54 research outputs found
Broadband THz study of excitonic resonances in the high-density regime
We report the first terahertz study of the intra-excitonic 1s-2p transition
at high excitation densities in GaAs/AlGaAs quantum wells. A strong shift,
broadening, and ultimately the disappearance of this resonance occurs with
increasing density, after ultrafast photoexcitation at the near-infrared
exciton line. Densities of excitons and unbound electron-hole pairs are
followed quantitatively using a model of the composite terahertz dielectric
response. Comparison with near-infrared absorption changes reveals a
significantly enhanced energy shift and broadening of the intra-excitonic
resonance.Comment: 4 pages, 4 figure
Decoherence-Free Subspaces for Multiple-Qubit Errors: (I) Characterization
Coherence in an open quantum system is degraded through its interaction with
a bath. This decoherence can be avoided by restricting the dynamics of the
system to special decoherence-free subspaces. These subspaces are usually
constructed under the assumption of spatially symmetric system-bath coupling.
Here we show that decoherence-free subspaces may appear without spatial
symmetry. Instead, we consider a model of system-bath interactions in which to
first order only multiple-qubit coupling to the bath is present, with
single-qubit system-bath coupling absent. We derive necessary and sufficient
conditions for the appearance of decoherence-free states in this model, and
give a number of examples. In a sequel paper we show how to perform universal
and fault tolerant quantum computation on the decoherence-free subspaces
considered in this paper.Comment: 18 pages, no figures. Major changes. Section on universal fault
tolerant computation removed. This section contained a crucial error. A new
paper [quant-ph/0007013] presents the correct analysi
Discovery of diverse and functional antibodies from large human repertoire antibody libraries
AbstractPhage display antibody libraries have a proven track record for the discovery of therapeutic human antibodies, increasing the demand for large and diverse phage antibody libraries for the discovery of new therapeutics. We have constructed naïve antibody phage display libraries in both Fab and scFv formats, with each library having more than 250billion clones that encompass the human antibody repertoire. These libraries show high fidelity in open reading frame and expression percentages, and their V-gene family distribution, VH-CDR3 length and amino acid usage mirror the natural diversity of human antibodies. Both the Fab and scFv libraries show robust sequence diversity in target-specific binders and differential V-gene usage for each target tested, supporting the use of libraries that utilize multiple display formats and V-gene utilization to maximize antibody-binding diversity. For each of the targets, clones with picomolar affinities were identified from at least one of the libraries and for the two targets assessed for activity, functional antibodies were identified from both libraries
Optical absorption of type-II superlattices.
International audienceOptical spectra of type-II superlattices are presented, including Coulomb interaction and continuum states. We clarify the relative importance of above- and below-barrier transitions. By gradually increasing the band-gap modulation, we visualize the transition from a bulk semiconductor to a type-I or type-II superlattice. We show that transitions that dominate the spectrum of a type-II superlattice are absent in the spectrum of a type-I superlattice and vice versa. The interplay of size and Landau quantization in the optical absorption is studied for both a type-I superlattice and a type-II superlattice in a perpendicular magnetic field
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Contactless terahertz probes of correlations and dynamics in low-dimensional electron-hole gases
Confinement of carriers in nanostructures entails strong modifications of their physical properties, offering a well-defined laboratory for investigating the complex many-body interactions between charge, lattice and spin degrees of freedom [1]. In particular, electron-hole (e-h) gases photoexcited into low-dimensional semiconductors are characterized by new optical transitions and strongly enhanced Coulomb interactions. Terahertz radiation offers a unique tool to measure low-energy excitations and transport properties in nanostructures without the need for an electrical contact. Along this path, we have recently developed a new, sensitive scheme to probe time-varying Coulomb correlations in confined carrier plasmas. Here, we discuss experiments that utilize this pulsed terahertz source to probe the dynamical interplay of bound and unbound e-h pairs on a picosecond timescale [2]. A new low-energy oscillator is observed directly after resonant creation of heavy-hole excitons in GaAs quantum wells. This peak arises from transitions between the hydrogen-like exciton bound states, most notably the 1s-2p level transition. The terahertz field probes excitons in a large range of in-plane momenta K in contrast to the usual restriction of interband probes close to K ~ 0. Owing to the strongly correlated motion of electrons and holes, charge-neutral excitons are electrically insulating up to a frequency that matches the separation between their lowest internal states. Above-bandgap excitation at elevated temperatures however induces unbound e-h pairs which represent a conducting ionized gas with a Drude-like response. The distinct responses of these extreme phases enable us to follow in time a metal-insulator transition that occurs upon formation of excitons out of a gas of unbound e-h pairs, as well as its reverse process of ionization. These are dynamical transitions which occur on different timescales. Ionization of excitons can occur within only a few picoseconds, and depends on the phonon occupation. Exci
Recommended from our members
Contactless terahertz probes of correlations and dynamics in low-dimensional electron-hole gases
Confinement of carriers in nanostructures entails strong modifications of their physical properties, offering a well-defined laboratory for investigating the complex many-body interactions between charge, lattice and spin degrees of freedom [1]. In particular, electron-hole (e-h) gases photoexcited into low-dimensional semiconductors are characterized by new optical transitions and strongly enhanced Coulomb interactions. Terahertz radiation offers a unique tool to measure low-energy excitations and transport properties in nanostructures without the need for an electrical contact. Along this path, we have recently developed a new, sensitive scheme to probe time-varying Coulomb correlations in confined carrier plasmas. Here, we discuss experiments that utilize this pulsed terahertz source to probe the dynamical interplay of bound and unbound e-h pairs on a picosecond timescale [2]. A new low-energy oscillator is observed directly after resonant creation of heavy-hole excitons in GaAs quantum wells. This peak arises from transitions between the hydrogen-like exciton bound states, most notably the 1s-2p level transition. The terahertz field probes excitons in a large range of in-plane momenta K in contrast to the usual restriction of interband probes close to K ~ 0. Owing to the strongly correlated motion of electrons and holes, charge-neutral excitons are electrically insulating up to a frequency that matches the separation between their lowest internal states. Above-bandgap excitation at elevated temperatures however induces unbound e-h pairs which represent a conducting ionized gas with a Drude-like response. The distinct responses of these extreme phases enable us to follow in time a metal-insulator transition that occurs upon formation of excitons out of a gas of unbound e-h pairs, as well as its reverse process of ionization. These are dynamical transitions which occur on different timescales. Ionization of excitons can occur within only a few picoseconds, and depends on the phonon occupation. Exci
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