Showing papers by "Herbert Walther published in 2002"
628 citations
TL;DR: Two-qubit logical gates are proposed on the basis of two atoms trapped in a cavity setup and commonly addressed by laser fields, which provides fidelity and a success rate very close to unity, suitable for performing quantum computation.
Abstract: Two-qubit logical gates are proposed on the basis of two atoms trapped in a cavity setup and commonly addressed by laser fields. Losses in the interaction by spontaneous transitions are efficiently suppressed by employing adiabatic transitions and the quantum Zeno effect. Dynamical and geometrical conditional phase gates are suggested. This method provides fidelity and a success rate of its gates very close to unity. Hence, it is suitable for performing quantum computation.
174 citations
TL;DR: In this paper, the authors demonstrate the generation of square and hexagonal lattices of optical vortices and reveal their propagation in a saturable nonlinear medium, showing that the lattices induce periodic modulation of the refractive index.
Abstract: We demonstrate experimentally the generation of square and hexagonal lattices of optical vortices and reveal their propagation in a saturable nonlinear medium. If the topological charges of the vortices have identical signs, the lattice exhibits rotation, whereas if their signs alternate between being the same and being opposite to each other, we observe stable propagation of the structures. In the nonlinear medium the lattices induce periodic modulation of the refractive index. Diffraction of a probe beam by this nonlinearity-induced periodic structure is observed.
49 citations
TL;DR: In this article, it was shown that the unitary evolution of a harmonic oscillator coupled to a two-level system can be undone by a suitable manipulation of the two level system, more specifically by a quasi-instantaneous phase change.
Abstract: We show that the unitary evolution of a harmonic oscillator coupled to a two-level system can be undone by a suitable manipulation of the two-level system---more specifically, by a quasi-instantaneous phase change. This enables us to isolate the dissipative evolution to which the oscillator may be exposed in addition. With this method we study the decoherence time of a photon mode in cavity QED, and that of the quantized harmonic motion of trapped ions. We comment on the relation to spin echoes and multipath interferometry.
42 citations
TL;DR: In this article, it was shown that in cases where the intensity interferences disappear the intensity-intensity correlations can display an interference pattern with a visibility of up to 100% depending on the polarization selected for the detection and is independent of the strength of the driving field.
Abstract: The different behavior of first-order interferences and second-order correlations are investigated for the case of two coherently excited atoms. For intensity measurements this problem is in many respects equivalent to Young's double-slit experiment and was investigated in an experiment by Eichmann et al. [Phys. Rev. Lett. 70, 2359 (1993)] and later analyzed in detail by Itano et al. [Phys. Rev. A 57, 4176 (1998)]. Our results show that in cases where the intensity interferences disappear the intensity-intensity correlations can display an interference pattern with a visibility of up to 100%. The contrast depends on the polarization selected for the detection and is independent of the strength of the driving field. The nonclassical nature of the calculated intensity-intensity correlations is also discussed.
32 citations
TL;DR: In this article, the frequency stabilisation of a Nd:YAG laser at 946 nm to the Hertz-level was reported, which was used for ultra-high resolution spectroscopy of the 5 s 2 1 S 0 − 5 s5p 3 P 0 transition in In+ and will ultimately serve as a local oscillator of an optical frequency standard based on a single trapped indium ion.
31 citations
TL;DR: In this article, a theoretical study on the fluorescence of an atom in front of a mirror is presented, on the assumption that the presence of a distant mirror and a lens imposes boundary conditions on the electric field in a plane close to the atom.
Abstract: Motivated by a recent experiment [J. Eschner et al., Nature (London) 413, 495 (2001)], we now present a theoretical study on the fluorescence of an atom in front of a mirror. On the assumption that the presence of the distant mirror and a lens imposes boundary conditions on the electric field in a plane close to the atom, we derive the intensities of the emitted light as a function of an effective atom-mirror distance. The results obtained are in good agreement with the experimental findings.
26 citations
TL;DR: In this article, a 100-kHz femtosecond amplifier system delivering pulses with a duration of 35 fs and an energy of 7 /spl mu/J was presented, which does not include a stretcher, since the large amount of dispersion accumulated during the amplification process is sufficient to prevent self-focusing.
Abstract: Presents a 100-kHz femtosecond amplifier system delivering pulses with a duration of 35 fs and an energy of 7 /spl mu/J. The system does not include a stretcher, since the large amount of dispersion accumulated during the amplification process is sufficient to prevent self-focusing. Compensation in approximately all orders is achieved through a combination of a prism compressor, chirped mirrors, and a liquid-crystal modulator, allowing the amplified pulses to be shortened to nearly the bandwidth limit.
