Advances towards the realization of silicon quantum computers through the efficient control of qubits

On June 12, Hitachi, Ltd. announced that it had proposed and confirmed the effectiveness of a "shuttling qubit method" that allows efficient control of qubits for the practical application of silicon quantum computers. Hitachi has also begun joint research with a group led by Professor Kenji Ohmori of the Institute for Molecular Science at the National Institutes of Natural Sciences on a "quantum operating system" suitable for controlling quantum computers, which included the aforementioned result.

Accelerating research for large-scale integration through joint research, aiming for early commercialization of silicon quantum computers. It is said that a quantum computer with a scale of ≥1 million qubits is needed to realize ultrafast calculations that are impossible with conventional computers, such as quantum chemical calculations of nitrogenase enzymes, which are attracting attention in the agricultural field. To realize such a large-scale quantum computer, it is necessary to implement technology that can efficiently control integrated quantum bits and error correction technology.

The silicon quantum computer being researched and developed by Hitachi is expected to have advantages over the superconducting type, which has been in development for some time, with regard to scalability. However, qubits are generally installed in fixed places, and it is necessary to connect operation and readout circuits to all qubits. Additionally, crosstalk (error) occurs between adjacent qubits. These factors hinder large-scale integration.

In response to these issues, Hitachi proposed a shuttling qubit method that allows qubits to be freely moved between pre-set areas for control such as computation and readout. This eliminates the need to connect arithmetic and readout circuits to all the qubits, simplifying the wiring structure of the silicon element. Additionally, the crosstalk (errors) is suppressed by performing operations through the evacuation of adjacent qubits.

In a silicon quantum computer, a single electron is trapped in a microstructure called a "quantum dot" formed in a silicon element, and its spin is used as a quantum bit. Conventionally, it has been assumed that its qubits (electrons) cannot be moved from within the quantum dots; however, Hitachi has focused on the fact that electrons in the array can be moved and has successfully conducted experiments based on this principle.

In addition, if it is possible to move (shuttling) while maintaining the quantum state, this will bring new possibilities for the control of operations and readout of qubits. Therefore, Hitachi proposed this control method as the "shuttling qubit method" and verified its effectiveness.

Furthermore, they constructed a simulator that incorporates these effects and confirmed that the shuttling qubit method can maintain a higher quantum computation accuracy (fidelity) than the conventional method with fixed qubits in large-scale quantum operations where the effect of crosstalk is significant. By moving qubits, operations can be performed between arbitrary qubits, which is expected to facilitate the implementation of error correction functions.

Omori summarized the findings: "This achievement is extremely important because it quantitatively demonstrates the effectiveness of 'dynamic' qubits in silicon-based quantum computers. Using this technology, arbitrary pairs of spatially separated qubits can be dynamically brought closer together in the middle of a computation, creating a "quantum entangled state," which is essential for quantum computing."

"In principle, it is also possible to put all the qubits that make up a quantum computer into a quantum entangled state. This is an operation that is not possible with hardware in which each qubit is spatially fixed, such as superconducting hardware and is expected to lead to revolutionary advances in computational accuracy and algorithms for quantum computers."

"Additionally, the Institute for Molecular Science is working on the implementation of dynamic qubits in its cooled-atom type system, and because there are many similarities in the control system between the two dynamic qubit systems, their joint development is expected to significantly accelerate the practical application of quantum computers in Japan."