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 computer1,2,3. Semiconductor quantum dots fabricated by strain driven self-assembly4 are particularly attractive for the realization of spin quantum bits, as they can be controllably positioned5, electronically coupled6 and embedded into active devices7,8,9,10. It has been predicted that the atomic-like electronic structure4 of such quantum dots suppresses coupling of the spin to the solid-state quantum dot environment11,12,13,14, thus protecting the ‘spin’ quantum information against decoherence15,16. 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.