Gate-defined Josephson junctions in magic-angle twisted bilayer graphene

Materials and devices for fundamental quantum science and quantum technologies

“…gates) can be used to control the electron filling via the electric field effect and alter the range of e-e interactions via screening [159]. With respect to point b), we mention for example "hyper-tunable" Josephson junctions [160,161] (where both the superconductor and weak link are realized within the same moiré material and they are both electrically tunable) and exquisitely sensitive quantum photodetectors [132,162,163], with unprecedented capabilities to detect single photons in the Terahertz and microwave regimes. A series of important steps have to be made by the community, though, to make sure that MQM becomes an established platform for quantum simulation.…”

Euler Obstructed Cooper Pairing in Twisted Bilayer Graphene: Nematic Nodal Superconductivity and Bounded Superfluid Weight

“…The nontrivial band topology is characterized by the C 2z T -protected nonzero Euler numbers [7] (or equivalently Wilson loop winding numbers [5]), where C nj is the spinless part of the nfold rotation about the j axis (with j = z out of plane) and T is the spinful time-reversal symmetry. When the nearly-flat bands are partially filled, superconductivity was observed in MATBG [2,[8][9][10][11][12][13][14][15][16], and the superconductivity may be nematic [12] (despite some debate [17]) and nodal [14,15] when there are 2∼3 holes per moiré unit cell. Here being nematic means breaking C 3z .…”

Building devices in magic-angle graphene

“…When two graphene sheets are stacked with their lattices misaligned by roughly 1.1°, the result is a material that can be a conductor, an insulator, a superconductor or even a ferromagnet, depending on the external electric field and the induced charge doping [1-3]. Writing in Nature Nanotechnology, Daniel Rodan-Legrain and co-workers [4] and Folkert de Vries and co-workers [5] independently report electronic devices that rely on interfacing these different electronic states on a submicrometer length scale. Specifically, Josephson junctions and quantum-dot constrictions are made entirely within the graphene bilayer using nearby gate electrodes to change the local charge density.…”

“…To create devices within the magic angle graphene, the authors designed electrodes above and below the layers and used them to induce the desired pattern of ground states by controlling the local charge doping. By inducing superconducting regions separated by a short, non-superconducting weak link, the two research groups were able to create planar Josephson junctions via purely electrostatic interfaces [4,5]. Josephson junctions linking superconducting reservoirs with different phases are the basis for technologies such as SQUID magnetometers, superconducting digital electronics, Josephson voltage standards and superconducting qubits.…”

“…They can also couple voltage to microwaves via the AC Josephson effect, a behaviour de Vries and co-workers confirmed in the magic angle junctions. The researchers exposed the device to a microwave excitation and observed fixed voltage Shapiro steps in the voltage ̶ current characteristic as the signature of the microwave photon absorption [5].…”