21 citations
TL;DR: In this article, the Fourier transform of the extended Lorentzian energy distribution becomes the exponential, but only for times $tg~0,$ a time asymmetry which is in conflict with the unitary group time evolution of standard quantum mechanics.
Abstract: The Fourier transform is often used to connect the Lorentzian energy distribution for resonance scattering to the exponential time dependence for decaying states. However, to apply the Fourier transform, one has to bend the rules of standard quantum mechanics; the Lorentzian energy distribution must be extended to the full real axis $\ensuremath{-}\ensuremath{\infty}lEl\ensuremath{\infty}$ instead of being bounded from below $0l~El\ensuremath{\infty}$ (Fermi's approximation). Then the Fourier transform of the extended Lorentzian becomes the exponential, but only for times $tg~0,$ a time asymmetry which is in conflict with the unitary group time evolution of standard quantum mechanics. Extending the Fourier transform from distributions to generalized vectors, we are led to Gamow kets, which possess a Lorentzian energy distribution with $\ensuremath{-}\ensuremath{\infty}lEl\ensuremath{\infty}$ and have exponential time evolution for $tg~{t}_{0}=0$ only. This leads to probability predictions that do not violate causality.
21 citations
TL;DR: In this article, two different methods are described to produce a fixed number of photons on demand: a micromaser operating under the conditions of a trapping state and a single ion in an optical cavity.
Abstract: Recent widely discussed applications in quantum information and quantum cryptography require radiation sources able to produce a fixed number of photons. In this paper we review the work performed in our laboratory to produce such fields on demand. Two different methods are described. The first one is based on the micromaser operating under the conditions of a trapping state. The second device uses a single ion in an optical cavity. The latter setup was recently realized in our laboratory.
10 citations
01 Jan 2002
TL;DR: In this article, a review of recent experiments performed in our laboratory dealing with the investigation of quantum phenomena in the radiation interaction of single atoms is presented, including experiments in single mode cavities using the one-atom maser or micromaser and in the second part experiments with ion traps.
Abstract: In this paper recent experiments performed in our laboratory are reviewed dealing with the investigation of quantum phenomena in the radiation interaction of single atoms. The first part describes experiments in single mode cavities using the one-atom maser or micromaser and in the second part experiments with ion traps are summarized. The latter experiments concentrate on the investigation of resonance fluorescence. In addition new experimental proposals using ultracold atoms in cavities and traps are discussed. In those future experiments the interplay between atomic waves and light waves is important and leads to new phenomena in radiationatom interaction.
TL;DR: In this paper, the authors show that the Jaynes-Cummings dynamics can be undone by a suitable manipulation of the two-level system, more specifically, by a quasi-instantaneous phase change.
Abstract: We show that the Jaynes–Cummings dynamics can be undone by a suitable manipulation of the two-level system—more specifically, by a quasi-instantaneous phase change. We discuss this effect in relation to decoherence measurements of a photon mode in cavity quantum electrodynamics and those of the quantized harmonic motion of trapped ions. We comment on the relation to spin echoes.
TL;DR: In this article, it was shown by specific examples and direct calculation, based primarily on breaking emission-absorption symmetry as in lasing without inversion, that cooling of internal states by external coherent control fields is possible.
Abstract: Cooling of internal atomic and molecular states via optical pumping and laser cooling of the atomic velocity distribution, rely on spontaneous emission. The outstanding success of such examples, taken together with general arguments, has led to the widely held notion that radiative cooling requires spontaneous emission. We here show by specific examples and direct calculation, based primarily on breaking emission-absorption symmetry as in lasing without inversion, that cooling of internal states by external coherent control fields is possible. We also show that such coherent schemes allow us to practically reach absolute zero in a finite number of steps, in contrast to some statements of the third law of thermodynamics.
01 Jan 2002
TL;DR: In this article, two different methods are discussed to produce a fixed number of photons on demand: the one-atom maser and the micromaser operating under the conditions of the so-called trapping states.
Abstract: The widely discussed applications in quantum information and quantum cryptography require radiation sources capable of producing a fixed number of photons. This paper reviews the work performed in our laboratory to produce these fields on demand. Two different methods are discussed. The first is based on the one-atom maser or the micromaser operating under the conditions of the so-called trapping states. In this situation the micromaser stabilises to a photon number state. The second device uses a single ion in an optical cavity. The latter setup was recently realised in our laboratory.
