Coherent Control of Isotope Separation in HD+ Photodissociation by Strong Fields

The photodissociation of the HD+ molecular ion in intense short- pulsed linearly polarized laser fields is studied using a time- dependent wave-packet approach where molecular rotation is fully included. We show that applying a coherent superposition of the fundamental radiation with its second harmonic can lead to asymmetries in the fragment angular distributions, with significant differences between the hydrogen and deuterium distributions in the long wavelength domain where the permanent dipole is most efficient. This effect is used to induce an appreciable isotope separation.Comment: Physical Review Letters, 1995 (in press). 4 pages in revtex format, 3 uuencoded figures. Full postcript version available at: http://chemphys.weizmann.ac.il/~charron/prl.ps or ftp://scipion.ppm.u-psud.fr/coherent.control/prl.p

Quantum key distribution and 1 Gbit/s data encryption over a single fibre

We perform quantum key distribution (QKD) in the presence of 4 classical channels in a C-band dense wavelength division multiplexing (DWDM) configuration using a commercial QKD system. The classical channels are used for key distillation and 1 Gbps encrypted communication, rendering the entire system independent from any other communication channel than a single dedicated fibre. We successfully distil secret keys over fibre spans of up to 50 km. The separation between quantum channel and nearest classical channel is only 200 GHz, while the classical channels are all separated by 100 GHz. In addition to that we discuss possible improvements and alternative configurations, for instance whether it is advantageous to choose the quantum channel at 1310 nm or to opt for a pure C-band configuration.Comment: 9 pages, 7 figure

A quantitative theory-versus-experiment comparison for the intense laser dissociation of H2+

A detailed theory-versus-experiment comparison is worked out for H$_2^+$ intense laser dissociation, based on angularly resolved photodissociation spectra recently recorded in H.Figger's group. As opposite to other experimental setups, it is an electric discharge (and not an optical excitation) that prepares the molecular ion, with the advantage for the theoretical approach, to neglect without lost of accuracy, the otherwise important ionization-dissociation competition. Abel transformation relates the dissociation probability starting from a single ro-vibrational state, to the probability of observing a hydrogen atom at a given pixel of the detector plate. Some statistics on initial ro-vibrational distributions, together with a spatial averaging over laser focus area, lead to photofragments kinetic spectra, with well separated peaks attributed to single vibrational levels. An excellent theory-versus-experiment agreement is reached not only for the kinetic spectra, but also for the angular distributions of fragments originating from two different vibrational levels resulting into more or less alignment. Some characteristic features can be interpreted in terms of basic mechanisms such as bond softening or vibrational trapping.Comment: submitted to PRA on 21.05.200

Ponderomotive effects in zero kinetic energy photoelectron spectroscopy with intense femtosecond pulses

Zero kinetic energy photoelectron (ZEKE) spectroscopy may be an interesting technique for monitoring the time evolution of state selected atomic products in ultrafast pump-probe experiments. We investigate here the effects of laser intensity by studying three-photon non-resonant ZEKE spectra of atomic xenon at the first ionization threshold with tunable femtosecond UV pulses. High intensity introduces a new broadening due to ponderomotive effects in the ZEKE spectrum. It is demonstrated that a three-photon ZEKE spectrum can be recorded at intensities where the broadenign due to these ponderomotive effects (i.e. the ac Stark shift for high-n Rydberg states) is small in comparison with the effective laser bandwidth.NRC publication: Ye

Space charge and plasma effects in zero kinetic energy (ZEKE)photoelectron spectroscopy

In photoelectron spectroscopy experiments it is generally assumed that the Coulomb force between charged particles is small compared with external fields, and that the free kinetic electrons will quickly leave the ions. This is the basis of the ZEKE photoelectron spectroscopy. However as the density of charged particles is increased, plasma physics effects begin to become important, and the kinetic electrons become trapped by the net positive charge and move so as to set up a self-field which can cancel any externally imposed electric fields. For high densities, fewer electrons than expected are able to escape the self-field. The production of self-consistent electric fields is studied by means of particle-in-cell plasma simulations and by N-body trajectory calculations, and simple expressions are derived for when plasma physics effects become significant. An experimental illustration of plasma effects in ZEKE is presented.Peer reviewed: YesNRC publication: Ye