Quantum information storage using tunable flux qubits.

We present details and results for a superconducting quantum bit (qubit) design in which a tunable flux qubit is coupled strongly to a transmission line. Quantum information storage in the transmission line is demonstrated with a dephasing time of T(2)∼ 2.5 µs.

Model for 1/f Flux noise in SQUIDs and Qubits.

We propose a model for 1/f flux noise in superconducting devices (f is frequency). The noise is generated by the magnetic moments of electrons in defect states which they occupy for a wide distribution of times before escaping. A trapped electron occupies one of the two Kramers-degenerate ground states, between which the transition rate is negligible at low temperature.

50 omega characteristic impedance low-pass metal powder filters.

We have fabricated several 50 omega characteristic impedance low-pass metal powder filters. The filters are made with bronze or copper metal powder with varying amounts of metal powder in a metal powder/epoxy mixture. Our goal is to make a filter with a characteristic impedance Z = 50 omega at frequencies up to 10 GHz.

Experimental demonstration of an oscillator stabilized Josephson flux qubit.

We experimentally demonstrate the use of a superconducting transmission line, shorted at both ends, to stabilize the operation of a tunable flux qubit. Using harmonic-oscillator stabilization and pulsed dc operation, we have observed Larmor oscillations with a single shot visibility of 90%. In another qubit, the visibility was 60% and there was no measurable visibility reduction after 35 ns.

Escape or switching at short times.

In the standard Arrhenius picture [S. Arrhenius, Z. Phys.

Current-induced excitations in single cobalt ferromagnetic layer nanopillars.

Current-induced excitations in Cu/Co/Cu single ferromagnetic layer nanopillars ( approximately 50 nm in diameter) have been studied experimentally as a function of Co layer thickness at low temperatures for large applied fields perpendicular to the layers. For asymmetric junctions current-induced excitations are observed at high current densities for only one polarity of the current and are absent at the same current densities in symmetric junctions. These observations confirm recent predictions of spin-transfer torque induced spin-wave excitations in single layer junctions with a strong asymmetry in the spin accumulation in the leads.

Time-resolved reversal of spin-transfer switching in a nanomagnet.

Time-resolved measurements of spin-transfer-induced (STI) magnetization reversal were made in current-perpendicular spin-valve nanomagnetic junctions subject to a pulsed current bias. These results can be understood within the framework of a Landau-Lifshitz-Gilbert equation that includes STI corrections and a Langevin random field for finite temperature. Comparison of these measurements with model calculations demonstrates that spin-transfer induced excitation is responsible for the observed magnetic reversal in these samples.

Current-induced magnetization reversal in high magnetic fields in Co/Cu/Co nanopillars.

Current-induced magnetization dynamics in Co/Cu/Co trilayer nanopillars (approximately 100 nm in diameter) have been studied experimentally at low temperatures for large applied fields perpendicular to the layers. At 4.2 K an abrupt and hysteretic increase in resistance is observed at high current densities for one polarity of the current, comparable to the giant magnetoresistance effect observed at low fields.

Coarse graining in micromagnetics.

Numerical solutions of the micromagnetic Landau-Lifshitz-Gilbert equations provide valuable information at low temperatures (T), but produce egregious errors at higher T. For example, Curie temperatures are often overestimated by an order of magnitude. We show that these errors result from the use of block or coarse-grained variables, without a concomitant renormalization of the system parameters to account for the block size.

Direct investigation of superparamagnetism in Co nanoparticle films.

A direct probe of superparamagnetism was used to determine the complete anisotropy energy distribution of Co nanoparticle films. The films were composed of self-assembled lattices of uniform Co nanoparticles of 3 or 5 nm in diameter, and a variable temperature scanning-SQUID microscope was used to measure temperature-induced spontaneous magnetic noise in the samples. Accurate measurements of anisotropy energy distributions of small volume samples will be critical to magnetic optimization of nanoparticle devices and media.

Low-frequency magnetic noise in micron-scale magnetic tunnel junctions.

We have observed low-frequency noise due to quasiequilibrium thermal magnetization fluctuations in micron-scale magnetic tunnel junctions (MTJs). This strongly field-dependent magnetic noise occurs within the magnetic hysteresis loops, either as 1/f or Lorentzian (random telegraph) noise. We attribute it to the thermally excited hopping of magnetic domain walls between pinning sites.

Thermally assisted magnetization reversal in submicron-sized magnetic thin films.

We have measured the rate of thermally assisted magnetization reversal of submicron-sized magnetic thin films. For fields H just less than the zero-temperature switching field H(C), the probability of reversal, P(exp)(s)(t), increases for short times t, achieves a maximum value, and then decreases exponentially. Micromagnetic simulations exhibit the same behavior and show that the reversal proceeds through the annihilation of two domain walls that move from opposite sides of the sample.

All the observed universe has contributed to life.

This paper presents evidence that virtually all electrons and nuclei of the atoms that are or have been part of living matter on Earth came from almost all stars in our and nearby galaxies and even from all other galaxies in the Universe that have produced observed high-energy gamma rays. However, a standard 70 kg human is always making about 7 3He, 600 40Ca, and 3000 14N nuclei every second by radioactive decay of 3H, 40K, and 14C, respectively.