Suppression of dephasing by qubit motion in superconducting circuits

We suggest and demonstrate a protocol which suppresses dephasing due to the low-frequency noise by qubit motion, i.e., transfer of the logical qubit of information in a system of $n \geq 2$ physical qubits. The protocol requires only the nearest-neighbor coupling and is applicable to different qubit structures. We further analyze its effectiveness against noises with arbitrary correlations. Our analysis, together with experiments using up to three superconducting qubits, shows that for the realistic uncorrelated noises, qubit motion increases the dephasing time of the logical qubit as $\sqrt{n}$. In general, the protocol provides a diagnostic tool to measure the noise correlations.Comment: 5 pages with 3 embedded figures, plus supplementary informatio

Rates of Neutrino Absorption on Nucleons and the Reverse Processes in Strong Magnetic Fields

The rates of electron neutrino capture on neutron, electron anti-neutrino capture on proton, and their reverse processes are important for understanding the production of heavy elements in the supernova environment above the protoneutron star. Observations and theoretical considerations suggest that some protoneutron stars may be born with strong magnetic fields. We develop a numerical method to calculate the above rates in supernova environments with magnetic fields up to 10^16 G. This method is accurate to the order of one over nucleon mass. We show that our results have the correct behavior in the limit of high neutrino energy or small magnetic field. Based on comparison of our results with various approximations, we recommend efficient estimates of the above rates for use in models of supernova nucleosynthesis in the presence of strong magnetic fields.Comment: 21 pages, 4 figures. Some explaination and references are added in the second versio

First-principles theory of ferroelectric phase transitions for perovskites: The case of BaTiO3

We carry out a completely first-principles study of the ferroelectric phase transitions in BaTiO$_3$. Our approach takes advantage of two features of these transitions: the structural changes are small, and only low-energy distortions are important. Based on these observations, we make systematically improvable approximations which enable the parameterization of the complicated energy surface. The parameters are determined from first-principles total-energy calculations using ultra-soft pseudopotentials and a preconditioned conjugate-gradient scheme. The resulting effective Hamiltonian is then solved by Monte Carlo simulation. The calculated phase sequence, transition temperatures, latent heats, and spontaneous polarizations are all in good agreement with experiment. We find the transitions to be intermediate between order-disorder and displacive character. We find all three phase transitions to be of first order. The roles of different interactions are discussed.Comment: 33 pages latex file, 9 figure

Static displacements and chemical correlations in alloys

Recent experiments in metallic solid solutions have revealed interesting correlations between static pair-displacements and the ordering behavior of these alloys. This paper discusses a simple theoretical model which successfully explains these observations and which provides a natural framework for analyzing experimental measurements of pair-displacements and chemical correlations in solid solutions. The utility and scope of this model is demonstrated by analyzing results of experiments on $Ni-Fe$ and $Cr-Fe$ alloys and results of simulations of $Cu-Au$ and $Cu-Ag$ alloys.Comment: 12 page

Concentration for One and Two Species One-Dimensional Reaction-Diffusion Systems

We look for similarity transformations which yield mappings between different one-dimensional reaction-diffusion processes. In this way results obtained for special systems can be generalized to equivalent reaction-diffusion models. The coagulation (A + A -> A) or the annihilation (A + A -> 0) models can be mapped onto systems in which both processes are allowed. With the help of the coagulation-decoagulation model results for some death-decoagulation and annihilation-creation systems are given. We also find a reaction-diffusion system which is equivalent to the two species annihilation model (A + B ->0). Besides we present numerical results of Monte Carlo simulations. An accurate description of the effects of the reaction rates on the concentration in one-species diffusion-annihilation model is made. The asymptotic behavior of the concentration in the two species annihilation system (A + B -> 0) with symmetric initial conditions is studied.Comment: 20 pages latex, uuencoded figures at the en

Fluid pumping system based on electrochemically-induced surface tension changes

A pump for exerting a pumping force on a fluid in an attached conduit, the pump being actuated by a voltage from a power supply, the pump comprising a liquid metal pool, a containment vessel for confining the pool, an inner tubular member in hydraulic contact with the attached conduit and with an open end disposed in the pool to divide a free upper surface of the pool into an actuation surface and a working surface, an electrolyte in contact with the actuation surface, an electrode in contact with the electrolyte, and an electrode in contact with the pool, such that application of a voltage to the electrodes causes a surface tension change in the actuation surface and a resulting height change in the working surface, whereby a pumping action is transferred to the fluid in the attached conduit