[1] Heriot Watt Univ, Sch Engn & Phys Sci, CM DTC, Edinburgh EH14 4AS, Midlothian, Scotland
[2] Heriot Watt Univ, Sch Engn & Phys Sci, Edinburgh EH14 4AS, Midlothian, Scotland
[3] Univ Turku, Dept Phys & Astron, Turku Ctr Quantum Phys, FI-20014 Turun, Finland