1Laboratory of Physical Chemistry and optETH, ETH Zurich, 8093 Zurich, Switzerland
Correspondence and requests for materials should be addressed to V.S. ([email protected]).
Received 19 March; accepted 14 May 2009.
Acknowledgements This work was supported by the Swiss National Foundation (SNF) and ETH Zurich (QSIT, grant no. PP-01 07-02). We thank I. Gerhardt and G. Wrigge for their contributions to this project. V.S. thanks X. S. Xie for his hospitality at Harvard University during the writing process of this manuscript.
