This article as been adapted from this Quantum Systems Accelerator article.
An active area of multidisciplinary exploration at the Quantum Systems Accelerator (QSA), a multi-institution QIS Research Center co-lead by Berkeley Lab and Sandia National Laboratories, is finding suitable materials to build increasingly complex quantum processors. Many of the current roadblocks in quantum information science and technology stem from disturbances and defects in the materials used to fabricate qubits across the leading platforms: neutral atoms, trapped ions, and superconducting circuits.
QSA scientists, like the Foundry’s Sinéad Griffin, study sophisticated two-dimensional (2D) materials that could potentially increase the coherence time of qubits in superconducting circuits. Their research leverages broad expertise at different QSA partner institutions and U.S. Department of Energy (DOE) research facilities, including the Advanced Light Source and the Molecular Foundry.
Leveraging 2D Versatility
Josephson junctions – the quantum equivalents of a transistor – are the building blocks of superconducting qubits. They result from a thinly layered sandwich of a non-superconducting material (insulator) between two layers of superconducting material, enabling the movement of an electrical charge through the material with little to no resistance. A vast majority of Josephson junctions typically use aluminum oxide as the insulator sandwiched between layers of aluminum. However, the traditional bulk materials – aluminum and niobium, used on classical silicon chips and Josephson Junctions – are potentially not the best materials for fabricating superconducting circuit processors. The material’s defects hinder the flow of electrical currents.
In contrast, 2D materials are extremely thin and versatile, with an average thickness of a few nanometers (nm) or less, and they tend to form in layers. Transition metal dichalcogenides (TMDs) hold great promise as a 2D material set for superconducting processors. TMDs can naturally be fabricated into very thin layers with a well-defined crystalline structure, so they can be stacked one on top of the other as a single crystal. This approach builds a well-defined nanostructure with potentially far fewer defects.
TMDs also have a series of important chemical and electronic properties that are often associated with reducing the impacts of defects. The dynamics are often subject to topological protection – a geometric property that increases the robustness of the quantum system itself.
Therefore, building Josephson Junctions with TMDs enables the fabrication of a superconducting quantum device without introducing new defects into the system.
The Foundry’s Sinéad Griffin develops theoretical and computational methods to describe quantum materials. Aided by the advances in supercomputing capabilities, Griffin studies the variety of fundamental properties in 2D materials, including TMDs.
“We don’t really know yet what is the best material to use or the best combination of 2D materials because there are a lot of options available. There’s a lot of space to actively explore the fundamental questions that can impact what properties you want your qubit to have,” she said.
“In fact, although we’ve had 2D materials for a long time, getting them to the fabrication stage and to the high quality that we need for quantum computing applications requires us to both study what the fundamental properties of these 2D materials are and also start engineering them to be in the right form and the right properties. Many of the 2D materials’ electronic properties can change when stacking them one over the other. ”
Griffin explained how popular 2D materials, such as graphene, are not suitable for Josephson junctions on their own because they’re not inherently superconducting, even if stacked one over another. However, scientists recently found that when the angle between two graphene layers is twisted, the material becomes superconducting.
“It’s really exciting to think about what sort of designer properties we can get in these 2D materials where you’re not stuck (unlike with bulk materials) within a small region of space to engineer and enhance the functionality. We’ll be able to pick and choose the combination of materials and properties we want in our superconducting qubits,” said Griffin.
With advances in quantum information science, high-performance classical computing, and heterogeneous quantum-classical computing, QSA researchers such as Analytis and Griffin can study and experiment with broader materials systems and shorter feedback loops, translating to better materials for quantum hardware. Thus, QSA’s broad mandate to co-design the solutions for quantum advantage will increasingly require cutting-edge materials science and innovation.
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