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 $T_2^*$ 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 fil

Controlled Dephasing of a Quantum Dot: From Coherent to Sequential Tunneling

Resonant tunneling through identical potential barriers is a textbook problem in quantum mechanics. Its solution yields total transparency (100% tunneling) at discrete energies. This dramatic phenomenon results from coherent interference among many trajectories, and it is the basis of transport through periodic structures. Resonant tunneling of electrons is commonly seen in semiconducting 'quantum dots'. Here we demonstrate that detecting (distinguishing) electron trajectories in a quantum dot (QD) renders the QD nearly insulating. We couple trajectories in the QD to a 'detector' by employing edge channels in the integer quantum Hall regime. That is, we couple electrons tunneling through an inner channel to electrons in the neighboring outer, 'detector' channel. A small bias applied to the detector channel suffices to dephase (quench) the resonant tunneling completely. We derive a formula for dephasing that agrees well with our data and implies that just a few electrons passing through the detector channel suffice to dephase the QD completely. This basic experiment shows how path detection in a QD induces a transition from delocalization (due to coherent tunneling) to localization (sequential tunneling)

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

Isolated Ballistic Non-Abelian Interface Channel

Non-abelian anyons are prospective candidates for fault-tolerant topological quantum computation due to their long-range entanglement. Curiously these quasiparticles are charge-neutral, hence elusive to most conventional measurement techniques. A proposed host of such quasiparticles is the $\nu$=5/2 quantum Hall state. The gapless edge modes can provide the topological order of the state, which in turn identifies the chirality of the non-abelian mode. Since the $\nu$=5/2 state hosts a variety of edge modes (integer, fractional, neutral), a robust technique is needed to isolate the fractional channel while retaining its original non-abelian character. Moreover, a single non-abelian channel can be easily manipulated to interfere, thus revealing the state's immunity to decoherence. In this work, we exploit a novel approach to gap-out the integer modes of the $\nu$=5/2 state by interfacing the state with integer states, $\nu$=2 & $\nu$=3 (1). The electrical conductance of the isolated interface channel was 0.5e$^2$/h, as expected. More importantly, we find a thermal conductance of 0.5$\kappa_0$T (with $\kappa_0$=$\pi^2k_B^2$/3h), confirming unambiguously the non-abelian nature of the $\nu$=1/2 interface channel and its Particle-Hole Pfaffian topological order. Our result opens new avenues to manipulate and test other exotic QHE states and braid, via interference, the isolated fractional channels.Comment: 20 pages, 4 main figure

Spin dynamics of electrons in the first excited subband of a high-mobility low-density 2D electron system

We report on time-resolved Kerr rotation measurements of spin coherence of electrons in the first excited subband of a high-mobility low-density two-dimensional electron system in a GaAs/Al0.35Ga0.65As heterostructure. While the transverse spin lifetime (T2*) of electrons decreases monotonically with increasing magnetic field, it has a non-monotonic dependence on the temperature, with a peak value of 596 ps at 36 K, indicating the effect of inter-subband electron-electron scattering on the electron spin relaxation. The spin lifetime may be long enough for potential device application with electrons in excited subbands