Spin qubits show promising control in emerging quantum computing research
Robert Shriver
Berlin, Germany (SPX) July 29, 2024
QuTech researchers have developed a new way to control spin qubits that could simplify the management of large-scale semiconductor qubit arrays. Their groundbreaking work on hopping spin was published in Nature Communications, and their innovative approach on somersaulting spin was published in Science.
In 1998, Roth and DiVincenzo introduced the concept of quantum computing with quantum dots, proposing spin hopping as the basis for qubit logic. The idea remained theoretical for more than 20 years, but researchers at QuTech, a collaboration between Delft University of Technology and TNO, have now demonstrated the feasibility of “hopping gates” with high performance.
Quantum dot-based qubits have been widely researched for quantum computing. Traditionally, these qubits are controlled by trapping a single electron, applying a large magnetic field, and using microwave signals for spin manipulation. However, QuTech’s new research shows that universal qubit control can be achieved without microwave signals. Instead, baseband signals and a small magnetic field are sufficient, greatly simplifying the control electronics of future quantum processors.
From bouncing qubits to tumbling qubits
To control the spin of an electron, the electron needs to hop from one quantum dot to another and rotate. The original proposal by Ross and DiVincenzo involved a type of magnet that would be difficult to implement experimentally. The QuTech team found a solution in germanium, a semiconductor that naturally enables spin rotation, first suggested in a study published in Nature Communications. Researchers Floor van Riggelen-Dooleman and Corentin Despres demonstrated that germanium enables the hopping of spin qubits, laying the foundation for a quantum link and revealing early signs of spin rotation.
For this analogy, think of the quantum dot array as a trampoline park, with electron spins bouncing between the trampolines. Germanium’s unique properties allow these bouncing spins to somersault, enabling effective qubit control. “Germanium has the advantage of aligning the spins in different directions in different quantum dots,” says Chien-An Wang, first author of the Science paper. The team achieved error rates of less than 1,000 for one-qubit gates and less than 100 for two-qubit gates.
Evolution of somersaulting qubit control
The researchers extended control of the two spins in the four-quantum-dot system to multiple quantum dots, like a person flipping on multiple trampolines.
“Quantum computing requires manipulating and coupling large numbers of qubits with high precision,” explains co-author Valentin John. Different quantum dots will exhibit different rotations, so understanding and managing this variation is key. Co-author Francesco Borsoi added, “We established a control routine that enables hopping spins on any quantum dot in our 10-quantum-dot array, allowing us to explore key qubit metrics in our extended system.”
Lead researcher Menno Feldhorst expressed pride in his team’s efforts: “I am proud of the result of our teamwork. In a period of one year, observing the rotation of qubits by hopping has become a tool used by the whole group. Developing efficient control schemes is essential for the operation of future quantum computers and we think this new approach is promising.”
Research report: Semiconductor quantum processor operated by hopping spins
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