Avalanche amplification of a single exciton in a semiconductor nanowire

Interfacing single photons and electrons is a crucial ingredient for sharing quantum information between remote solid-state qubits. Semiconductor nanowires offer the unique possibility to combine optical quantum dots with avalanche photodiodes, thus enabling the conversion of an incoming single photon into a macroscopic current for efficient electrical detection. Currently, millions of excitation events are required to perform electrical read-out of an exciton qubit state. Here we demonstrate multiplication of carriers from only a single exciton generated in a quantum dot after tunneling into a nanowire avalanche photodiode. Due to the large amplification of both electrons and holes (> 10^4), we reduce by four orders of magnitude the number of excitation events required to electrically detect a single exciton generated in a quantum dot. This work represents a significant step towards single-shot electrical read-out and offers a new functionality for on-chip quantum information circuits

Assessment of motor functioning in the preschool period

The assessment of motor functioning in young children has become increasingly important in recent years with the acknowledgement that motor impairment is linked with cognitive, language, social and emotional difficulties. However, there is no one gold standard assessment tool to investigate motor ability in children. The aim of the current paper was to discuss the issues related to the assessment of motor ability in young pre-school children and to provide guidelines on the best approach for motor assessment. The paper discusses the maturational changes in brain development at the preschool level in relation to motor ability. Other issues include sex differences in motor ability at this young age, and evidence for this in relation to sociological versus biological influences. From the previous literature it is unclear what needs to be assessed in relation to motor functioning. Should the focus be underlying motor processes or movement skill assessment? Several key assessment tools are discussed that produce a general measure of motor performance followed by a description of tools that assess specific skills, such as fine and gross motor, ball and graphomotor skills. The paper concludes with recommendations on the best approach in assessing motor function in pre-school children

Strained-layer superlattices physics

SEMICONDUCTORS & SEMIMETALS V3

Voltage-tunable near-infrared photodetector: Versatile component for optical communication systems

We present a voltage-tunable wavelength-selective photodetector (VWP) for the near-infrared (NIR), realized by engineering the optical properties of Si and Ge. The structure exhibits a wide tunability by varying the applied voltage and weighting the photocurrents generated by two back-to-back diodes. We demonstrate its versatility in deconvolving signals of different wavelengths and a preliminary study on the applications of the VWP as a solid-state spectrum analyzer, The spectrum of NIR light impinging the device can be resolved from a set of photocurrent measurements for different bias voltages. (C) 1998 American Vacuum Society

Photocurrent spectroscopy measurements of Si-Ge alloys and superlattices

We used photocurrent spectroscopy at 300 K, 77 K and 4.2 K to investigate the optical absorption processes of (001) Si-Ge strained layer superlattices and alloys with an average composition of 50%Si-50%Ge. We observed an evolution toward higher energies of the threshold in the photocurrent spectra as the period of the superlattices decreases, with the spectrum of the shortest period superlattices (2:2) approaching that of the alloy. The energy dependence of absorption in these Si-Gr heterostructures is quite distinct from that measured in elemental silicon