Optimal Dynamical Decoherence Control of a Qubit

A theory of dynamical control by modulation for optimal decoherence reduction is developed. It is based on the non-Markovian Euler-Lagrange equation for the energy-constrained field that minimizes the average dephasing rate of a qubit for any given dephasing spectrum.Comment: 6 pages, including 2 figures and an appendi

POTENTIATION OF THE T-LYMPHOCYTE RESPONSE TO MITOGENS : I. THE RESPONDING CELL

Human and mouse lymphoid cells, stimulated by phytohemagglutinin (PHA) or lipopolysaccharide W (LPS), release supernatant factor(s) which are mitogenic for mouse thymocytes and which potentiate their responses to PHA or concanavalin A (Con A), The term LAF (lymphocyte-activating factor) is proposed for this activity. LAF not only enhances the mitotic responses of the less dense thymus subpopulations (A, B, and C) separable on discontinuous bovine serum albumin (BSA) gradients but also gives substantial responses in the otherwise inert cells of the denser fractions D and P. LAF does not exert a potentiating stimulatory effect on the responses of unfractionated mouse spleen cells, but does act synergistically with PHA on nonadherent spleen cells and on spleen cells of mice of several strains 5 days after irradiation and injection of thymocytes. Similarly LAF, which has no visible effect on unfractionated human peripheral blood cells, strongly potentiates the PHA response of column-purified lymphocytes, when these are cultured at low concentration. We conclude that LAF stimulates both central and peripheral T lymphocytes and enhances their responses to other stimulants

Nonradiative interaction and entanglement between distant atoms

We show that nonradiative interactions between atomic dipoles placed in a waveguide can give rise to deterministic entanglement at ranges much larger than their resonant wavelength. The range increases as the dipole-resonance approaches the waveguide's cutoff frequency, caused by the giant density of photon modes near cutoff, a regime where the standard (perturbative) Markov approximation fails. We provide analytical theories for both the Markovian and non-Markovian regimes, supported by numerical simulations, and discuss possible experimental realizations.Comment: 9 pages, 2 figure

Moments of quadratic twists of elliptic curve L-functions over function fields

We calculate the first and second moments of L-functions in the family of quadratic twists of a fixed elliptic curve E over F_q[x], asymptotically in the limit as the degree of the twists tends to infinity. We also compute moments involving derivatives of L-functions over quadratic twists, enabling us to deduce lower bounds on the correlations between the analytic ranks of the twists of two distinct curves.Comment: 32 page

Symmetric photon-photon coupling by atoms with Zeeman-split sublevels

We propose a simple scheme for highly efficient nonlinear interaction between two weak optical fields. The scheme is based on the attainment of electromagnetically induced transparency simultaneously for both fields via transitions between magnetically split F=1 atomic sublevels, in the presence of two driving fields. Thereby, equal slow group velocities and symmetric cross-coupling of the weak fields over long distances are achieved. By simply tuning the fields, this scheme can either yield giant cross-phase modulation or ultrasensitive two-photon switching.Comment: Modified scheme, 4 pages, 1 figur

Generation of macroscopic quantum-superposition states by linear coupling to a bath

We demonstrate through an exactly solvable model that collective coupling to any thermal bath induces effectively nonlinear couplings in a quantum many-body (multi-spin) system. The resulting evolution can drive an uncorrelated large-spin system with high probability into a macroscopic quantum-superposition state. We discuss possible experimental realizations.Comment: 4 pages, 2 figures, Physical Review Letters (in press

Universal dynamical decoherence control of noisy single-and multi-qubit systems

In this article we develop, step by step, the framework for universal dynamical control of two-level systems (TLS) or qubits experiencing amplitude- or phase-noise (AN or PN) due to coupling to a thermal bath. A comprehensive arsenal of modulation schemes is introduced and applied to either AN or PN, resulting in completely analogous formulae for the decoherence rates, thus underscoring the unified nature of this universal formalism. We then address the extension of this formalism to multipartite decoherence control, where symmetries are exploited to overcome decoherence.Comment: 28 pages, 4 figure

Charged Dilaton Black Holes with Unusual Asymptotics

We present a new class of black hole solutions in Einstein-Maxwell-dilaton gravity in $n \ge 4$ dimensions. These solutions have regular horizons and a singularity only at the origin. Their asymptotic behavior is neither asymptotically flat nor (anti-) de Sitter. Similar solutions exist for certain Liouville-type potentials for the dilaton.Comment: 24 pages, harvmac.tex, no figure