One of the key challenges of creating quantum computers is to keep the information of the qubits intact from the decoherence of the environment. In the fight against the loss of coherence, a new alternative is to engineer quantum states states with topological protection, which will be insensitive to conventional decoherence effects (such as charge fluctuations). Such topological quantum states can be created, for example, in nano-circuits formed from a combination of semiconductor nanowires and superconductors. One of the topologically protectected exotic excitations is called Majorana state and first signatures of such states have been experimentally found. Certain topological protected operations are possible using Majorana Fermions, but to cover all computational basis operations, more complex states, parafermion states are needed.
Nanowires coupled to superconductors can host Majorana states, and by coupling two nanowires together using a superconducting island Parafermions are expected to appear under certain experimental conditions. One of the key requirements is that the coupling between the two wire should be mainly mediated by crossed Andreev reflection. Also, important and open questions are the parity lifetime of the system, the effect of quasi-particle poisoning, and the appearance of helical gaps. The states can be probed using spectroscopy techniques or by measurement of superconducting current phase relation using high frequency techniques.
During the PhD work the candidate will investigate the building blocks to realize Majorana and parafermion states in semiconducting nanowire systems. The circuits will be realized using electron-beam lithography and will be measured at ultra-low temperatures using both low and high frequency techniques. The work is done in close collaboration with European universities.
|Left: Double nanowire system with parafermions (red rings) probed using quantum dots. Right: Parafermions coupled to an RF readout circuit.|
Knowledge of solid state physics, motivation for experimental work, English knowledge, basic programming and measurement automation skills