Researchers are advancing quantum computing by developing qubits based on the spins of electrons and holes, and a recent breakthrough at the University of Basel demonstrated controlled interactions between qubits using the spin of holes. These advances suggest a promising future for scalable, efficient quantum computers using existing silicon technology.
University of Basel advances in qubit technology lead to scalable Quantum ComputingIt uses electron and hole spins to enable precise qubit control and interaction.
The pursuit of practical quantum computers is in full swing, with researchers around the world investigating various qubit technologies. Despite considerable efforts, there is still no consensus on which type of qubit is best suited to unlock the full potential of quantum information science.
Qubits are the foundation of quantum computers. They are responsible for processing, transmitting, and storing data. To be effective, qubits must store information reliably and process it quickly. This requires stable and rapid interactions between large numbers of qubits that can be precisely controlled by an external system.
Today's state-of-the-art quantum computers have only a few hundred qubits, so they can only perform calculations that classical computers can already perform, and often more efficiently. To advance quantum computing, researchers must find a way to fit millions of qubits on a single chip.
Electrons and holes
To solve the problem of arraying and linking thousands of quantum bits, researchers from the University of Basel and NCCR SPIN rely on a type of quantum bit that uses the spin (intrinsic angular momentum) of electrons or holes. Holes are essentially missing electrons in a semiconductor. Both holes and electrons have spin, which can be in one of two states, up or down, similar to the 0 and 1 of classical bits. Compared to electron spin, hole spin has the advantage that it can be controlled entirely electrically, without the need for additional components such as micro-magnets on the chip.
Two interacting hole-spin qubits. When a hole (magenta/yellow) tunnels from one site to the other, its spin (arrow) rotates due to so-called spin-orbit coupling, resulting in an anisotropic interaction indicated by the surrounding bubbles. Credit: NCCR SPIN
In 2022, physicists from Basel demonstrated that it is possible to capture the hole spins of existing electronic devices and use them as quantum bits. These “FinFETs” (fin field effect transistors) are built into modern smartphones and are produced in widespread industrial processes. Now, a team led by Dr. Andreas Kuhlmann has succeeded for the first time in achieving a controllable interaction between two quantum bits within this setup.
Fast and precisely controlled spin flips
To perform calculations, quantum computers require “quantum gates,” which represent operations that manipulate quantum bits and couple them together. As the researchers report in the journal Natural PhysicsIn , they were able to couple two qubits and flip the spin of one in a controlled way depending on the spin state of the other – known as controlled spin flip. “Hall spin allows us to make fast, high-fidelity two-qubit gates. This principle also enables us to couple many more qubit pairs,” says Kuhlman.
The coupling of two spin qubits is based on the exchange interaction that occurs between two indistinguishable particles interacting electrostatically. Remarkably, the exchange energy of the hole is not only electrically controllable but also strongly anisotropic. This is a consequence of spin-orbit coupling, which means that the spin state of the hole is affected by the hole's motion in space.
To explain this observation with a model, experimental and theoretical physicists from the University of Basel and NCCR SPIN joined forces. “The anisotropy enables two-qubit gates without the usual trade-off between speed and fidelity,” says Kuhlmann. “Not only do qubits based on Hall spin leverage the proven manufacturing of silicon chips, they are also highly scalable and have proven to be fast and robust in experiments.” The research highlights that this approach has great potential in the race to develop large-scale quantum computers.
Reference: Simon Geyer, Bence Hetényi, Stefano Bosco, Leon C. Camenzind, Rafael S. Eggli, Andreas Fuhrer, Daniel Loss, Richard J. Warburton, Dominik M. Zumbühl, Andreas V. Kuhlmann, “Anisotropic exchange interactions in two Hall spin qubits,” May 6, 2024, Natural Physics.
Publication date: 10.1038/s41567-024-02481-5
