303 research outputs found
Measurement induced entanglement and quantum computation with atoms in optical cavities
We propose a method to prepare entangled states and implement quantum
computation with atoms in optical cavities. The internal state of the atoms are
entangled by a measurement of the phase of light transmitted through the
cavity. By repeated measurements an entangled state is created with certainty,
and this entanglement can be used to implement gates on qubits which are stored
in different internal degrees of freedom of the atoms. This method, based on
measurement induced dynamics, has a higher fidelity than schemes making use of
controlled unitary dynamics.Comment: 4 pages including 2 figures. v2+3: minor change
Spin-Orbit Mediated Control of Spin Qubits
We propose to use the spin-orbit interaction as a means to control electron
spins in quantum dots, enabling both single qubit and two qubit operations.
Very fast single qubit operations may be achieved by temporarily displacing the
electrons. For two qubit operations the coupling mechanism is based on a
combination of the spin-orbit coupling and the mutual long-ranged Coulomb
interaction. Compared to existing schemes using the exchange coupling, the
spin-orbit induced coupling is less sensitive to random electrical fluctuations
in the electrodes defining the quantum dots.Comment: 4 pages, 3 figures; minor changes - final version as published in
Phys. Rev. Let
Spin-orbit induced spin-qubit control in nanowires
We elaborate on a number of issues concerning our recent proposal for
spin-qubit manipulation in nanowires using the spin-orbit coupling. We discuss
the experimental status and describe in further detail the scheme for
single-qubit rotations. We present a derivation of the effective two-qubit
coupling which can be extended to higher orders in the Coulomb interaction. The
analytic expression for the coupling strength is shown to agree with numerics.Comment: 5 pages, 3 figures, Contribution to ICN+T2006, Basel, Switzerland,
July-August, 200
Fractional quantum Hall states of atoms in optical Lattices
We describe a method to create fractional quantum Hall states of atoms
confined in optical lattices. We show that the dynamics of the atoms in the
lattice is analogous to the motion of a charged particle in a magnetic field if
an oscillating quadrupole potential is applied together with a periodic
modulation of the tunneling between lattice sites. We demonstrate that in a
suitable parameter regime the ground state in the lattice is of the fractional
quantum Hall type and we show how these states can be reached by melting a Mott
insulator state in a super lattice potential. Finally we discuss techniques to
observe these strongly correlated states.Comment: 4+epsilon pages including 3 figures. V2: Changes in the presentatio
Spin-Photon Entangling Diode
We propose a semiconductor device that can electrically generate entangled
electron spin-photon states, providing a building block for entanglement of
distant spins. The device consists of a p-i-n diode structure that incorporates
a coupled double quantum dot. We show that electronic control of the diode bias
and local gating allow for the generation of single photons that are entangled
with a robust quantum memory based on the electron spins. Practical performance
of this approach to controlled spin-photon entanglement is analyzed.Comment: 4 pages, 2 figures; figures update
A Hybrid Long-Distance Entanglement Distribution Protocol
We propose a hybrid (continuous-discrete variable) quantum repeater protocol
for distribution of entanglement over long distances. Starting from entangled
states created by means of single-photon detection, we show how entangled
coherent state superpositions, also known as `Schr\"odinger cat states', can be
generated by means of homodyne detection of light. We show that
near-deterministic entanglement swapping with such states is possible using
only linear optics and homodyne detectors, and we evaluate the performance of
our protocol combining these elements.Comment: 4 pages, 3 figure
Molecular Basis of Enhanced Activity in Factor VIIa-Trypsin Variants Conveys Insights into Tissue Factor-mediated Allosteric Regulation of Factor VIIa Activity
The complex of coagulation factor VIIa (FVIIa), a trypsin-like serine protease, and membrane-bound tissue factor (TF) initiates blood coagulation upon vascular injury. Binding of TF to FVIIa promotes allosteric conformational changes in the FVIIa protease domain and improves its catalytic properties. Extensive studies have revealed two putative pathways for this allosteric communication. Here we provide further details of this allosteric communication by investigating FVIIa loop swap variants containing the 170 loop of trypsin that display TF-independent enhanced activity. Using x-ray crystallography, we show that the introduced 170 loop from trypsin directly interacts with the FVIIa active site, stabilizing segment 215â217 and activation loop 3, leading to enhanced activity. Molecular dynamics simulations and novel fluorescence quenching studies support that segment 215â217 conformation is pivotal to the enhanced activity of the FVIIa variants. We speculate that the allosteric regulation of FVIIa activity by TF binding follows a similar path in conjunction with protease domain N terminus insertion, suggesting a more complete molecular basis of TF-mediated allosteric enhancement of FVIIa activity
Density-matrix renormalisation group approach to quantum impurity problems
A dynamic density-matrix renormalisation group approach to the spectral
properties of quantum impurity problems is presented. The method is
demonstrated on the spectral density of the flat-band symmetric single-impurity
Anderson model. We show that this approach provides the impurity spectral
density for all frequencies and coupling strengths. In particular, Hubbard
satellites at high energy can be obtained with a good resolution. The main
difficulties are the necessary discretisation of the host band hybridised with
the impurity and the resolution of sharp spectral features such as the
Abrikosov-Suhl resonance.Comment: 16 pages, 6 figures, submitted to Journal of Physics: Condensed
Matte
A systematic approach for evaluating the role of surface-exposed loops in trypsin-like serine proteases applied to the 170 loop in coagulation factor VIIa
Proteases play a major role in many vital physiological processes. Trypsin-like serine proteases (TLPs), in particular, are paramount in proteolytic cascade systems such as blood coagulation and complement activation. The structural topology of TLPs is highly conserved, with the trypsin fold comprising two β-barrels connected by a number of variable surface-exposed loops that provide a surprising capacity for functional diversity and substrate specificity. To expand our understanding of the roles these loops play in substrate and co-factor interactions, we employ a systematic methodology akin to the natural truncations and insertions observed through evolution of TLPs. The approach explores a larger deletion space than classical random or directed mutagenesis. Using FVIIa as a model system, deletions of 1â7 amino acids through the surface exposed 170 loop, a vital allosteric regulator, was introduced. All variants were extensively evaluated by established functional assays and computational loop modelling with Rosetta. The approach revealed detailed structural and functional insights recapitulation and expanding on the main findings in relation to 170 loop functions elucidated over several decades using more cumbersome crystallization and single deletion/mutation methodologies. The larger deletion space was key in capturing the most active variant, which unexpectedly had a six-amino acid truncation. This variant would have remained undiscovered if only 2â3 deletions were considered, supporting the usefulness of the methodology in general protease engineering approaches. Our findings shed further light on the complex role that surface-exposed loops play in TLP function and supports the important role of loop length in the regulation and fine-tunning of enzymatic function throughout evolution
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