Dutch researchers have successfully developed nanowires allowing individual electrons to be captured by a ‘quantum dot’ on which superconductivity can take place. These nanowires could make powerful quantum computers.
The combination of a quantum dot and superconductivity makes it possible to create ‘Majorana fermions’, exotic particles that are their own antiparticle and which are regarded as an important component in the quantum computers of the future.
This is not the first time that scientists have succeeded in creating nanowires with quantum dots on them in which superconductivity can occur. It is, however, the first time that this has been done using nanowires having a germanium core and a silicon shell. According to researcher Joost Ridderbos the principal advantage of this material, besides its quantum properties, is that it is extremely well defined; that is to say, it can be manufactured with great precision, with every single atom in the right place.
The researchers first produced a wire with a diameter of about 20 nanometers. They then fitted it with minuscule aluminum electrodes. At a temperature of 0.02 degrees Celsius above absolute zero (minus 273.15 degrees Celsius) they succeeded in passing superconducting electricity through this wire, and with the help of an external electric field they created a quantum dot containing exactly one ‘electron hole’.
A Ge–Si core–shell nanowire is used to realize a Josephson field‐effect transistor with highly transparent contacts to superconducting leads. By changing the electric field, access to two distinct regimes, not combined before in a single device, is gained: in the accumulation mode the device is highly transparent and the supercurrent is carried by multiple subbands, while near depletion, the supercurrent is carried by single‐particle levels of a strongly coupled quantum dot operating in the few‐hole regime. These results establish Ge–Si nanowires as an important platform for hybrid superconductor–semiconductor physics and Majorana fermions.