We are working on germanium spin qubits, a promising and versatile base material to engineer spin quantum bits. In these 'heterostructures', holes are hosted in a germanium layer sandwiched between two layers of silicon/germanium. These holes exhibit a very high mobility and unlike electron spins which are only sensitive to magnetic fields, hole spins can be manipulated by an electric field, ie by voltages on a gate. The all-electrical control comes with its own drawback: spins become sensitive to electrical, and therefore charge noise in the devices. The germanium heterostructures feature metallic top gates that mostly screen the charge noise from defects they covered; however, in regions not covered by top gates, unscreened charges are responsible for charge noise limiting the coherence time.
We are acquiring a world unique cleanroom equipment combining atomic layer deposition and atomic layer etching, which will allow for the development of original structures where the gates are penetrating deep within the heterostructure, in order to circumvent the effect of these lone charges on the surface in the case of top gates. With this novel scheme, the definition and manipulation of quantum dots will be extremely simplified, and we plan to obtain two-qubit gate devices well within the scope of this PhD.