Revolutionary Qubit technology paves the way for practical quantum computing

Researchers are making advances in quantum computing by developing qubits based on spins of electrons and holes, with recent discoveries at the University of Basel showing controlled interactions between qubits using spins of holes. These advances suggest a promising future for scalable and efficient quantum computers using existing silicon technology.

Advances in qubit technology at the University of Basel show promise for scalability quantum computingusing electron and hole spins to achieve precise qubit control and interactions.

The pursuit of a practical quantum computer is in full swing, with researchers around the world exploring a wide range of qubit technologies. Despite extensive efforts, there is still no consensus on which type of qubit best maximizes the potential of quantum information science.

Qubits are the foundation of a quantum computer. They are responsible for data processing, transfer and storage. Effective qubits must store and rapidly process information reliably. This requires stable and fast interactions between large numbers of qubits that external systems can precisely control.

Today’s most advanced quantum computers possess only a few hundred qubits. This limits them to performing calculations that conventional computers are already capable of and can often do more efficiently. For quantum computing to advance, researchers must find a way to accommodate millions of qubits on a single chip.

Electrons and holes

To solve the problem of arranging and connecting thousands of qubits, researchers at the University of Basel and NCCR SPIN rely on a type of qubit that uses the spin (intrinsic angular momentum) of an electron or a hole. A hole is essentially an electron missing from a semiconductor. Both holes and electrons have spin, which can assume one of two states: up or down, analogous to 0 and 1 in classical bits. Compared to an electron spin, a hole spin has the advantage that it can be controlled entirely electrically without the need for additional components such as on-chip micromagnets.

Two rotating cubes with interacting holes. When a hole (magenta/yellow) tunnels from one site to another it rotates (arrow) due to so-called spin-orbit coupling, leading to anisotropic interactions described by the surrounding bubbles. Credit: NCCR SPIN

In 2022, Basel physicists demonstrated that hole spins in an existing electronic device can be trapped and used as qubits. These “FinFETs” (field-effect transistors) are built into modern smartphones and are manufactured in widespread industrial processes. Now, a team led by Dr. Andreas Kuhlmann has for the first time managed to achieve a controllable interaction between two qubits within this configuration.

Fast and precisely controlled rotation

A quantum computer needs “quantum gates” to perform calculations. These represent operations that manipulate qubits and join them together. As researchers report in the journal Nature Physics, they were able to pair two qubits and make a controlled spin of one of their spins, depending on the spin state of the other – known as controlled spin. “Hole rotations allow us to create two-qubit gates that are fast and highly reliable. This principle now also makes it possible to couple a larger number of qubit pairs,” says Kuhlmann.

The binding of two spin qubits is based on their exchange interaction, which occurs between two indistinguishable particles that interact with each other electrostatically. Surprisingly, the hole exchange energy is not only electrically controllable, but also highly anisotropic. This is a consequence of spin-orbit coupling, which means that the spin state of a hole is affected by its motion through space.

To describe this observation in a model, experimental and theoretical physicists at the University of Basel and NCCR SPIN combined forces. “Anisotropy makes two-qubit gates possible without the usual trade-off between speed and fidelity,” says Kuhlmann. “Qubits based on hole spins not only use tried and tested silicon chip fabrication, they are also highly scalable and have been proven to be fast and robust in experiments.” The study highlights that this approach has a strong chance in the race to develop a large-scale quantum computer.

Reference: “Anisotropic exchange interaction of two qubits with hole spin” by Simon Geyer, Bence Hetényi, Stefano Bosco, Leon C. Camenzind, Rafael S. Eggli, Andreas Fuhrer, Daniel Loss, Richard J. Warburton, Dominik M. Zumbühl and Andreas V. Kuhlmann, May 6, 2024, Nature Physics.
DOI: 10.1038/s41567-024-02481-5