S. Wallentowitz

High-order nonlinearities in the motion of a trapped atom

Blackett Laboratory, Imperial College, London SW7 2BZ, United Kingdom(June 10, 1998)We study the counterpart to the multi-photon down conversion in the quantised motion of a trappedatom. The Lamb–Dicke approximation leads to a divergence of the mean motional excitation in afinite interaction time for k-quantum down conversions with k ≥ 3, analogous to the situation inthe parametric approximation of nonlinear optics. We show that, in contrast to the Lamb–Dickeapproximation, the correct treatment of the overlap of the atomic center-of-mass wave function andthe driving laser waves leads to a proper dynamics without any divergence problem. That is, thewavy nature of both matter and light is an important physical property which cannot be neglectedfor describing the motional dynamics of a trapped atom, even for small Lamb–Dicke parameters.PACS numbers: 42.50.Vk, 32.80.Lg, 42.65.-k, 03.65.-wI. INTRODUCTION

DETERMINATION OF ENTANGLED QUANTUM STATES OF A TRAPPED ATOM

(September 18, 1996)We propose a method for measuring entangled vibronicquantum states of a trapped atom. It is based on the nonlin-ear dynamics of the system that appears by resonantly drivinga weak electronic transition. The proposed technique allowsthe direct sampling of a Wigner-function matrix, displayingall knowable information on the quantum correlations of themotional and electronic degrees of freedom of the atom. Itopens novel possibilities for testing fundamental predictionsof the quantum theory concerning interaction phenomena.PACS numbers: 03.65.Bz, 42.50.Vk, 32.80.PjI. INTRODUCTION

Unambiguous ultrashort pulse reconstruction from double spectrograms alone

We report a spectrographic technique for amplitude and phase measurements of ultrashort laser pulses (above 10 fs). Pulse information is obtained directly from two different spectrograms, using the mathematical relations between Wigner–Ville function projections. Pulses are reconstructed rapidly and unambiguously without stagnation. This non-interferometric method is demonstrated experimentally for the successful characterization of 100 fs pulses.

Semiclassical dynamics of a rigid rotor: SO(3) covariant approach

A semi-classical analysis of the quantum rigid-rotor motion based on a phase-space description of the rotation in terms of a SO(3) covariant Wigner-like distribution is presented. The results are applied to the description of the intense-field alignment of an anisotropically polarizable molecule with high rotational excitation.

Local sampling of the quantum phase-space distribution of a continuous-wave optical beam

It is shown how the quantum phase-space distribution of a continuous-wave (cw) optical beam can be obtained independently at each point in phase space by a combination of unbalanced homodyne and balanced-heterodyne techniques. The unbalanced homodyning allows for the local sampling of phase space, whereas the heterodyne part, although introducing a loss of detection efficiency, provides a highly efficient reduction of 1/f excess noise that is required for the sampling of cw fields. The method complements well-known techniques for light pulses by providing a robust method for cw optical beams.

Robust ¿trapping states¿ in the motion of a trapped ion

Abstract A novel robust mechanism for the generation of “trapping states” is shown to exist in the coupling of a two-level system with an oscillator, which is based on nonlinearities in the laser-induced vibronic coupling. This mechanism is exemplified with an ion confined in the potential well of a trap, where the nonlinearities are due to Franck–Condon type overlap integrals of the laser waves with the ionic centre-of-mass wavefunction. In contrast to the coherent trapping mechanism known from micro-maser theory, this mechanism works also in an incoherent regime operated by noisy lasers and is therefore much more robust against external decoherence effects. These features favour the incoherent regime, in particular for the preparation of highly excited trapping states.

Rotational master equation for cold laser-driven molecules

The equations of motion for the molecular rotation are derived for vibrationally cold dimers that are polarized by off-resonant laser light. It is shown that, by eliminating electronic and vibrational degrees of freedom, a quantum master equation for the reduced rotational density operator can be obtained. The coherent rotational dynamics is caused by stimulated Raman transitions, whereas spontaneous Raman transitions lead to decoherence in the motion of the quantized angular momentum. As an example the molecular dynamics for the optical Kerr effect is chosen, revealing decoherence and heating of the molecular rotation.

Passively Mode-locked Nd:Cr:YAG Laser pumped by a Modular Scalable Solar Light Concentrator

We demonstrate a passively mode-locked Nd:Cr:YAG laser which is solar-pumped by optical fibers. With the achievement of a higher beam quality a semiconductor saturable absorber mirror can be used to generate picosecond pulses.

Modular Scalable Solar Light Concentrator for Pumping a Nd:Cr:YAG Laser Medium

A solar-pumped Nd:Cr:YAG ceramic laser is presented that is pumped by Fresnel lens sunlight concentrators via polymer optical fibers. Focusing into the laser medium via aspheric lenses allows to improve the laser beam quality.

Optimal lossy quantum interferometry in phase space

We analyse the phase space representation of the optimal measurement of a phase shift in an interferometer with equal photon loss in both its arms. In the local phase estimation scenario with a fixed number of photons, we identify features of the spin Wigner function that warrant sub-shot noise precision, and discuss their sensitivity to losses. We derive the asymptotic form of an integral kernel describing the process of photon loss in the phase space in the limit of large photon numbers. The analytic form of this kernel allows one to assess the ultimate precision limit for a lossy interferometer. We also provide a general lower bound on the quantum Fisher information in terms of spin Wigner functions.