaneous excitation of pairs of ground-state magnetic sublevels to common excited magnetic sublevels is performed. As a result, coherent population trapping resonance is detected at only a given modulation frequency, whose value gives a measure of the magnetic field. The parameters of the resonances are measured in order to determine the ultimate accuracy of the magnetic field measurement. Received: 6 December 2002 / Revised version: 24 February 2003 / Published online: 2 June 2003 RID="*" ID="*"Corresponding author. Fax: +39-0577/23-4689, E-mail: biancalana@unisi.it     

Lasing action in air induced by ultra-fast laser filamentation

The backscattered fluorescence of nitrogen from filaments generated by intense ultra-fast Ti:sapphire laser pulses propagating in air is studied. The backscattered fluorescence from N₂ molecules and ions show an exponential increase with increasing filament length, indicating amplified spontaneous emission. Received: 11 October 2002 / Revised version: 3 December 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +1-418/6562-623, E-mail: qluo@phy.ulaval.ca     

Quantum efficiency and the photonic control of molecular fluorescence in the solid state

We investigate the role of quantum efficiency in determining the extent to which the local photonic environment may alter the spontaneous emission rate of organic dye molecules with broad luminescence spectra. Comparison of theory with experimental results shows that the quantum efficiency is a key determining factor in such control, low quantum efficiencies leading to poor control. These results help establish a firm basis for characterising near-fields in nano-optics and controlling fluorescing species. Received: 31 May 2002 / Revised version: 27 June 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +44-1392/264-111, E-mail: S. Astilean@exeter.ac.uk     

Geometry,commutation relations and the quantum fictitious force

We express the commutation relation between the operators of the momentum and the radial unit vectors in D dimensions in differential and integral form. We connect this commutator with the quantum fictitious potential emerging in the radial Schr?dinger equation of an s-wave. Received: 6 August 2002 / Revised version: 30 October 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-731/502-3086, E-mail: markus.cirone@physik.uni-ulm.de     

Photofabrication of periodic microstructures in azodye-doped polymers by interference of laser beams

Volume holographic gratings and two-dimensional periodic microstructures in azodye-doped polymethylmethacrylate were fabricated, respectively, by interference of two coherent beams of a femtosecond laser and by interference of three coherent beams of a nanosecond laser. The dependence of the first-order Bragg diffraction efficiency and the photoinduced refractive-index modulation of the gratings on the intensity of the writing light was investigated. The measurements of the absorption spectra before and after irradiation with the writing light suggest that the photoinduced gratings were refractive-index-modulated gratings, which arose from a photoinduced decomposition reaction of the azodye molecules through multiphoton absorption. In the experiments involving the interference of three beams, the period of the two-dimensional periodic microstructures was changed by adjusting the angle between the three writing beams. Received: 10 July 2002 / Revised version: 5 September 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-774/955206, E-mail: jhsi@photon.jst.go.jp     

Characterization of nonlinear optical parameters of polymethine dyes

We investigated nonlinear refraction, nonlinear absorption, and saturable absorption of polymethine dyes by the Z-scan technique (λ=1064 nm). The analysis of simultaneous appearance of several nonlinear optical processes in dye solutions excited by picosecond pulses was carried out. The saturable absorption was analyzed taking into account various models. Nonlinear refractive indices, nonlinear absorption coefficients, and saturation intensities of various polymethine dyes were measured. Received: 27 December 2002 / Revised version: 6 March 2003 / Published online: 5 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-471/363-366, E-mail: r_ganeev@issp.u-tokyo.ac.jp RID="**" ID="**"Present address: The Institute for Solid State Physics, The Tokyo University, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan     

<Emphasis Type="Italic">Z</Emphasis>-scan technique through beam radius measurements

A modification to the well-known z-scan technique for measuring optical non-linearities is introduced. It is based on directly measuring the beam radius in the far field instead of the transmittance of the irradiance through an aperture, as in the original version. It has the advantage of being insensitive to beam pointing instability and is almost insensitive to power fluctuations. Furthermore, the calculations required for the determination of the non-linear parameters are simplified. For demonstrating the advantages of the modified method, beam radius and transmittance measurements were simultaneously taken in the standard non-linear optical material CS₂. Separate fittings of these measurements gave almost the same values for the non-linearities, quite similar to those in the literature. A common fitting has been applied to both sets of measurements, enhancing the accuracy of the method. Received: 8 July 2002 / Revised version: 18 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +30-2610/997470, E-mail: gianetas@physics.upatras.gr     

Is quantum computing with solid state devices possible?

