The silicon quantum computer system chip used in this research study. Credit: RIKEN

Scientists from RIKEN and QuTech– a cooperation in between TU Delft and TNO– have attained a crucial turning point towards the advancement of a fault-tolerant quantum computer. They were able to demonstrate a two-qubit gate fidelity of 99.5 percent– higher than the 99 percent considered to be the threshold for constructing fault-tolerant computers– using electron spin qubits in silicon, which are promising for large-scale quantum computers as the nanofabrication innovation for developing them already exists.

The world is currently in a race to develop massive quantum computer systems that might significantly surpass classical computers in specific locations. However, these efforts have been hindered by a variety of factors, consisting of in particular the issue of decoherence, or sound generated in the qubits. This problem becomes more severe with the number of qubits, hindering scaling up. In order to achieve a massive computer system that could be utilized for useful applications, it is thought that a two-qubit gate fidelity of a minimum of 99 percent to carry out the surface code for error correction is required. This has been attained in certain kinds of computer systems, utilizing qubits based upon superconducting circuits, trapped ions, and nitrogen-vacancy centers in diamond, but these are tough to scale up to the millions of qubits needed to implement useful quantum calculation with an error correction.

To do the current work, released in Nature, the group chose to experiment with a quantum dot structure that was fabricated by nanofabrication on a stretched silicon/silicon germanium quantum well substrate, utilizing a controlled-NOT (CNOT) gate. To improve the gate speed, they carefully designed the device and tuned the device operation condition by voltages used to gate electrodes to integrate established fast single-spin rotation method utilizing micromagnets and a big two-qubit coupling.

Through the work, they found that a residential or commercial property called the Rabi frequency– a marker of how the qubits change states in response to an oscillating field– is key to the performance of the system, and they found a series of frequencies for which the single-qubit gate fidelity was 99.8 percent and the two-qubit gate fidelity was 99.5 percent, clearing the required limit.

Through this, they showed that they might accomplish universal operations, meaning that all the standard operations that make up quantum operations, including a single qubit operation and a two-qubit operation, could be performed with eviction fidelities above the mistake correction limit.

To test the ability of the brand-new system, the researchers implemented a two-qubit Deutsch-Jozsa algorithm and the Grover search algorithm. On both algorithms output right outcomes with a high fidelity of 96-97%, showing that silicon quantum computers can perform quantum estimations with high precision.

Akito Noiri, the very first author of the study, says, “We are really delighted to have achieved high-fidelity universal quantum gate set, among the key challenges for silicon quantum computers.”

Seigo Tarucha, leader of the research groups, stated, “The presented result makes spin qubits, for the first time, competitive versus superconducting circuits and ion traps in terms of universal quantum control performance. This research study shows that silicon quantum computers are appealing candidates, in addition to superconductivity and ion traps, for research study and advancement towards the awareness of large-scale quantum computers.

In the very same issue of Nature, speculative demonstrations of likewise high-fidelity universal quantum gate sets accomplished in silicon qubits are likewise reported from 2 independent research groups. A group at QuTech also utilized electron spin qubits in quantum dots (Quantum reasoning with spin qubits crossing the surface area code threshold). Another team at UNSW Sydney (University of New South Wales) utilized a set of ion-implanted phosphorous nuclei in silicon as nuclear spin qubits (Precision tomography of a three-qubit donor quantum processor in silicon).

Referral: “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.

The world is presently in a race to develop large-scale quantum computers that could greatly outperform classical computer systems in particular areas. To do the existing work, published in Nature, the group decided to experiment with a quantum dot structure that was produced by nanofabrication on a strained silicon/silicon germanium quantum well substrate, utilizing a controlled-NOT (CNOT) gate. In the very same issue of Nature, experimental demonstrations of likewise high-fidelity universal quantum gate sets attained in silicon qubits are likewise reported from two independent research study teams. A group at QuTech also used electron spin qubits in quantum dots (Quantum logic with spin qubits crossing the surface area code threshold).