Led by Prof. Sven Rogge, a Chief Investigator at the Centre for Quantum Computation and Communication Technology (CQC2T), a research team has achieved a considerable increase in the coherence time of a spin-orbit qubit in silicon, which enables the preservation of quantum information for a longer time.
The study outcomes offer a new means to scale up silicon quantum computers.
A team of international researchers has substantially increased the duration for which quantum information can be retained by a spin-orbit qubit in silicon, opening a new door to make silicon quantum computers more functional and scalable.
For more than a decade, scientists have investigated spin-orbit qubits as an option to scale up the number of qubits in a quantum computer. This is because they are easy to control and couple over huge distances. But they have always exhibited very limited coherence times, which is too short for quantum technologies.
The study recently reported in Nature Materials demonstrates that long coherence times can be achieved if the spin-orbit coupling is sufficiently strong. The team achieved 10,000 times longer coherence time compared to what was recorded earlier for spin-orbit qubits, thus making them a perfect candidate for scaling up silicon quantum computers.
We turned the conventional wisdom on its head by demonstrating exceptionally long coherence times—~10 milliseconds—and therefore, that spin-orbit qubits can be remarkably robust.
Sven Rogge, Chief Investigator, Centre for Quantum Computation and Communication Technology
Rogge, who led the research team, is also a Professor at the University of New South Wales (UNSW).
Strong Coupling is Key
The length of time for which a qubit can preserve quantum information is governed by its stability.
In spin-orbit qubits, information is stored on the electron’s spin as well as its motion—how it “orbits” atoms in the chip’s lattice. The strength of the coupling between the two spins maintains the qubit stable and makes it less prone to degradation by electric noise in devices.
“The quantum information in most spin-orbit qubits is extremely fragile. Our spin-orbit qubit is special because quantum information stored in it is very robust,” says lead author Dr Takashi Kobayashi, who performed the research at UNSW and is now at Tohoku University.
The information is stored in the orientation of the spin and orbit of the electron, not just the spin. The circular orbit of the charged electron and the spin are locked together like gears due to the very strong attraction in the spin-orbit coupling. Increasing the strength of that spin-orbit coupling lets us achieve the significantly longer coherence times we’ve published today.
Dr Takashi Kobayashi, Study Lead Author, Tohoku University
Engineering Longer Coherence Times
The researchers increased the coherence time by first developing spin-orbit qubits through the introduction of impurities, known as acceptor dopant atoms, in a silicon crystal. Then, they modified the strain in the chip’s silicon lattice structure to produce spin-orbit coupling of various levels.
“The crystal is special because it only contains the isotope of silicon with no nuclear spin. This eliminates magnetic noise, and because it is strained sensitivity to electric noise is also reduced,” noted Kobayashi.
“Our chip was fixed on to a material that at a low temperature stretches out the silicon—like a rubber band. Stretching the lattice to the correct tension allowed us to tune the spin-orbit coupling to the optimal value,” he added.
The result was more than 10,000 times longer coherence times than observed earlier in spin-orbit qubits. This implies that quantum information is preserved for a longer period, thus enabling many more operations to be carrying—an crucial platform for scaling up quantum computers.
Scaling Up with Spin-Orbit Coupling
If a quantum computer must outshine a classical computer, it is essential for a large number of qubits to work together to carry out complex calculations.
The stability of our spin-orbit qubit to electric fields is unique, proving a robust new pathway to make scalable quantum computers.
Dr Joe Salfi, Study Co-Author, University of British Columbia
Dr Salfi performed the study while at CQC2T and is now affiliated with the University of British Columbia.
The study outcomes eventually offer new techniques to control individual qubits and coupling qubits over considerably larger distances, which will render the chip fabrication process highly flexible.
Moreover, the electrical interaction enables coupling to other quantum systems, offering the potential for hybrid quantum systems. Previous studies reported in Science Advances by the UNSW research group demonstrated that spin-orbit coupling in silicon offers several benefits for scaling up to a large number of qubits.
“Spins in silicon are very attractive for scalable quantum information devices because they’re stable and compatible with current computer processing techniques, making those devices easy to manufacture. Now that we’ve demonstrated long coherence times, spin-orbit qubits make a strong candidate for a large-scale quantum processor in silicon,” added Prof. Rogge.
Kobayashi, T., et al. (2020) Engineering long spin coherence times of spin-orbit qubits in silicon. Nature Materials. doi.org/10.1038/s41563-020-0743-3.