Realization of efficient metal grating couplers for membrane-based integrated photonics.

Grating couplers are widely used to couple light between photonic integrated circuits and optical fibers. Here, we fabricate and characterize a device based on a buried metal grating. In contrast to dielectric gratings, simulations predict strongly reduced parasitic leakage of light to the substrate and are performance independent of the optical buffer thickness, while using standard fabrication processes with high yield. The gratings show a 3 dB bandwidth of 61 nm and chip-to-fiber coupling efficiency of 54%, which makes them attractive building blocks for on-wafer testing and dense optical interconnects.

An exchange-coupled donor molecule in silicon.

We present a combined experimental-theoretical demonstration of the energy spectrum and exchange coupling of an isolated donor pair in a silicon nanotransistor. The molecular hybridization of the atomic orbitals leads to an enhancement of the one- and two-electron binding energies and charging energy with respect to the single donor case, a desirable feature for quantum electronic devices. Our hydrogen molecule-like model based on a multivalley central-cell corrected effective mass theory incorporating a full configuration interaction treatment of the 2-electron spectrum matches the measured data for an arsenic diatomic molecule with interatomic distance R = 2.3 ± 0.5 nm.

Optically programmable electron spin memory using semiconductor quantum dots.

The spin of a single electron subject to a static magnetic field provides a natural two-level system that is suitable for use as a quantum bit, the fundamental logical unit in a quantum computer. Semiconductor quantum dots fabricated by strain driven self-assembly are particularly attractive for the realization of spin quantum bits, as they can be controllably positioned, electronically coupled and embedded into active devices. It has been predicted that the atomic-like electronic structure of such quantum dots suppresses coupling of the spin to the solid-state quantum dot environment, thus protecting the 'spin' quantum information against decoherence. Here we demonstrate a single electron spin memory device in which the electron spin can be programmed by frequency selective optical excitation. We use the device to prepare single electron spins in semiconductor quantum dots with a well defined orientation, and directly measure the intrinsic spin flip time and its dependence on magnetic field. A very long spin lifetime is obtained, with a lower limit of about 20 milliseconds at a magnetic field of 4 tesla and at 1 kelvin.