Bacterial Hsp70 resolves misfolded states and accelerates productive folding of a multi-domain protein

The ATP-dependent Hsp70 chaperones (DnaK in E. coli) mediate protein folding in cooperation with J proteins and nucleotide exchange factors (E. coli DnaJ and GrpE, respectively). The Hsp70 system prevents protein aggregation and increases folding yields. Whether it also enhances the rate of folding remains unclear. Here we show that DnaK/DnaJ/GrpE accelerate the folding of the multi-domain protein firefly luciferase (FLuc) 20-fold over the rate of spontaneous folding measured in the absence of aggregation. Analysis by single-pair FRET and hydrogen/deuterium exchange identified inter-domain misfolding as the cause of slow folding. DnaK binding expands the misfolded region and thereby resolves the kinetically-trapped intermediates, with folding occurring upon GrpE-mediated release. In each round of release DnaK commits a fraction of FLuc to fast folding, circumventing misfolding. We suggest that by resolving misfolding and accelerating productive folding, the bacterial Hsp70 system can maintain proteins in their native states under otherwise denaturing stress conditions. The Hsp70 system prevents protein aggregation and increases folding yields, but it is unknown whether it also enhances the rate of folding. Here the authors combine refolding assays, FRET and hydrogen/deuterium exchange-mass spectrometry measurements to study the folding of firefly luciferase and find that the bacterial Hsp70 actively promotes the folding of this multi-domain protein

Dynamics of Quantum Dot Nuclear Spin Polarization Controlled by a Single Electron

We present an experimental study of the dynamics underlying the buildup and decay of dynamical nuclear spin polarization in a single semiconductor quantum dot. Our experiment shows that the nuclei can be polarized on a time scale of a few milliseconds, while their decay dynamics depends drastically on external parameters. We show that a single electron can very efficiently depolarize the nuclear spins and discuss two processes that can cause this depolarization. Conversely, in the absence of a quantum dot electron, the lifetime of nuclear spin polarization is on the time scale of a second, most likely limited by the non-secular terms of the nuclear dipole-dipole interaction. We can further suppress this depolarization rate by 1-2 orders of magnitude by applying an external magnetic field exceeding 1 mT.Comment: 5 pages, 3 figure

Effective cross-Kerr nonlinearity and robust phase gates with trapped ions

We derive an effective Hamiltonian that describes a cross-Kerr type interaction in a system involving a two-level trapped ion coupled to the quantized field inside a cavity. We assume a large detuning between the ion and field (dispersive limit) and this results in an interaction Hamiltonian involving the product of the (bosonic) ionic vibrational motion and field number operators. We also demonstrate the feasibility of operation of a phase gate based on our hamiltonian. The gate is insensitive to spontaneous emission, an important feature for the practical implementation of quantum computing.Comment: Included discussion of faster gates (Lamb-Dicke regime), Corrected typos, and Added reference

Knight Field Enabled Nuclear Spin Polarization in Single Quantum Dots

We demonstrate dynamical nuclear spin polarization in the absence of an external magnetic field, by resonant circularly polarized optical excitation of a single electron or hole charged quantum dot. Optical pumping of the electron spin induces an effective inhomogeneous magnetic (Knight) field that determines the direction along which nuclear spins could polarize and enables nuclear-spin cooling by suppressing depolarization induced by nuclear dipole-dipole interactions. Our observations suggest a new mechanism for spin-polarization where spin exchange with an electron reservoir plays a crucial role. These experiments constitute a first step towards quantum measurement of the Overhauser field.Comment: 5 pages, 3 figure

Perturbation Theory for Quantum Computation with Large Number of Qubits

We describe a new and consistent perturbation theory for solid-state quantum computation with many qubits. The errors in the implementation of simple quantum logic operations caused by non-resonant transitions are estimated. We verify our perturbation approach using exact numerical solution for relatively small (L=10) number of qubits. A preferred range of parameters is found in which the errors in processing quantum information are small. Our results are needed for experimental testing of scalable solid-state quantum computers.Comment: 8 pages RevTex including 2 figure

Entanglement and four wave mixing effects in the dissipation free nonlinear interaction of two photons at a single atom

We investigate the nonlinear interaction between two photons in a single input pulse at an atomic two level nonlinearity. A one dimensional model for the propagation of light to and from the atom is used to describe the precise spatiotemporal coherence of the two photon state. It is shown that the interaction generates spatiotemporal entanglement in the output state similar to the entanglement observed in parametric downconversion. A method of generating photon pairs from coherent pump light using this quantum mechanical four wave mixing process is proposed.Comment: 10 pages, including 3 figures, correction in eq.(7), updated references, final version for publication in PR

Nonperturbative Coherent Population Trapping: An Analytic Model

Coherent population trapping is shown to occur in a driven symmetric double-well potential in the strong-field regime. The system parameters have been chosen to reproduce the $0^{-}\leftrightarrow 3^{+}$ transition of the inversion mode of the ammonia molecule. For a molecule initially prepared in its lower doublet we find that, under certain circumstances, the $3^{+}$ level remains unpopulated, and this occurs in spite of the fact that the laser field is resonant with the $0^{-}\leftrightarrow 3^{+}$ transition and intense enough so as to strongly mix the $0^{+}$ and $0^{-}$ ground states. This counterintuitive result constitutes a coherent population trapping phenomenon of nonperturbative origin which cannot be accounted for with the usual models. We propose an analytic nonperturbative model which accounts correctly for the observed phenomenon.Comment: 5 pages, 2 figure

The extracellular chaperone Clusterin enhances Tau aggregate seeding in a cellular model

Spreading of aggregate pathology across brain regions acts as a driver of disease progression in Tau-related neurodegeneration, including Alzheimer’s disease (AD) and frontotemporal dementia. Aggregate seeds released from affected cells are internalized by naïve cells and induce the prion-like templating of soluble Tau into neurotoxic aggregates. Here we show in a cellular model system and in neurons that Clusterin, an abundant extracellular chaperone, strongly enhances Tau aggregate seeding. Upon interaction with Tau aggregates, Clusterin stabilizes highly potent, soluble seed species. Tau/Clusterin complexes enter recipient cells via endocytosis and compromise the endolysosomal compartment, allowing transfer to the cytosol where they propagate aggregation of endogenous Tau. Thus, upregulation of Clusterin, as observed in AD patients, may enhance Tau seeding and possibly accelerate the spreading of Tau pathology

Infrared generation in low-dimensional semiconductor heterostructures via quantum coherence

A new scheme for infrared generation without population inversion between subbands in quantum-well and quantum-dot lasers is presented and documented by detailed calculations. The scheme is based on the simultaneous generation at three frequencies: optical lasing at the two interband transitions which take place simultaneously, in the same active region, and serve as the coherent drive for the IR field. This mechanism for frequency down-conversion does not rely upon any ad hoc assumptions of long-lived coherences in the semiconductor active medium. And it should work efficiently at room temperature with injection current pumping. For optimized waveguide and cavity parameters, the intrinsic efficiency of the down-conversion process can reach the limiting quantum value corresponding to one infrared photon per one optical photon. Due to the parametric nature of IR generation, the proposed inversionless scheme is especially promising for long-wavelength (far- infrared) operation.Comment: 4 pages, 1 Postscript figure, Revtex style. Replacement corrects a printing error in the authors fiel