The silicon quantum computer chip used in this study. Photo credit: RIKEN
Researchers from RIKEN and QuTech – a collaboration between TU Delft and TNO – have reached an important milestone in the development of a fault-tolerant quantum computer. They were able to demonstrate two-qubit gate accuracy of 99.5 percent — higher than the 99 percent considered the threshold for building fault-tolerant computers — using electron spin qubits in silicon, which show promise for large-scale quantum computing using nanofabrication technology their production already exists.
The world is currently in a race to develop large quantum computers that could far outperform classical computers in certain areas. However, these efforts have been hampered by a number of factors, most notably the issue of decoherence, or the noise generated in the qubits. This problem becomes more severe with the number of qubits and hinders scaling. In order to achieve a large scale computer that could be used for useful applications, it is believed that a two-qubit gate accuracy of at least 99 percent is required to implement the surface code for error correction. This has been achieved in certain types of computers using qubits based on superconducting circuits, trapped ions, and nitrogen vacancy centers in diamond, but these are difficult to scale to the millions of qubits needed to implement practical quantum computations with an error correction.
For the current work, published in Nature, the group decided to experiment with a quantum dot structure made by nanofabrication on a strained silicon/silicon-germanium quantum well substrate using a controlled NOT gate (CNOT). In previous experiments, the gate accuracy was limited due to the slow gate speed. To improve the gate speed, they carefully designed the device and tuned the device’s operating conditions by applying voltages to the gate electrodes to match the established fast single-spin rotation technique using micromagnets and a large two-qubit coupling combine. This allows them to increase the gate speed by a factor of 10 compared to the previous work. Interestingly, it used to be believed that increasing gate speed would always result in better fidelity, but they found that there was a limit and beyond that, increasing speed actually degraded fidelity.
Through the work, they discovered that a property called the Rabi frequency — a marker for how the qubits change state in response to an oscillating field — is key to the system’s performance, and they found a range of frequencies for which the Single frequency qubit gate accuracy was 99.8 percent and two qubit gate accuracy was 99.5 percent, exceeding the required threshold.
By doing so, they demonstrated that they can achieve universal operations, meaning that all fundamental operations that represent quantum operations, consisting of a single qubit operation and a two-qubit operation, can be performed with a gate accuracy above the error correction threshold.
To test the performance of the new system, researchers implemented a two-qubit Deutsch-Jozsa algorithm and the Grover search algorithm. Both algorithms give correct results with a high accuracy of 96-97%, showing that silicon quantum computers can perform quantum calculations with high accuracy[{” attribute=””>accuracy.
Akito Noiri, the first author of the study, says, “We are very happy to have achieved high-fidelity universal quantum gate set, one of the key challenges for silicon quantum computers.”
Seigo Tarucha, leader of the research groups, said, “The presented result makes spin qubits, for the first time, competitive against superconducting circuits and ion traps in terms of universal quantum control performance. This study demonstrates that silicon quantum computers are promising candidates, along with superconductivity and ion traps, for research and development toward the realization of large-scale quantum computers.
In the same issue of Nature, experimental demonstrations of similarly high-fidelity universal quantum gate sets achieved in silicon qubits are also reported from two independent research teams. A team at QuTech also used electron spin qubits in quantum dots (Quantum logic with spin qubits crossing the surface code threshold). Another team at UNSW Sydney (University of New South Wales) used a pair of ion-implanted phosphorous nuclei in silicon as nuclear spin qubits (Precision tomography of a three-qubit donor quantum processor in silicon).
Reference: “Fast universal quantum gate above the fault-tolerance threshold in silicon” by Akito Noiri, Kenta Takeda, Takashi Nakajima, Takashi Kobayashi, Amir Sammak, Giordano Scappucci and Seigo Tarucha, 19 January 2022, Nature.
DOI: 10.1038/s41586-021-04182-y
See Major Breakthrough As Quantum Computing in Silicon Hits 99% Accuracy for more on this research.
