A new quantum component made of graphene

Less than 20 years ago, Konstantin Novoselov and Andre Geim first created two-dimensional crystals consisting of a single layer of carbon atoms. Known as graphene, this material has had quite a career since then.

Due to its exceptional strength, graphene is currently used to reinforce products such as tennis rackets, car tires or aircraft wings. But it’s also an interesting topic for fundamental research, as physicists are constantly discovering new, surprising phenomena that haven’t been observed in other materials.

The right twist

Bilayer graphene crystals, in which the two atomic layers are slightly rotated around each other, are of particular interest to researchers. About a year ago, a team of researchers led by Klaus Ensslin and Thomas Ihn at ETH Zurich’s Laboratory for Solid State Physics were able to demonstrate that twisted graphene could be used to create Josephson junctions, the fundamental building blocks of superconducting devices.

Based on this work, researchers have now been able to produce the first superconducting quantum interference device, or SQUID, from twisted graphene to demonstrate the interference of superconducting quasi-particles. Conventional SQUIDs are already used, for example, in medicine, geology and archaeology. Their sensitive sensors are capable of measuring even the smallest changes in magnetic fields. However, SQUIDs only work in conjunction with superconducting materials, so they require liquid helium or nitrogen cooling when operating.

In quantum technology, SQUIDs can host quantum bits (qubits). that is, as evidence for performing quantum operations. “SQUIDs are to superconductivity what transistors are to semiconductor technology—the fundamental building blocks for more complex circuits,” Ensslin explains.

The spectrum is widening

The graphene SQUIDs created by PhD student Elías Portolés are no more sensitive than their conventional aluminum counterparts and must also be cooled to temperatures below 2 degrees above absolute zero. “So it’s not a breakthrough for SQUID technology per se,” says Ensslin. However, it significantly expands the application range of graphene. “Five years ago, we were already able to show that graphene could be used to make single-electron transistors. Now we’ve added superconductivity,” says Ensslin.

What is remarkable is that the behavior of graphene can be controlled in a targeted manner by pushing an electrode. Depending on the applied voltage, the material can be an insulator, conductor, or superconductor. “The rich range of opportunities offered by solid-state physics is available to us,” says Enslin.

Also interesting is that the two fundamental building blocks of a semiconductor (transistor) and a superconductor (SQUID) can now be combined in a single material. This makes it possible to create new control functions. “Normally, the transistor is made of silicon and the SQUID is made of aluminum,” says Ensslin. “These are different materials that require different processing technologies.”

An extremely demanding production process

Superconductivity in graphene was discovered by an MIT research team five years ago, yet there are only a dozen experimental groups worldwide investigating this phenomenon. Even fewer are capable of turning superconducting graphene into a functional component.

The challenge is that the scientists must perform several delicate steps of work one after the other: First, they must align the graphene sheets at precisely the right angle to each other. Then the next steps include connecting electrodes and etching holes. If the graphene were to be heated, as is often the case during clean room processing, the two layers realign the twist angle disappears. “The entire standard semiconductor technology has to be retrofitted, making this an extremely difficult job,” says Portolés.

The vision of hybrid systems

Enslin thinks one step ahead. A wide variety of different qubit technologies are currently being evaluated, each with their own advantages and disadvantages. This is mostly done by various research groups within the National Center of Competence in Quantum Science and Technology (QSIT). If scientists manage to connect two of these systems using graphene, it may be possible to combine their benefits as well. “The result would be two different quantum systems in the same crystal,” Enslin says.

This would also create new possibilities for superconductivity research. “With these components, we could better understand how superconductivity in graphene arises in the first place,” he adds. “All we know today is that there are different superconducting phases in this material, but we don’t yet have a theoretical model to explain them.”

The study is published in Nature Nanotechnology.