1. Quantum Measurement Group, MIT, Cambridge, Massachusetts 02139, United States
2. Department of Nuclear Science and Engineering, MIT, Cambridge, Massachusetts 02139, United States
3. School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States
4. Department of Physics, MIT, Cambridge, Massachusetts 02139, United States
5. Department of Chemistry, MIT, Cambridge, Massachusetts 02139, United States
6. Department of Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts 02139, United States