### Abstract

The electron spin of quantum dots is among the most promising candidates for quantum information processing in the solid state. Optical selection rules make it possible to control and measure spins in isolated quantum dots optically. The implementation of large-scale quantum algorithms requires architectures of coupled quantum dots to implement two-qubit gates. Constructing an efficient interface between spin and photon quantum states is crucial for applications in quantum communication.

We will discuss recent experimental and theoretical work on molecularly coupled quantum dots [1,2] and quantum dot cavity-QED [3]. Using time-resolved Faraday rotation, we have recently demonstrated coherent transfer of electron spin states between quantum dots coupled by conjugated molecules [1]. A simple transfer-Hamiltonian ansatz allows one to derive analytical expressions for the Faraday rotation signal of coupled quantum dots as a function of the probe frequency and captures many of the essential experimental features [2]. Recent progress in solid state microcavity design has led to mode volumes close to the theoretical limit and Q-factors of order 5000, approaching the strong-coupling regime for quantum dot cavity-QED. A quantum dot interacting with two resonant cavity modes is described by a two-mode Jaynes-Cummings model. Depending on the quantum dot energy level scheme, the interaction of a singly doped quantum dot with a cavity photon generates entanglement of electron spin and cavity states or allows one to implement a SWAP gate for spin and photon states [3]. This system thus provides a natural interface between quantum information schemes based on electron spins and linear optics, respectively, and potentially enables the integration of computation and communication.

[1] M. Ouyang and D. D. Awschalom, Science 301, 1074 (2003). [2] F. Meier, V. Cerletti, O. Gywat, D. Loss, and D. D. Awschalom, Phys. Rev. B 69, 195315 (2004). [3] F. Meier and D. D. Awschalom, cond-mat/0405342.