Designing interacting many-body systems with desirable properties is a frontier of modern quantum and condensed matter physics, chemistry, materials science, and engineering. Recently, heterostructures of atomically thin van der Waals (vdW) layers, such as graphene and transition metal dichalcogenides, have been proposed as a complementary material platform that is naturally endowed with a tunable nanoscale, periodic quantum landscape through the moiré superlattice. A compelling but open question is whether the moiré superlattice in vdW heterostructures can serve as a platform for “construction” of quantum many-body interactions between various types of solid-state qubits residing at moiré superlattice sites. Such qubits can take the form of electronic spins, quantum defects, or optical excitations (excitons: bound electron-hole pairs). The most remarkable and unique aspect of these materials is their ability to be assembled into complex 3D structures with emergent properties not found anywhere else in nature. Hence, the qubit-moiré quantum metamaterial provides a customizable and addressable platform that is the solid-state analog of a lattice of ultracold atoms. The InTriQATe NRT program will create opportunities for graduate students to become adept at the techniques for vdW heterostructure assembly, characterization, and theoretical modeling. It will prepare students with the skills and knowledge needed for this emerging and rapidly advancing field of quantum metamaterials. The realization of tunable, interacting moiré qubits would provide many exciting opportunities for studying exotic quantum phases of matter in a highly configurable and versatile solid-state system with applications in topological electronics, quantum light generation, and chip-scale tests of model Hamiltonians and simulations with open quantum systems. By stacking two layers of lattice-matched vdW semiconductors, MoSe2 and WSe2 , with a few-degree relative twist angle, periodic variations in the interlayer atomic alignment result in a moiré superlattice. The moiré potential depth and period are highly controllable via material combination and interlayer alignment with bandgap modulation from 10-100 meV across each 10-100 nm moiré supercell. Recently, members of our team discovered the existence of arrays of long-lived excitons confined in the moiré superlattice that can serve as an array of nominally identical single-photon emitters. Inspired by these exciting proof-of-principle experiments, we aim to address the many open questions related to moiré quantum materials, including: What are the fundamental optical and electronic properties and dynamics of interacting moiré qubits? What are the spectroscopic signatures of correlated behavior and topological phenomena? Can one control nearest-neighbor interactions through exciton or spin occupancy or filling factor of the moiré superlattice? How does the choice of materials and layer number affect moiré qubit dynamic response? Through a close-knit experiment-theory feedback loop, we will answer these open questions, validate theoretical models, elucidate the role of quantum and classical noise, strain, and disorder in large many-body quantum systems, and evaluate their potential for quantum technologies. This project seeks to not only understand decoherence, dissipation, and how novel macroscopic phenomena emerge from correlated behavior in moiré superstructures but also to devise ways to take advantage of and control them. An essential element for exploring qubit connectivity and quantum many-body phenomena in moiré materials is the layer-by-layer stacking with precise control over the vdW surface preparation, the interlayer twist angle, and the interlayer spacing. Using mechanical exfoliation, dry transfer stacking, and imaging tools within the UCSB vdW assembly lab, we will create multi-layer vdW heterostructures comprising a whole gamut of material combinations, such as graphene (conductor), MoSe2 and WSe2 (semiconductors), hexagonal boron nitride (hBN, insulator), and NiPS3 (magnet). Specific examples of moiré superstructures include hBN-encapsulated MoSe2 /WSe2 with graphene electrostatic gates (moiré excitons), graphene bilayers coupled to NiPS3 (moiré spins), and graphene bilayers coupled to WSe2 or hBN atomic-scale defects (moiré defects). We will use atomic-force and transmission electron microscopy to directly image the moiré superlattice and determine its periodicity and uniformity. Observing quantum many-body effects in moiré superlattices requires minimizing thermal fluctuations that destroy long-range order; consequently, experiments must be performed at cryogenic temperatures. Below ∼10 K, moiré exciton superlattices are expected to leave unique fingerprints on coherent spin and optical responses. UCSB has a comprehensive set of facilities available for graduate student training and research on moiré quantum materials. Trainees in this area will have the opportunity to experiment with optical microscopy at 25 mK in a dilution refrigerator; several quantum optical spectroscopy experiments with open- and closed-cycle optical cryostats to study coherent optical emission, photon-photon correlations, and single emitter emission statistics; cryogenic Raman spectroscopy as a probe of interlayer coupling; and low-temperature electrically detected magnetic resonance and electron paramagnetic resonance spectroscopy for probing spin dynamics and inter-band transitions at the Terahertz facility.
