Hole spin orbit qubits in Ge quantum wells

Electronic devices are getting smaller and smaller, since it has been possible for the semiconductor industry to shrink the fundamental element (transistor) by 6 orders of magnitude from about 1cm in 1950 to a bit more than 10nm nowadays. However, it is clear that this miniaturization is reaching its limits. Alternative principles of computation need to be devised. One of these ideas is to use quantum mechanics for this purpose. Rather than using the charge of an electron in the transistors it has been proposed that one should use the electron spin, a quantum mechanical property. By using the spin degree of freedom researchers started to move from classical bits towards quantum bits, i.e. qubits. In the early 2000 it was believed that Si and Ge would not play any significant role in the field of spin qubits. Too many problems existed in the realization of quantum devices and too low was the quality of the existing material. However, in the past few years it has become clear that actually Si and Ge seem to be of the most promising materials for the creation of a new generation of devices, which will work based on the principles of quantum mechanics. In this project we are aiming to study Ge quantum devices. In contrast to the majority of the works reported so far we are aiming to study hole spin qubits. Holes are nothing else then missing electrons. But like an air bubble in a glass of mineral water has different properties than a water molecule (the air bubble moves upwards, the water molecule downwards), the same is also true for holes. Special properties of holes suggest that hole spin qubits might be actually very promising. In this work we will create different versions of hole qubits and try to understand for how long quantum information can be stored in them. In parallel we are going to try to enlarge our knowledge about the fundamental properties of holes.

The invention of the transistor in 1947 led to a technological revolution as it became the building block of the first reliable computers in the 60's. Since then its size has continuously decreased boosting therefore the computational power. Today transistors have become so small that quantum physics makes their operation challenging. Therefore, researchers in basic research are investigating new concepts, which would allow information processing to operate on completely different principles. In this line, spins have been suggested as elementary quantum bits (qubits) to realize a quantum computer. In this project we have investigated hole spins confined in Germanium, the material from which the first transistor was realized. Such hole spins very predicted to have very promising qubit properties. We started with Ge/SiGe heterostructures, a layer of Ge which is sandwiched between two SiGe layers. In a next step we have created metallic electrodes on such heterostructures by means of nanofabrication. This allows us to apply electric fields and therefore spatially localize the holes, in so-called quantum dots. We have created two such quantum-dots, i.e. a double quantum dot. This double quantum dot allows to host a two-level system which we used as a qubit. The qubit is operated by localizing and separating, within a nanosecond, two spins between the two quantum dots. Our experiments demonstrated a singlet-triplet qubit, which can be operated already at fields below 1mT. Such fields do not destroy superconductivity, i.e. the state of matter with zero resistance, paving therefore the way to combine spin qubits with superconducting circuits.