Self-assembled semiconductor quantum dots show remarkable optical and spin coherence properties, which have lead to a concerted research effort examining their potential as a quantum bit for quantum information science1-6. Here, we present an alterna
tive application for such devices, exploiting recent achievements of charge occupation control and the spectral tunability of the optical emission of quantum dots by electric fields7 to demonstrate high-sensitivity electric field measurement. In contrast to existing nanometer-scale electric field sensors, such as single electron transistors8-11 and mechanical resonators12,13, our approach relies on homodyning light resonantly Rayleigh scattered from a quantum dot transition with the excitation laser and phase sensitive lock-in detection. This offers both static and transient field detection ability with high bandwidth operation and near unity quantum efficiency. Our theoretical estimation of the static field sensitivity for typical parameters, 0.5 V/m/ surd Hz, compares favorably to the theoretical limit for single electron transistor-based electrometers. The sensitivity level of 5 V/m/ surd Hz we report in this work, which corresponds to 6.4 * 10-6 e/ surd Hz at a distance of 12 nm, is worse than this theoretical estimate, yet higher than any other result attained at 4.2 K or higher operation temperature.
We temporally resolve the resonance fluorescence from an electron spin confined to a single self-assembled quantum dot to measure directly the spins optical initialization and natural relaxation timescales. Our measurements demonstrate that spin init
ialization occurs on the order of microseconds in the Faraday configuration when a laser resonantly drives the quantum dot transition. We show that the mechanism mediating the optically induced spin-flip changes from electron-nuclei interaction to hole-mixing interaction at 0.6 Tesla external magnetic field. Spin relaxation measurements result in times on the order of milliseconds and suggest that a $B^{-5}$ magnetic field dependence, due to spin-orbit coupling, is sustained all the way down to 2.2 Tesla.
