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2D Materials for Quantum Technology

Periodic Reporting for period 2 - 2D4QT (2D Materials for Quantum Technology)

Reporting period: 2021-03-01 to 2022-08-31

Quantum technology is an exciting field of research at the crossover between physics and engineering. Apart for the well-known example of quantum computing, other active areas of research where quantum concepts play a fundamental role are quantum cryptography, quantum sensing, quantum simulations and quantum metrology. In all these fields, material aspects play a fundamental role as soon as it comes to the physical implementation of theoretical concepts. Major advancements in quantum technological applications are thus often closely linked to breakthroughs in identifying and optimizing suitable material platforms.

There is the wide spread believe that two-dimensional (2D) materials – such as graphene, bilayer graphene and transition metal dichalcogenides – can contribute decisively to advance quantum technologies. These materials possess a wealth of physical properties and phenomena that make them an attractive platform for this scope. Moreover, the possibility of stacking different 2D materials on top of each other to form artificial solids with tailored properties, the so-called van der Waals (vdW) heterostructures, offers an immense range of opportunities, including the possibility of realizing systems with unique quantum-mechanical properties.

The overarching goal of the project 2D4QT is to assess experimentally the potential of graphene-based vdW-heterostructures for quantum technology applications. Specifically, we aim at implementing experimentally a number of theoretical proposals for exploiting the spin and valley degrees of freedom in graphene and bilayer graphene for quantum information purposes, to be able to say whether these materials hold to their promises or still hide surprises.

A fundamental aspect of the project is also technology development. In fact, to be able to perform the envisioned experiments, we need to substantially advance the technology for fabricating complex vdW heterostructures, turning today’s “art” of making samples into a technology that allows systematic improvements. Such a technology-push is likely to have a far-reaching impact well beyond 2D4QT, since vdW heterostructures have huge potential not only for quantum technological applications, but also for innovative solutions for THz electronics, energy harvesting and storage, telecommunication, ultra-broad band detectors.
Since the beginning of the project, we made substantial progress in fabricating and characterizing electrostatically defined quantum devices in bilayer graphene (BLG) – especially quantum dots.

Quantum dots are key elements for spin-based quantum computation: they are solid-state devices with typical size of few tens of nanometer, which can be used to confine and manipulate single electrons. The investigation of graphene-based quantum dots dates back to 2008, but until 2018 all devices were fundamentally mined by disorder. The real breakthrough occurred in 2018, when the team of Klaus Ensslin showed the key technological ingredients for confining electrons in bilayer graphene using electrostatic gates. This allowed to reach for the first time the single-electron occupation in graphene-based quantum dots (or better, in BLG-based quantum dots), and to investigate spin and valley states. We were quick to take up these advances, and published our first results on gate-defined quantum dots in BLG also in 2018. Since then the progress has been fast.

Within the 2D4QT project, we established the process to make highly tunable quantum dot devices, allowing full control over the charge occupation and the tunneling barriers. We demonstrated the implementation of electron and hole quantum dots in close proximity, which is a unique possibility of BLG. We performed systematic low-temperature quantum transport experiments to gain a thorough understanding of the single- and two- particle spectrum of quantum dots in BLG. In addition, we demonstrated high-frequency manipulation of the states in the quantum dots, dispersive charge- readout, as well as spin-valley Pauli blockade. Each of these results represents an important step towards the demonstration of a spin qubit in BLG quantum dots, which are now within reach.

These progresses have also made us one of the two world-leading groups in the field of BLG based quantum devices, together with the Ensslin group at ETH Zurich.
The project 2D4QT wants to set the technological basis for exploiting the exceptional properties of graphene and bilayer graphene (BLG) for technology applications, in particular for what concerns the exploitation of the spin and valley degrees of freedom and the possibility of engineering topologically protected states.

To do so, we are making a major investment in technology in order to substantially advance the methods for fabricating high-quality graphene and BLG devices. This effort is driven by the vision of turning sample fabrication from an "art" into a technology, meaning that we do not only aim at realizing the specific, sophisticated samples needed for our experiments, but we also develop clear fabrication recipes and new tools that allow for systematic improvements. The long-term vision is the complete automation of the whole fabrication process.

The other scientific objectives of 2D4QT are the exploration of spin- and valley-based devices in graphene and BLG, with a strong focus on the requirements set by quantum information applications (e.g. coherence time and addressability). At the end of this project, we want to be able to say whether these systems are the horse-worth-betting-on predicted by theory, or whether there are fundamental problems that hinder their application. Should the latter be the case this would be very interesting from the perspective of basic research, as it would indicate that, despite a decade of intense investigations, graphene can still hide surprises.

Based on the results achieved so far, BLG is proving a very promising platform hosting spin-qubits and for quantum information applications – as predicted. Compared to other platforms such as Silicon, it is still 8-12 years beyond in terms of technological maturity, but it might offer some major advantages in the middle to long term, e.g. for overcoming some of the roadblocks or challenges encountered by other platforms, such as spin to photon coupling or a nearly impossible lifting of the valley degeneracy.
False colored scanning electron microscopy (SEM) image of BLG QDs.
Artistic illustration of the main goals of the 2D4QT goals.