This breakthrough paves the way for the integration of millions of such qubits on a single chip, utilising established manufacturing techniques. The global competition to develop a practical quantum computer is intensifying, with researchers exploring a wide array of qubit technologies. However, there is yet no consensus on which type of qubit will best unlock the full potential of quantum information science.
Qubits are the fundamental building blocks of a quantum computer, responsible for processing, transferring, and storing data. To function effectively, qubits must both store information reliably and process it swiftly. The key to rapid information processing lies in the stable and fast interaction between a large number of qubits, whose states can be reliably controlled externally.
For a quantum computer to be viable, it must be capable of accommodating millions of qubits on a single chip. The most advanced quantum computers today are equipped with only a few hundred qubits, limiting their capacity to perform calculations that can already be executed, often more efficiently, by traditional computers.
Electrons and holes
Addressing the challenge of arranging and linking thousands of qubits, researchers at the University of Basel and the NCCR SPIN have focused on a type of qubit based on the spin (intrinsic angular momentum) of an electron or a hole. A hole, in this context, refers to the absence of an electron in a semiconductor. Both electrons and holes possess spin, which can adopt one of two states—up or down—analogous to the binary states of 0 and 1 in classical bits. A key advantage of using hole spin is that it can be entirely controlled electrically, without the need for additional components such as micromagnets on the chip.
In 2022, physicists from Basel demonstrated that hole spins could be trapped and utilised as qubits in an existing electronic device. These devices, known as FinFETs (fin field-effect transistors), are already used in modern smartphones and are manufactured through well-established industrial processes. Now, under the leadership of Dr Andreas Kuhlmann, the team has, for the first time, achieved a controllable interaction between two qubits within this framework.
Fast and precise controlled spin-flip
To perform computations, a quantum computer requires "quantum gates," which are operations that manipulate qubits and link them together. As reported in the journal Nature Physics, the researchers successfully coupled two qubits and induced a controlled flip of one qubit's spin, depending on the state of the other—an operation known as a controlled spin-flip. "Hole spins enable us to create two-qubit gates that are both fast and high-fidelity. This principle now allows us to couple a larger number of qubit pairs," explains Kuhlmann.
The coupling of two spin qubits is based on their exchange interaction, which occurs between two indistinguishable particles interacting electrostatically. Notably, the exchange energy of holes is not only electrically controllable but also strongly anisotropic—a consequence of spin-orbit coupling, where the spin state of a hole is influenced by its movement through space.
To model this observation, experimental and theoretical physicists at the University of Basel and the NCCR SPIN collaborated closely. "The anisotropy enables two-qubit gates without the usual compromise between speed and fidelity," summarises Dr Kuhlmann.
"Qubits based on hole spins not only take advantage of the proven silicon chip fabrication techniques but are also highly scalable and have demonstrated speed and robustness in experiments." This study highlights the significant potential of this approach in the ongoing race to develop a large-scale quantum computer.
