Reduced Sensitivity to Charge Noise in Semiconductor Spin Qubits via Symmetric Operation.

We demonstrate improved operation of exchange-coupled semiconductor quantum dots by substantially reducing the sensitivity of exchange operations to charge noise. The method involves biasing a double dot symmetrically between the charge-state anticrossings, where the derivative of the exchange energy with respect to gate voltages is minimized. Exchange remains highly tunable by adjusting the tunnel coupling. We find that this method reduces the dephasing effect of charge noise by more than a factor of 5 in comparison to operation near a charge-state anticrossing, increasing the number of observable exchange oscillations in our qubit by a similar factor. Performance also improves with exchange rate, favoring fast quantum operations.

What Is Hatching in the Egg?

We report VLA detections at 1.3, 2, and 3.6 cm of the Egg Nebula, RAFGL 2688. We resolve the source and find an inner 9&arcsec; diameter core that is produced by approximately 0.01 M middle dot in circle of dust. It seems that the Egg Nebula experienced a major mass ejection, losing approximately 1 M middle dot in circle during the past

The Detached Dust Shell around the Massive Star HD 179821.

We have used the Keck I telescope to resolve at three mid-IR wavelengths the emission from HD 179821 (= RAFGL 2343), a G-type supergiant of perhaps 30 M middle dot in circle with a detached dust shell. The shell is very approximately circular in shape with an inner diameter of approximately 3&farcs;3, corresponding to 3.0x1017 cm. We estimate that the star was losing approximately 4x10-4 M middle dot in circle yr-1 until about 1800 yr ago, when the mass loss slowed dramatically. During the past approximately 104 yr, the star has lost approximately 10% of its initial mass. The star lies about 0&farcs;35 off center and is closer to the brighter, northern hemisphere of the nebula, which can be explained if the outflow velocity Vinfinity deviates by +20% from the average in the southern hemisphere and -20% from the average in the northern hemisphere. The mass-loss rate M&d2;(straight theta) may have been inversely correlated with the outflow velocity so that the momentum outflow p&d2; was isotropic during the mass-loss phase. It also seems that M&d2;totalVinfinity was within a factor of 2 of L*/c, where L* is the current luminosity of the star; the mass loss may have been driven by radiation pressure. These results may help characterize the asymmetric circumstellar winds into which supernova explosions propagate.

A mysterious dust clump in a disk around an evolved binary star system.

The discovery of planets in orbit around the pulsar PSR1257+12 shows that planets may form around post-main-sequence stars. Other evolved stars, such as HD44179 (an evolved star which is part of the binary system that has expelled the gas and dust that make the Red Rectangle nebula), possess gravitationally bound orbiting dust disks. It is possible that planets might form from gravitational collapse in such disks. Here we report high-angular-resolution observations at millimetre and submillimetre wavelengths of the dusk disk associated with the Red Rectangle. We find a dust clump with an estimated mass near that of Jupiter in the outer region of the disk. The clump is larger than our Solar System, and far beyond where planet formation would normally be expected, so its nature is at present unclear.

Search for interstellar superheavy hydrogen.

Models for fundamental physical interactions allow for the existence of stable or nearly stable elementary particles much heavier than the proton. Stellar spectra were searched for a positively charged superheavy particle, X(+), which, with a bound electron, should appear as apparently superheavy neutral hydrogen in the interstellar medium. An upper limit for the abundance of X relative to normal hydrogen in the line of sight toward the bright star gamma Cassiopeiae is 2 x 10(-8).

Differentiation of organic from nonorganic failure to thrive syndrome in infancy.

Differentiation between organic and nonorganic failure to thrive (FTT) often requires prolonged, complicated hospital evaluation. This study was aimed at developing a more direct diagnostic method for distinguishing hospitalized organic from nonorganic FTT infants on the basis of quantifiable social behaviors. An easily administered 1- to 7-point approach-withdrawal scale was created to monitor daily a standardized sequence of brief social interactions between examiner and infant. Behavioral profiles of three matched groups of patients aged 6 to 16 months were compared: eight infants with nonorganic FTT, ten infants with organic FTT, and seven normally grown infants hospitalized for medical reasons. An underlying behavioral pattern systematically and statistically distinguished nonorganic FTT infants from organic FTT infants and the control patients, despite the enormous variability in illness and social distortions in all three groups. Infants with nonorganic FTT predictably prefer distant social encounters and inanimate objects, while organic FTT infants and medically ill control patients consistently respond most positively to close, personal interactions such as touching and holding. Aside from helping to differentiate types of FTT, this study provides a reliable and clinically useful observational tool for easily monitoring ongoing socioemotional status of hospitalized infants.