Qubits, the core of quantum computing, are crucial for data processing, transfer, and storage. They must reliably store information and facilitate swift data processing, which depends on stable, rapid interactions among a substantial number of externally controllable qubits.

For quantum computing to advance, accommodating millions of qubits on a single chip is necessary. Current top-tier quantum computers house merely a few hundred qubits, which restrict them to computations that conventional computers can often execute more effectively.

Electrons and holes
Addressing the challenge of organizing and connecting thousands of qubits, researchers at the University of Basel and the NCCR SPIN are focusing on a qubit type utilizing the spin of an electron or a hole - an empty space left by a missing electron in a semiconductor. Spins, capable of adopting up or down states, can be entirely controlled electrically in hole spins, unlike electron spins which require additional elements like micromagnets.

University of Basel physicists had demonstrated by 2022 that hole spins in FinFETs - transistors integrated into modern smartphones - could act as qubits. Dr. Andreas Kuhlmann's team has now first successfully managed a controllable interaction between two such qubits.

Fast and precise controlled spin-flip
Quantum computers operate using "quantum gates," which manipulate qubits and link them. In a study published in Nature Physics, the team achieved a controlled spin-flip in one qubit based on another's state - a key operation for quantum computing. "Hole spins enable two-qubit gates that are both rapid and precise. This setup can potentially link more qubit pairs," stated Kuhlmann.

This qubit coupling derives from an electrostatic interaction known as exchange interaction, notable for its strong anisotropy due to spin-orbit coupling affecting the hole's spin by its spatial motion.

The combined efforts of experimental and theoretical physicists at the University of Basel and NCCR SPIN have modeled this behavior. Kuhlmann summarized, "This anisotropy allows for two-qubit gates without compromising speed for fidelity."

Hole-spin qubits, relying on established silicon chip production techniques, show significant scalability and robustness in tests, positioning them as strong contenders in developing large-scale quantum computers.

Research Report:Anisotropic exchange interaction of two hole-spin qubits

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