Entanglement is the characteristic trait of quantum physics. It has no counterpart in the classical world of our daily experience and it is not completely understood by the experts either. And yet we know that it plays a crucial role in the power of quantum communication and quantum computing. Our progress in dissecting entanglement and applying it as a tool has been spurred by the development of increasingly better sources. For a long time entanglement creation was the sole domain of nonlinear optics, but recently we have begun to generate entanglement from and in other systems, atomic and solid state.

Quantum dots are "artificial atoms" in which charge carriers - electrons and holes - are confined in a nanometer-sized semiconductor region. Their advantages over actual atoms include that they are fixed in a semiconductor matrix and thus fixed in space and that they can be designed to have specific properties. The downsides are that not all properties can be controlled perfectly and that the semiconductor matrix is not as clean an environment as a vacuum chamber. Environmental influences can arm the coherence of carriers in the dot and the emitted photons.

We mostly work with InAs/GaAs quantum dots grown and/or provided by our various collaborators: P. Michler (U. Stuttgart), E. Pelucchi (Tyndall National Institute), P. Poole (NRC Ottawa), A. Rastelli (U. Linz), G. Solomon (NIST Gaithersburg).

Our goal of achieving entanglement from semiconductor quantum dots requires that we have excellent control. To this end we realized coherent two-photon excitation of a single quantum dot (see image), which allowed us to create time-bin entangled photon pairs from the quantum dot.