Periodic Reporting for period 2 - SUPERGRAPH (Topological Superconductivity in Graphene)
Reporting period: 2022-04-01 to 2023-09-30
In recent years, a considerable stream of work from the condensed matter community has been focusing on hybrid systems coupling superconductors to various topological states of matter. Such a heterogeneous coupling is pivotal in enabling the emergence of new excitations –the Majorana or parafermions— that could be used as quantum bits (qubits) with unique properties of non-locality and immunity to external perturbations, essential to encode and manipulate quantum information in a robust and stable fashion. Such quantum excitations could serve as new quantum bits for designing next generation of fault-tolerant quantum computers. Nevertheless, topological insulators that can be efficiently hybridized with superconductors and enable reliable coherent manipulation are still missing.
This project aims at demonstrating the quantum Hall topological insulator state that emerges in charge neutral graphene when subjected to a strong perpendicular magnetic field, as a new platform for topological superconductivity. Its novelty hinges on an unprecedented substrate engineering that profoundly modifies the quantum Hall ground state of neutral graphene. The ensuing robust quantum Hall phase harbors spin-filtered, helical edge states that can be easily coupled to superconducting electrodes for investigating novel hybrid superconducting quantum circuits.
The versatility of graphene will enable us designing locally gated quantum devices to demonstrate control of helical edge channels, tunnelling experiments, and time-resolved microwave spectroscopy to unveil Majoranas or parafermions in hybrid superconducting quantum Hall devices. Ultimately, quantum coherent manipulation of Majorana qubits in hybrid devices will be performed, providing a major breakthrough in the way of fault-tolerant quantum computers.
This project aims at demonstrating the quantum Hall topological insulator state that emerges in charge neutral graphene when subjected to a strong perpendicular magnetic field, as a new platform for topological superconductivity. Its novelty hinges on an unprecedented substrate engineering that profoundly modifies the quantum Hall ground state of neutral graphene. The ensuing robust quantum Hall phase harbors spin-filtered, helical edge states that can be easily coupled to superconducting electrodes for investigating novel hybrid superconducting quantum circuits.
The versatility of graphene will enable us designing locally gated quantum devices to demonstrate control of helical edge channels, tunnelling experiments, and time-resolved microwave spectroscopy to unveil Majoranas or parafermions in hybrid superconducting quantum Hall devices. Ultimately, quantum coherent manipulation of Majorana qubits in hybrid devices will be performed, providing a major breakthrough in the way of fault-tolerant quantum computers.
We have extended our understanding of the quantum Hall topological insulator state and developed a new fabrication technique. The quantum Hall topological insulator state requires graphene to be in a specific dielectric environment, with a screening electrode at a distance smaller than the magnetic length. Our new approach uses a metallic screening electrode and a specific contact fabrication method, producing ultra-high mobility samples that exhibit helical edge transport with excellent conductance quantization at charge neutrality and low magnetic fields. These samples are perfectly suitable for the next course of actions of the project, which focuses on proximitizing these helical edge channels with superconducting electrodes.
In parallel, we investigated the coupling between superconductivity and more standard quantum Hall edge channels. We realized a Josephson junction operating up to 8 teslas in the quantum Hall regime of graphene, which represents extremely harsh conditions for the standard Josephson effect. Operating at a filling factor of two, the Josephson supercurrent develops on a single chiral quantum Hall channel and exhibits unique chiral properties. We also provided a consistent understanding of the limiting decoherence mechanism involved in these new hybrid devices. This breakthrough [H. Vignaud et al. Nature 624, 545 (2023)] not only represents a major advance for superconducting quantum Hall hybrids but also sets the exact parameters required for proximitizing the helical phase with our newly developed screened graphene devices.
In parallel, we investigated the coupling between superconductivity and more standard quantum Hall edge channels. We realized a Josephson junction operating up to 8 teslas in the quantum Hall regime of graphene, which represents extremely harsh conditions for the standard Josephson effect. Operating at a filling factor of two, the Josephson supercurrent develops on a single chiral quantum Hall channel and exhibits unique chiral properties. We also provided a consistent understanding of the limiting decoherence mechanism involved in these new hybrid devices. This breakthrough [H. Vignaud et al. Nature 624, 545 (2023)] not only represents a major advance for superconducting quantum Hall hybrids but also sets the exact parameters required for proximitizing the helical phase with our newly developed screened graphene devices.
The demonstration and understanding of Josephson junctions in the quantum Hall regime represent substantial progress beyond the state of the art. Coupled with our newly developed quantum Hall topological insulator platform in charge-neutral graphene, this should provide an unprecedented testbed for realizing helical Josephson junctions and topological superconductivity. We expect to characterize such junctions in the second part of the project using state-of-the-art quantum transport measurements and, in parallel, develop a suitable high-field microwave measurement platform for direct spectroscopy of those junctions.