HYBRIDQUANTSYS · An Efficient Optical Interface between Quantum Dots and Ultracold Atoms

An efficient optical interface between quantum dots and ultracold atoms

The principle objective of the Marie Curie IIF Project HYBRIDQUANTSYS is the storage and retrieval of a quantum dot (QD) generated single photon in an ultracold ensemble of rubidium (Rb) atoms. This project aims to advance quantum information science by combining two leading technologies. QDs are mesoscopic, atomic-like systems in the solid-state that are capable of producing quantum states of light in a broadband, triggered fashion. As such, they are being developed as a resource for quantum information processing. However, it has been shown that in order to perform quantum information tasks over long distances, like quantum key distribution or distributed computation, quantum light sources must be complemented by quantum memories. Quantum memories are devices that can store quantum states of light for a controlled amount of time. A leading implementation of a quantum memory is based on dense ensembles of three-level atoms. Since Rb atoms can be laser-cooled and trapped into dense ensembles and have exceptionally long ground state coherence times, they have emerged as a natural choice for a quantum memory. Thus, this project aims to take the next steps towards integrating a QD photon source with a Rb quantum memory. This includes the development of a complete theoretical description of the system, an experimental study of QDs with optical transitions near 780 nm or 795 nm, and the development of a high optical depth Rb ensemble to be used as a quantum memory. Since the project was initiated in March 2012, we have been working hard to implement these objectives, which form the foundation for integrating QD single photon sources with ultracold atom quantum memories. The detailed theoretical investigation has been completed, yielding the anticipated efficiencies of the storage/retrieval processes as well as the necessary control fields. This investigation required the extension of existing theoretical models to account for the large bandwidth of the QD photon. In addition, various hyperfine levels of Rb for implementation of the memory were compared using the newly-developed theory and clear choices for optimization were determined. Experimental imperfections and non-idealities were also taken into account and were shown to only cause minor reductions in efficiencies. Simultaneously with this effort, we collaborated with the group of Prof. Richard Warburton to investigate the properties of QDs with emission near 780 nm. While initial results showed QDs with too much inhomogeneous broadening, clear avenues for getting better samples have been identified and work is ongoing. The high optical depth Rb ensemble apparatus is also being developed. By using a 2D magneto-optical trap (MOT) to load a 3D-MOT, we have been able to achieve high atom numbers, fast loading times, and long trap lifetimes all of which are important parameters for the quantum memory. Currently, we are optimizing the setup and beginning to implement an optical lattice trap to further reduce effects of atomic motion in the quantum memory. The main result achieved so far is the development of an extended theoretical treatment of broadband photon storage in Rb, which we have submitted for publication. We feel that the results are of significant interest to the quantum information community and will spur further collaboration between the QD community and the cold atoms community. In addition, work on QD spectroscopy in the 780 nm band should result in a forthcoming publication. The project is progressing nicely and further experimental results are anticipated in the coming months, in particular an investigation of improved quantum dot samples that are expected to show spectrally narrower emission. Notably, the researcher Matthew Rakher has accepted a permanent scientific position in his home country (the United States) and will terminate his Marie Curie fellowship after 13 months (March 2013). By this time, sufficient transfer of knowledge has taken place between the fellow and the other members of the host group such that the experiments can smoothly continue. Furthermore, a major objective has been completed as obtaining a permanent scientific position in the researcher's home country is one of the primary goals of the Marie Curie Fellowship.

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