This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Quantum computers need qubits that can be controlled both quickly and reliably. Yet making semiconductor spin qubits faster often makes them more sensitive to electrical noise, creating a difficult trade-off.
Researchers at the University of Osaka and their collaborators have now demonstrated a way to loosen this constraint by adding one extra quantum dot, allowing speed and noise resistance to be tuned more independently after the device has been made. The research is published in the journal npj Quantum Information. Quantum dots are tiny regions in a semiconductor that can confine individual electrons.
In conventional flopping-mode spin qubits, two quantum dots are used to move an electron between locations, enabling fast electrical control of its spin. However, the same conditions that increase operating speed can also make the qubit more vulnerable to small voltage fluctuations. The researchers added a third, auxiliary quantum dot whose energy could be adjusted electrically.
By tuning this extra dot, they reshaped the electron's orbital energy structure, a strategy they call orbital band engineering. This created a broader operating region that was less sensitive to voltage noise while preserving the strong coupling needed for fast spin control. The team achieved operating frequencies above 100 MHz, reaching 271 ± 20 MHz under another tuning condition.
They also combined the device design with machine learning–based feedback that continuously corrected slow shifts in qubit frequency. This extended the estimated dephasing time from about 52 nanoseconds to 580 nanoseconds. Randomized benchmarking showed a 4-nanosecond gate fidelity of 99.56% with feedback, rising to 99.72% with additional pulse optimization.
"In GaAs quantum dots, magnetic noise makes fast electrical control particularly important, while electrical control also makes voltage noise relevant," says Takafumi Fujita. "By adding an auxiliary quantum dot, we introduced an extra degree of freedom for tuning the qubit frequency and could explore operating regions that are less sensitive to electrical noise." The approach could provide a useful design principle for correcting device-to-device variations after manufacturing, a key challenge when scaling semiconductor quantum computers to very large numbers of qubits. The researchers expect that the concept could also be applied to silicon- and germanium-based spin qubits.
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