What’s New in Electronics Mar/Apr 2022

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QUANTUM COMPUTING IN SILICON SURPASSES 99% ACCURACY Three new studies, published back-to-back in the journal Nature, have reported high-fidelity (>99%) two-qubit gates based on spin qubits in silicon, confirming that robust, reliable quantum computing in silicon is now a reality. Together, the trio of results amplifies the promise of semiconductor spin qubits.

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he world is currently in a race to develop large-scale quantum computers that could vastly outperform classical computers in certain areas. However, these efforts have been hindered by a number of factors, including in particular the problem of decoherence, or noise generated in the qubits. This problem becomes more serious with the number of qubits, hampering scaling up. In order to achieve a large-scale computer that could be used for useful applications, it is believed that a two-qubit gate fidelity (accuracy) of at least 99% to implement the surface code for error correction is required, as the laws of quantum physics pose severe restrictions on how the correction takes place in a quantum computer. This has been achieved in certain types of computers, using qubits based on superconducting circuits, trapped ions and nitrogen-vacancy centres in diamond, but these are hard to scale up to the millions of qubits required to implement practical quantum computation with an error correction. Semiconductor spin qubits in silicon are stable enough to hold quantum information

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MARCH/APRIL 2022

for long periods and can be scaled up using techniques familiar from existing advanced semiconductor manufacturing technology, but have difficulty performing quantum logic operations with sufficiently high accuracy. The higher the accuracy of the operations, the higher the likelihood that near-term applications for quantum computers come in reach, and the higher the likelihood that errors can be corrected faster than they appear. The first of the new studies, by RIKEN and QuTech (a collaboration between TU Delft and TNO, the Netherlands Organisation for Applied Scientific Research), saw researchers demonstrate a two-qubit gate fidelity of 99.5% — higher than the 99% considered to be the threshold for building

fault-tolerant computers — using electron spin qubits in silicon. To carry out their work, the researchers experimented with a quantum dot structure that was fabricated by nanofabrication on a strained silicon/silicon germanium quantum well substrate, using a controlled-NOT (CNOT) gate. In previous experiments, the gate fidelity was limited due to slow gate speed. To improve the gate speed, the team carefully designed the device and tuned the device operation condition by voltages applied to gate electrodes to combine the established fast single-spin rotation technique using micromagnets and a large two-qubit coupling. This allows them to enhance the gate speed by a factor of 10 compared to previous work.

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