Development of Spin-Qubit Devices Based on ZnSe/(Zn,Mg)Se Quantum Structures

• Applicants: Privatdozent Dr. Alexander Pawlis; Dr. Lars Reiner Schreiber
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 337456818

The main goal of this research project is the implementation and optimization of electrostatically defined quantum dots (EDQD) based on (Zn,Mg)Se/ZnSe quantum structures for application of electrically controlled spin qubits. Such devices are founded on field-effect transistors (FET) with a high mobility 2-dimensional electron gas (2DEG) located in the ZnSe quantum well. Prototypes of those FETs were recently implemented but revealed charge instabilities and a reduced mobility mainly caused by the present device architecture with a remote n-type doping of the (Zn,Mg)Se below the gate region. At this starting point we revise the FET device architecture to make the remote doping obsolete. For that purpose a novel shadow mask approach is established in order to spatially decouple the ohmic contacts from the gate region of the FET, so that both can be independently optimized. Moreover, we plan to tailor the heterostructure accordingly, such that at cryogenic temperatures the charge state of the ZnSe quantum well can be controlled using either a global top- or back-gate. Using this technique EDQDs are then fabricated and their general properties are analyzed. Based on the results the device is tweaked to minimize charge fluctuations in the system. Then, we will realize spin qubits with EDQDs in (Zn,Mg)Se/ZnSe for the first time and demonstrate qubit readout and manipulation. The spin-orbit interaction, spin relaxation and dephasing time will be benchmarked. Thus, we investigate the impact of the nuclear spin background in the host matrix on the coherence, which for example in III/V based EDQDs is known to limit the coherence and fidelity of the qubits. In the last stage of the project, the EDQD device processing is transferred to isotopically enriched 64-Zn80-Se and (64-Zn,24-Mg)80-Se all exhibiting zero nuclear spin in order to extend the spin dephasing time further. These results are crucial to estimate the potential of EDQDs based on II/VI semiconductors as an alternative and particularly versatile qubit platform compared to other presently available systems.

• DFG Programme: Research Grants