Experiments with a few qubits, the basic elements of a quantum computer, using the methods of nuclear magnetic resonance (NMR) have demonstrated that quantum computing is possible. A useful quantum computer would need to use many qubits, while it appears that NMR with molecules is limited to about ten qubits. The easiest way to assemble a large number of qubits would be to use the existing solid state technology. However, the accuracy with which large numbers of solid state devices can be fabricated does not match the high-precision methods that have made quantum computing with magnetic resonance possible. Quantum computing with solid state devices must expect to encounter a new set of problems posed by differences between nominally identical devices. The difficulties are illustrated with examples of proposed qubits. Specific questions that must be addressed in attempts to use solid state devices for quantum computing are suggested. Received: 25 July 2002 / Accepted: 31 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +1-914/945-2141, E-mail: rkeyes@us.ibm.com     

EPR entangled states and complex fractional Fourier transformation

Starting from the Einstein-Podolsky-Rosen entangled state representations of continuous variables we derive a new formulation of complex fractional Fourier transformation (CFFT). We find that two-variable Hermite polynomials are just the eigenmodes of the CFFT. In this way the CFFT is linked to the appropriate operator transformation between two kinds of entangled states in the context of quantum mechanics. In so doing, the CFFT of quantum mechanical wave functions can be derived more directly and concisely. Received 21 February 2002 / Received in final form 1st June 2002 Published online 24 September 2002 RID="*" ID="*"Work supported by National Natural Science Foundation of China under grant 10175057 and the President Foundation of Chinese Academy of Science. RID="a" ID="a"e-mail: fhym@ustc.edu.cn     

Characterization and control of phase fluctuations in elongated Bose–Einstein condensates

Quasi-one-dimensional Bose–Einstein condensates (BECs) in elongated traps exhibit significant phase fluctuations even at very low temperatures. We present recent experimental results on the dynamic transformation of phase fluctuations into density modulations during time of flight and show the excellent quantitative agreement with the theoretical prediction. In addition we confirm that, under our experimental conditions, in the magnetic trap density modulations are strongly suppressed even when the phase fluctuates. We also discuss our theoretical results on control of the condensate phase by employing a time-dependent perturbation. Our results set important limitations on future applications of BECs in precision atom interferometry and atom optics, but at the same time suggest pathways to overcome these limitations. Received: 17 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +49-511/762-3023, E-mail: Helge.Kreutzmann@ITP.uni-hannover.de     

Spiral and curved periodic crack patterns in sol-gel films

Thin silicate sol-gel films with four different crack patterns were created reproducibly by controlling the film deposition parameters. The crack geometry, periodicity, and amplitude were studied experimentally as a function of the film thickness, curing time, and temperature. Direct evidence was found that the physical interplay between stress relief through film cracking and stress relief through film warping results in sawtooth, spiral, closed loop, or straight line crack trajectories. Received: 25 March 2002 / Accepted: 1 July 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +1-941-359-4396, Email: sendova@ncf.edu     

Simultaneous measurement of absolute OH concentration,temperature and flow velocity in a flame by photothermal deflection spectroscopy

We demonstrate the use of photothermal deflection spectroscopy for a simultaneous measurement of absolute minority-species concentration, temperature and flow velocity in a flame from the analysis of a single data trace. The ability to make absolute concentration measurements, as well as the ability to measure three important combustion parameters simultaneously, may make this technique quite useful in many circumstances. Received: 27 August 2002 / Revised version: 26 September 2002 / Published online: 11 December 2002 RID="*" ID="*"Corresponding author. Fax: +1-479/575-4580, E-mail: rgupta@uark.edu     

Far-field distribution of third-harmonic generation by two crossed beams

The far-field intensity distribution of the third harmonic generated in a thin silica plate by two laser beams crossed at a small angle comprises four peaks approximately symmetric with respect to the bisector. The measurement of the intensity ratio of the inner to the outer peaks yielded values scattered from 10 to 20, surprisingly larger than the combinatorial value 9. The phase-matching factor, as determined by the full three-dimensional calculations required by the absence of axial symmetry, accounts for the observed ratio and its fluctuations. Received: 14 November 2002 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +7-95/1941-994, E-mail: fomichev@imp.kiae.ru