[1] Univ Southampton, Dept Phys & Astron, Southampton SO17 1BJ, Hants, England
[2] Univ Sheffield, Dept Elect & Elect Engn, EPSRC Cent Facil 3 5 Semicond, Sheffield S1 3JD, S Yorkshire, England
[3] ETH Honggerberg, HPT, Swiss Fed Inst Technol, Inst Quantum Elect, CH-8093 Zurich, Switzerland