Development of high-fidelity readout method for semiconductor qubits

Various candidates for hardware in quantum computers are being studied around the world. One of the leading candidates employs a method in which a single electron is confined in a tiny semiconductor (quantum dot), and its spin is used as an information carrier. In a qubit that uses the spin of a single electron (spin qubit), the information regarding the spin direction is converted into information about the presence or absence of an electric charge (spin-to-charge conversion), and the spin qubit is read out by detecting this charge information. Various spin-to-charge conversion methods have been reported to date, and high-fidelity readout of over 99% has been achieved. However, the performance and operating environment requirements for quantum dots to achieve high-fidelity readout are very strict, posing a major constraint on future practical application.

A research group led by Associate Professor Haruki Kiyama of the Graduate School of Information Science and Electrical Engineering at Kyushu University, and Professor Akira Oiwa of the Institute of Scientific and Industrial Research at Osaka University, has improved the existing single latching mechanism of spin-to-charge conversion and devised a new spin-to-charge conversion method (double latching mechanism), which can considerably reduce readout errors regardless of the spin qubit performance. The results are published online in npj Quantum Information.

By applying this method to actual quantum dots, they achieved a high-fidelity readout comparable to the world's highest fidelity to date. Moreover, there were concerns that the fidelity of conventional readout methods degrades in the large-scale arrays of quantum dots, and the development of a high-fidelity readout method that would be useful for large-scale arrays was a challenge. However, the double latching mechanism makes it possible to perform high-fidelity readout even in large-scale arrays.

For example, with the single latching mechanism, the readout fidelity strongly depends on the energy difference between the qubit states, and the readout fidelity may even fall below 70% in some devices. However, with the double latching mechanism, it is possible to substantially improve the readout fidelity regardless of the device.

The newly developed method is expected to promote research and development of the large-scale arrays of semiconductor spin qubits and to enable advancements toward the realization of large-scale semiconductor quantum computers.

The research group plans to apply the double latching mechanism to actual medium-scale quantum dot arrays to demonstrate that high readout fidelity is maintained. The research will then be expanded to perform readout in larger arrays and verify compatibility with other fundamental technologies such as qubit control, with the aim of realizing a semiconductor quantum computer within the next 20-30 years.

Journal Information
Publication: npj Quantum Information
Title: High-fidelity spin readout via the double latching mechanism
DOI: 10.1038/s41534-024-00882-1