Could you briefly outline the key findings of your paper?

We fabricated and investigated the very first superconductor-nanowire hybrid devices with tunnel barriers grown during the nanowire growth. Two tunnel barriers (blue) form a quantum dot (red), often referred to as an artificial atom, which constitutes one of the most fundamental building blocks in nano and quantum electronics. These quantum dots are electrically and spatially very well defined, which allows us to perform systematic tunnel spectroscopy measurements of the evolution of the superconducting proximity region of a nanowire segment close to the superconducting contact. In particular, when tuning the electron density in the nanowire segment, we observe a transition from the short to the long junction limit of Andreev bound states, the states that form at an interface between a normal and a superconducting material.

What is your role in this work?

This work is the result of a strong collaboration of the Quantum- and Nanoelectronics group, led by Prof. C. Schönenberger and Dr. A. Baumgartner at the University of Basel (Switzerland), and the groups of Prof. K. A. Dick and Dr. C. Thelander at the University of Lund and the Center for Analysis and Synthesis in Lund (Sweden). Our collaborators in Lund grew the InAs nanowires with integrated quantum dots, defined by atomically sharp tunnel barriers. We fabricated the devices and carried out the measurements with colleagues in the Nanoelectronics group. Subsequently, we analyzed the data and worked on the interpretation of the data, which in addition were very nicely reproduced in a tight-binding model by Dr. D. Chevallier from the Condensed Matter Theory group at the University of Basel.

What was the genesis of this paper?  How did you come to this particular problem?

The superconducting proximity effect is the underlying mechanism of topologically non-trivial boundary states, like Majorana bound states, in semiconducting nanowires with a large spin orbit interaction. These protected states are potentially useful for quantum information technologies. First experiments on Majorana bound states have been reported, which are controversially discussed, since an unambiguous proof has not been reported, yet. A major reason for this is that in standard nanowires the position and strength of the tunnel barriers necessary for tunneling spectroscopy are badly defined and difficult to control, with random potential modulations governing the potential landscape. We have worked on a variety of platforms before and often encountered the problem that to do proper transport spectroscopy in electronic devices, we need well understood and reproducible tunnel barriers that are not a part of the unknowns in an experiment.

In this paper, we present the first experimental demonstration of systematic spectroscopy of the proximity induced gap by using built-in axial-grown quantum dots in indium arsenide nanowires, presently one of the main platforms for topological quantum states in solids. The integrated quantum dots are defined by two potential barriers that form when the nanowire crystal structure is changed from zincblende to wurtzite, which can be achieved with atomic precision by controlling the growth parameters. These QDs are electrically and spatially well defined, which allows us to probe the induced superconducting gap at a precise distance from the QD and with predictable coupling parameters. This new type of platform allows us to provide fundamental characteristics of the proximity gap and to demonstrate the large tunability of this quantum system. This novel spectroscopy tool is well suited to study superconducting bound states in semiconducting nanowires, and might be useful to tackle fundamental limitations found in recent studies of Majorana bound states.

What is the most empowering implication of your results?

Compared to previous experiments investigating the spatial distribution and length scales of the superconducting proximity effect in metals,semiconducting nanowires also allow us to control the carrier density and all related parameters, like the Fermi velocity, in the system. We can perform two types of tunnel spectroscopy distinguished by the transport mechanism through the integrated quantum dot, both probing the nanowire density of states: in the cotunneling regime, where the QD can be seen as a single tunnel barrier, and in the sequential tunneling regime, where the QD acts as an energy filter with Coulomb blockade resonances. These complementary measurements allow us to draw a clear picture of how the proximity gap forms in a nanowire segment. Thereby we are able to understand the observed evolution of the proximity induced gap qualitatively as a gate-tunable transition of Andreev bound states forming the nanowire segment from the long to the short junction limit. This qualitative picture is supported by numerical calculations confirming our findings quantitatively.

And now, what’s next?

We plan to investigate also discrete sub-gap states in a nanowire lead segment as well as devices with two superconducting contacts, with the possibility to tune the NW segments from the short junction limit to a Josephson junction, with coupling of different Andreev bound states mediated by the integrated quantum dot. On the long run, we would like to use integrated quantum dots to investigate topological states, such as Majorana bound states, hopefully resulting in new insights in their lifetime, spin texture and parity.

Original journal article: 
Spectroscopy of the superconducting proximity effect in nanowires using integrated quantum dots
C. Jünger et al., Communications Physics 2, 76 (2019)