1. Clarendon Laboratory, Department of Physics, University of Oxford, OX1 3PU Oxford, U.K.
2. Department of Engineering Science, University of Oxford, OX1 3PJ Oxford, U.K.
3. School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, United States
4. Asylum Research, Oxford Instruments KK, Tokyo 103-0006, Japan
5. Asylum Research – An Oxford Instruments Company, Santa Barbara, California 93117, United States