Dynamics of polymer chain collapse into compact states

Molecular dynamics simulation methods are used to study the folding of polymer chains into packed cubic states. The polymer model, based on a chain of linked sites moving in the continuum, includes both excluded volume and torsional interactions. Different native-state packing arrangements and chain lengths are explored; the organization of the native state is found to affect both the ability of the chain to fold successfully and the nature of the folding pathway as the system is gradually cooled. An order parameter based on contact counts is used to provide information about the folding process, with contacts additionally classified according to criteria such as core and surface sites or local and distant site pairs. Fully detailed contact maps and their evolution are also examined.Comment: 11 pages, 11 figures (some low resolution

Secure quantum key distribution using squeezed states

We prove the security of a quantum key distribution scheme based on transmission of squeezed quantum states of a harmonic oscillator. Our proof employs quantum error-correcting codes that encode a finite-dimensional quantum system in the infinite-dimensional Hilbert space of an oscillator, and protect against errors that shift the canonical variables p and q. If the noise in the quantum channel is weak, squeezing signal states by 2.51 dB (a squeeze factor e^r=1.34) is sufficient in principle to ensure the security of a protocol that is suitably enhanced by classical error correction and privacy amplification. Secure key distribution can be achieved over distances comparable to the attenuation length of the quantum channel.Comment: 19 pages, 3 figures, RevTeX and epsf, new section on channel losse

Correlated Errors in Quantum Error Corrections

We show that errors are not generated correlatedly provided that quantum bits do not directly interact with (or couple to) each other. Generally, this no-qubits-interaction condition is assumed except for the case where two-qubit gate operation is being performed. In particular, the no-qubits-interaction condition is satisfied in the collective decoherence models. Thus, errors are not correlated in the collective decoherence. Consequently, we can say that current quantum error correcting codes which correct single-qubit-errors will work in most cases including the collective decoherence.Comment: no correction, 3 pages, RevTe

Constraints on the uncertainties of entangled symmetric qubits

We derive necessary and sufficient inseparability conditions imposed on the variance matrix of symmetric qubits. These constraints are identified by examining a structural parallelism between continuous variable states and two qubit states. Pairwise entangled symmetric multiqubit states are shown here to obey these constraints. We also bring out an elegant local invariant structure exhibited by our constraints.Comment: 5 pages, REVTEX, Improved presentation; Theorem on neccessary and sufficient condition included; To appear in Phys. Lett.

Resonant cancellation of off-resonant effects in a multilevel qubit

Off-resonant effects are a significant source of error in quantum computation. This paper presents a group theoretic proof that off-resonant transitions to the higher levels of a multilevel qubit can be completely prevented in principle. This result can be generalized to prevent unwanted transitions due to qubit-qubit interactions. A simple scheme exploiting dynamic pulse control techniques is presented that can cancel transitions to higher states to arbitrary accuracy.Comment: 4 pages, Revtex, submitted for publicatio

Non-adiabatic geometrical quantum gates in semiconductor quantum dots

In this paper we study the implementation of non-adiabatic geometrical quantum gates with in semiconductor quantum dots. Different quantum information enconding/manipulation schemes exploiting excitonic degrees of freedom are discussed. By means of the Aharanov-Anandan geometrical phase one can avoid the limitations of adiabatic schemes relying on adiabatic Berry phase; fast geometrical quantum gates can be in principle implementedComment: 5 Pages LaTeX, 10 Figures include

Screening of qubit from zero-temperature reservoir

We suggest an application of dynamical Zeno effect to isolate a qubit in the quantum memory unit against decoherence caused by coupling with the reservoir having zero temperature. The method is based on using an auxiliary casing system that mediate the qubit-reservoir interaction and is simultaneously frequently erased to ground state. This screening procedure can be implemented in the cavity QED experiments to store the atomic and photonic qubit states.Comment: 4 pages, 5 figure

Spin-based all-optical quantum computation with quantum dots: understanding and suppressing decoherence

We present an all-optical implementation of quantum computation using semiconductor quantum dots. Quantum memory is represented by the spin of an excess electron stored in each dot. Two-qubit gates are realized by switching on trion-trion interactions between different dots. State selectivity is achieved via conditional laser excitation exploiting Pauli exclusion principle. Read-out is performed via a quantum-jump technique. We analyze the effect on our scheme's performance of the main imperfections present in real quantum dots: exciton decay, hole mixing and phonon decoherence. We introduce an adiabatic gate procedure that allows one to circumvent these effects, and evaluate quantitatively its fidelity

Quantitative Treatment of Decoherence

We outline different approaches to define and quantify decoherence. We argue that a measure based on a properly defined norm of deviation of the density matrix is appropriate for quantifying decoherence in quantum registers. For a semiconductor double quantum dot qubit, evaluation of this measure is reviewed. For a general class of decoherence processes, including those occurring in semiconductor qubits, we argue that this measure is additive: It scales linearly with the number of qubits.Comment: Revised version, 26 pages, in LaTeX, 3 EPS figure

Organic photovoltaic devices with enhanced efficiency processed from non-halogenated binary solvent blends

The development of processing routes to fabricate organic photovoltaic devices (OPVs) using non-halogenated solvents is a necessary step towards their eventual commercialisation. To address this issue, we have used Hansen solubility parameter analysis to identify a non-halogenated solvent blend based on a mixture of carbon disulphide and acetone. This solvent blend was then used to deposit a donor–acceptor polymer–fullerene thin-film that was then used as the active layer of bulk-heterojunction OPV. For the benchmark polymer:fullerene system PCDTBT:PC70BM, a power conversion efficiency of 6.75% was achieved; a 20% relative improvement over reference cells processed using the chlorinated-solvent chlorobenzene. Improvements in device efficiency are attributed to an increase in electron and hole conductivity resulting from enhanced fullerene crystallisation; a property that leads to enhanced device efficiency through improved charge extraction