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Towards spin qubits and Majorana fermions in Germanium self-assembled hut-wires

Final Report Summary - SPAJORANA (Towards spin qubits and Majorana fermions in Germanium self-assembled hut-wires)

The interest in hole spins as potential qubits has increased significantly over the past few years as the strong spin orbit coupling of hole spins should allow the realization of fully electrically controlled spin qubits. During this project the properties of hole spins in Ge were investigated. Ge was chosen as material because its spin orbit coupling is larger than that of Si but it is still fully compatible with Si technology.

In this project so-called Ge hut wires were investigated. Hut-wires are monolithically grown on Si without the use of any catalyst and have a triangular cross section with a height of about 2nm. Due to this very strong confinement the light-hole band and heavy-hole band should be very well separated.

In the initial stage of the project the properties of the confined hole states were investigated. By performing magnetotransport measurements the g-factors in the three main crystallographic directions were extracted. The experiments showed that the out of plane g-factors were about 10 times larger than the in plane g-factors. By combing the experiments with theory it could be concluded that the confined hole states have to 99% heavy-hole character. Such heavy-hole states are promising for the realization of hole spin qubits because for heavy-hole states the hyperfine interaction is of Ising type.

In order to realize a heavy-hole spin qubit double quantum dots were realized for reading out the qubit based on the Pauli spin blockade principle. Electric dipole spin resonance could be achieved and coherent spin rotations were demonstrated. Rabi frequencies exceeding 140MHz at about 100 mT were observed. By performing Ramsey experiments a dephasing time T2* of 130 could be extracted. In addition two axis control of the qubit was demonstrated. Due to the fact that the qubit was read out via Pauli spin blockade (current readout) spin echo and CPMG experiments could not be performed.

For hole systems T2 should be equal to 2T1. In order to determine this upper limit of the coherence time, devices with a charge sensor were realized. Spin relaxation times of about 100 microseconds at 500mT were extracted emphasizing the potential of hole spin qubits.

While self-assembled hut wires are a very interesting platform for studying the fundamental properties of hole spin qubits it is very difficult to move towards more complex devices. In the last months of the project site controlled Ge hut wires were investigated. Capacitive coupling between parallel wires could be demonstrated opening the path towards the realization of scalable quantum devices.