7 research outputs found
Enhancing the Coherence of a Spin Qubit by Operating it as a Feedback Loop That Controls its Nuclear Spin Bath
In many realizations of electron spin qubits the dominant source of
decoherence is the fluctuating nuclear spin bath of the host material. The
slowness of this bath lends itself to a promising mitigation strategy where the
nuclear spin bath is prepared in a narrowed state with suppressed fluctuations.
Here, this approach is realized for a two-electron spin qubit in a GaAs double
quantum dot and a nearly ten-fold increase in the inhomogeneous dephasing time
is demonstrated. Between subsequent measurements, the bath is prepared
by using the qubit as a feedback loop that first measures its nuclear
environment by coherent precession, and then polarizes it depending on the
final state. This procedure results in a stable fixed point at a nonzero
polarization gradient between the two dots, which enables fast universal qubit
control.Comment: Journal version. Improved clarity of presentation and more concise
terminology. 4 pages, 3 figures. Supplementary document included as ancillary
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Immunoglobulin domains in Escherichia coli and other enterobacteria: from pathogenesis to applications in antibody technologies
The immunoglobulin (Ig) protein domain is widespread in nature having a well-recognized role in proteins of the immune system. In this review, we describe the proteins containing Ig-like domains in
Escherichia coli and entero-bacteria, reporting their structural and functional properties, protein folding, and diverse biological roles. In addition, we cover the expression of heterolo-gous Ig domains in E. coli owing to its biotechnological application for expression and selection of antibody fragments and full-length IgG molecules. Ig-like domains in E. coli and enterobacteria are frequently found in cell surface proteins and fimbrial organelles playing important functions during host cell
adhesion and invasion of pathogenic strains, being structural components of pilus and nonpilus fimbrial systems and members of the intimin/invasin family of outer membrane (OM) adhesins. Ig-like domains are also found in periplasmic chaperones and OM usher proteins assembling fimbriae, in oxidoreductases and hydrolytic enzymes, ATP-binding cassette transporters, sugar-binding and metal-resistance proteins. The folding of most E. coli Ig-like domains is
assisted by periplasmic chaperones, peptidyl
prolylcis/transisomerases and disulfide bond catalysts that also participate in the folding of antibodies expressed in this bacterium. The technologies for expression and selection of recombinant antibodies in
E. coli are described along with their biotechnological potential.This work has been supported by Grants of the Spanish Ministry of Science and Innovation (BIO2008-05201; BIO2011-26689), the Autonomous Community of Madrid
(S-BIO-236-2006; S2010-BMD-2312), CSIC (PIE 2011 20E049), ‘la Caixa’ Foundation, and the VI Framework Program from the European Union (FP6-LSHB-CT-2005-512061 NoE ‘EuroPathogenomics’).Peer reviewe
Semi-classical model for the dephasing of a two-electron spin qubit coupled to a coherently evolving nuclear spin bath
We study electron spin decoherence in a two-electron double quantum dot due
to the hyperfine interaction, under spin-echo conditions as studied in recent
experiments. We develop a semi-classical model for the interaction between the
electron and nuclear spins, in which the time-dependent Overhauser fields
induced by the nuclear spins are treated as classical vector variables.
Comparison of the model with experimentally-obtained echo signals allows us to
quantify the contributions of various processes such as coherent Larmor
precession and spin diffusion to the nuclear spin evolution.Comment: 14 Pages, some equations were corrected; Published July 27, 201
Universal quantum control of two-electron spin quantum bits using dynamic nuclear polarization
One fundamental requirement for quantum computation is to perform universal
manipulations of quantum bits at rates much faster than the qubit's rate of
decoherence. Recently, fast gate operations have been demonstrated in logical
spin qubits composed of two electron spins where the rapid exchange of the two
electrons permits electrically controllable rotations around one axis of the
qubit. However, universal control of the qubit requires arbitrary rotations
around at least two axes. Here we show that by subjecting each electron spin to
a magnetic field of different magnitude we achieve full quantum control of the
two-electron logical spin qubit with nanosecond operation times. Using a single
device, a magnetic field gradient of several hundred milliTesla is generated
and sustained using dynamic nuclear polarization of the underlying Ga and As
nuclei. Universal control of the two-electron qubit is then demonstrated using
quantum state tomography. The presented technique provides the basis for single
and potentially multiple qubit operations with gate times that approach the
threshold required for quantum error correction.Comment: 11 pages, 4 figures. Supplementary Material included as ancillary
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Tuning Methods for Semiconductor Spin Qubits
We present efficient methods to reliably characterize and tune gate-defined semiconductor spin qubits. Our methods are developed for double quantum dots in GaAs heterostructures, but they can easily be adapted to other quantum-dot-based qubit systems. These tuning procedures include the characterization of the interdot tunnel coupling, the tunnel coupling to the surrounding leads, and the identification of various fast initialization points for the operation of the qubit. Since semiconductor-based spin qubits are compatible with standard semiconductor process technology and hence promise good prospects of scalability, the challenge of efficiently tuning the dot’s parameters will only grow in the near future, once the multiqubit stage is reached. With the anticipation of being used as the basis for future automated tuning protocols, all measurements presented here are fast-to-execute and easy-to-analyze characterization methods. They result in quantitative measures of the relevant qubit parameters within a couple of seconds and require almost no human interference