Qdev seminar by David van Driel, Technical University of Delft

Charge sensing and hybridization of Andreev bound states

Majorana zero modes appear at the ends of a Kitaev chain, which can be engineered by coupling quantum dots to superconductors. Even a minimal, two-site Kitaev chain is predicted to host Majorana zero modes in a parameter sweet spot, and was recently realized in a semiconductor-superconductor hybrid nanowire. 

Majorana qubits can be made by encoding the qubit states in the parity of a pair of Majorana zero modes. Parity readout is therefore crucial for any quantum information experiment. Charge sensing has been proposed as a readout technique, and requires a sensor dot being able to resolve the charge of a superconductor-semiconductor system. In this seminar, we demonstrate charge sensing of an Andreev Bound State (ABS) in a hybrid semiconductor-superconductor nanowire. 

First, we show that the charge sensor can detect the change of charge during the singlet-doublet transition of an ABS. Second, we resolve the continuously changing charge of an ABS which remains in the singlet state. Then, we couple the ABS to a quantum dot (QD) and sense the hybridisation of charge. 

In addition, we show that the ABS-QD system has two types of interaction; one between the odd parity states, the other between the even parity states. Changing the tunnel barrier between the two states changes the ratio of these interactions. We can tune the system into a parameter sweet spot, and realize a minimal Kitaev chain in the ABS-QD system.

Our results demonstrate that QDs can be used for charge sensing Kitaev chain systems. Furthermore, we show that QD-ABS systems can also constitute a minimal Kitaev chain.