Towards scalable hut wire quantum devices

Electronic devices are getting smaller and smaller, approaching their physical limits. As a consequence, alternative ideas of how computation could work need to be found. In this direction, many groups all around the world are studying quantum effects in semiconductor devices in the hope that such quantum effects could be used in the near future for new types of electronic devices. As significant progress has been made over the past decades in controlling some of the quantum properties, the time has come to try to create more complex quantum devices. This project focuses on exactly this goal. We are going to study a new material platform: Ge one dimensional quantum structures which can be perfectly positioned. Just as Lego bricks can be used in order to create complicated constructions, we are going to use these ordered Ge nanowires (our Lego bricks) in order to move towards more complex devices. By measuring these nanostructures at temperatures close to absolute zero, we are going to investigate their potential as quantum bits. In particular two types of quantum bits will be studied: On the one hand we are going to use external magnetic fields in order to use the spin degree of freedom as a qubit. On the other hand, by coupling the Ge nanostructures to superconductors (materials which show zero resistance at low temperatures) we aim to investigate if a new type of qubit can be realized. By investigating the properties of these advanced nanostructures at low temperatures, new insight into the quantum properties will be obtained.

Semiconductor transistors, fundamental to the operation of practically every electronic device, are getting smaller and smaller reaching a size where their operation fails as quantum mechanics kicks in. While industry is looking to continue this very successful miniaturization process, in basic research the possibilities of different functionalities is investigated. Such could be the realization of quantum bits in nanoscale transistors. In this project we have used Ge based semiconductors in order to investigate their potential for the realization of scalable quantum bits. By a combination of top-down nanofabrication and bottom-up self-assembled Ge hut wires with controllable position, length and distance in between them were realized. Nanoscale devices were created in such hut wires and the properties of single quantum dots were investigated by performing electronic transport measurement at -273.13C. Our measurements revealed the presence of spin-orbit coupling which can be tuned by an electric field. This is a key ingredient for electric spin manipulation. Furthermore, for two hut wires close by, we found that they electrostatically sense each other. The addition or removal of one charge in one hut wire can be sensed by the second one. Such is very important, as the above described charge sensing, is used widely in spin qubit experiments. For two dimensional Ge/SiGe heterostructures we found a reliable way to induce superconductivity. This was achieved by bringing Al close to it, so that it can inherit the superconducting properties of it, namely Ge does not show any more resistivity. In order to get the best possible results the Al has been deposited on the Ge/SiGe heterostructures at -196C. This also allows to grow very thin Al films which can sustain magnetic fields larger than 1 Tesla. This therefore allow us to combine semiconductor and superconductor qubits on the same device. In addition, we were able to create an ideal superconducting diode, which is the analog of a semiconductor diode. Depending on the direction of current flow the device is either resistive dissipationless, i.e. the current can flow without losses.