[1] Univ Vienna, Fac Phys, Vienna Ctr Quantum Sci & Technol, Boltzmanngasse 5, A-1090 Vienna, Austria
[2] Czech Acad Sci, Inst Phys, Cukrovarnicka 10, Prague 16200, Czech Republic
[3] Czech Tech Univ, Fac Elect Engn, Tech 2, Prague 16627, Czech Republic
[4] TU Wien, Inst Sensor & Actuator Syst, Guhaustr 27-29, A-1040 Vienna, Austria