01 May 2002
Journal Article•
TL;DR: In this article, the authors studied the dependence of photoelectron spectra on the intensity of the laser pulse and the contribution of specific electron trajectories to the photo-electron spectrum with a quantum mechanical model.
Abstract: We discuss two recent photoionization experiments with atoms in intense laser fields: First we study the dependence of photoelectron spectra on the intensity of the laser pulse. In a second experiment a photoelectron spectrum for elliptical polarization is discussed. With a quantum mechanical model the contribution of specific electron trajectories to the photoelectron spectra can be analyzed. In the presented cases, trajectories with long travel times are of significant importance. Despite of the distinctive quantum nature of the effects, some key features can be explained by classical electron trajectories.
01 Jan 2002
TL;DR: In this article, a single trapped ion in a linear Paul trap was used to investigate the optical field with atomic resolution. But the probe was a single ion in the trap, and the trap allowed the longitudinal direction to be scanned by displacing the ion along the trap axis.
Abstract: This paper describes the investigation of an optical field with atomic resolution The probe is a single trapped ion in a linear Paul trap The trap allows the longitudinal direction to be scanned by displacing the ion along the trap axis In the two transversal directions the ion is confined by the trap potential and therefore the optical field is scanned by displacement of the sample using a piezo drive The method is demonstrated by measuring particular modes of an optical cavity
01 May 2002
TL;DR: In den letzten Jahren gelangen mithilfe des Feynmanschen Pfadintegrals grose Fortschritte im Verstandnis der Effekte, die durch die Wechselwirkung intensiver Laserpulse mit Atomen entstehen as mentioned in this paper.
Abstract: Heute kann man Laserpulse erzeugen, deren Feldstarken diejenigen ubertreffen, welche die Atome zusammen halten. In den letzten Jahren gelangen mithilfe des Feynmanschen Pfadintegrals grose Fortschritte im Verstandnis der Effekte, die durch die Wechselwirkung intensiver Laserpulse mit Atomen entstehen. Die begriffliche Nahe des Pfadintegralformalismus zu klassischen Trajektorien erlaubt in vielen Fallen sogar ein intuitives Verstandnis des physikalischen Mechanismus.
24 May 2002
TL;DR: In this article, the influence of the absolute phase on photoelectron emission has been demonstrated experimentally using long pulses, where the angular distribution of emitted photoelectrons has inversion symmetry.
Abstract: Summary form only given. In this work we have employed a technique which allows us to demonstrate experimentally the influence of the absolute phase on photoelectron emission. Photoionization of atoms in intense laser fields is characterized by above-threshold ionization (ATI). Using long pulses the angular distribution of emitted photoelectrons has inversion symmetry, since the electric field of long pulses exhibits such a symmetry. For ultrashort pulses, depending on the absolute phase, the generation of photoelectrons violates inversion symmetry. The absolute phase is thus expected to cause an anticorrelation in the number of electrons escaping in opposite directions, if these are investigated shot by shot.
16 Jun 2002
TL;DR: Using active vibration isolation of a high-finesse reference cavity, a 946 nm laser was used for high-resolution spectroscopy of the indium ion clock transition as mentioned in this paper.
Abstract: Summary form only given. Using active vibration isolation of a high-finesse reference cavity, we demonstrate a Hz-level linewidth of a 946 nm laser, used for high-resolution spectroscopy of the indium ion clock transition. Recent experimental results are presented.
12 May 2002
TL;DR: In this article, the phase of the carrier frequency with respect to the envelope (absolute phase) determines the variation of the laser electric field in time, which is important for many different topics in laser physics.
Abstract: Progress in femtosecond laser technology led to the production of laser pulses whose envelope varies on a time scale comparable to that of the electromagnetic field itself [1]. Such pulses consist of less than two optical cycles in FWHM. Under these conditions, the phase of the carrier frequency with respect to the envelope (absolute phase) determines the variation of the laser electric field in time. Since all effects in strong-field laser interaction are driven by the electromagnetic field of the laser, the absolute phase is important for many different topics in laser physics ranging from coherent control of chemical reactions and generation of attosecond pulses to photoionization, production of coherent soft X-rays and particle acceleration. Photoionization using intense laser pulses is characterized by the fact that an atom may absorb more photons than necessary for its ionization. The effect is known as above-threshold ionization (ATI). ATI has found applications for the characterization of few-cycle and attosecond pulses [2-3].