Research: graphene-based hybrid quantum systems

Superconducting graphene for radiofrequency applications

The electronic properties of graphene, an atomic monolayer of graphite, are now relatively well understood in the low frequency (DC) regime after 10 years of intense research. On the other hand, its properties in the high-frequency limit (above 1GHz for example) have only recently at-tracted attention. Its ultimately thin thickness together with its electrical tunability [Nov04] makes graphene an ideal platform for integration into future nanoelectronics systems. Graphene has a strong advantage over other semiconductors: being only one atom thick and inert it can a priori be integrated on a large scale into any existing platforms and architectures.

Graphene can become superconducting over macroscopic scales when decorated with an array of tin (Sn) islands [Kes10,Han14]. Electric tunability (in particular the superconducting transition temperature) of this material has been shown [Kes10]. This hybrid material can be seen to some extent as a network of Josephson junctions, i.e. an array of a superconducting material con-nected by a non-superconducting material. Josephson junctions intrinsically present a large in-ductance of kinetic nature, i.e. which does not depend only on the system geometry. As there is a strong link between carrier density and kinetic inductance in a superconductor, it should be possible to tune the total inductance of the system with an electric field, hence building an electrically tunable inductor. The new system that we introduce here (tin decorated graphene) has thus a very strong potential to be used in the radiofrequency regime.

The two main scientific questions that we are currently addressing are:
(1) Is tin decorated graphene a suitable superconductor for transmitting radiofrequency radiations, especially in terms of losses?
(2) Can it be used to build an electrically tunable inductor?

[Han14] Z. Han et al Nature Physics 10, 380 (2014)
[Kes10] B. M. Kessler et al Phys. Rev. Lett. 104, 047001 (2010)
[Nov04] K.S. Novoselov et al Science 306, 666 (2004)

Graphene based superconducting quantum bit: the Graphmon

The future of nanoelectronics will be quantum. The downscaling in electronics has now reached a point where the size of the devices (less than 10 nm) means that their quantum behavior must be taken into account. While this might be seen by some industries as a major problem this also gives a real opportunity to imagine and build devices with new quantum functionalities. A key building block for future quantum electronics systems is the quantum bit. Such system has two possible states (0 and 1) that follow the laws of quantum mechanics. One example is that one might build any superposition of 0 and 1. This will have implications for building future quantum computers.

In this work we want to build a new type of device to implement a quantum bit that would have strong advantages over other competing systems. The idea is to use the know-how that has been developed in the superconducting quantum bit community over the past 20 years and integrate in the core of the system a semiconducting material to bring novel electrical functionality to the device, in the form of a voltage-tunable energy. We will use graphene, a two-dimensional zero-band-gap semiconductor, because of the potential scalability of such approach. Such device is expected to behave as a quantum two-level system with an energy structure that can be tuned with an electric field (gate) thanks to graphene (see figure).

More about the other topics (quantum transport in graphene/boron nitride heterostructures, transition metal dichalcogenides heterostructures, artificial Dirac matter in III-V materials, NEMS coupled to microwave cavities...) soon!