Dr. Shigeki Takeuchi, Professor, Dr. Ryo Okamoto, Assistant Professor, and their colleagues in the Research Institute for Electronic Science, Hokkaido University have realized a fundamental quantum gate operation for an optical quantum computer by combining "nonlinear optical switches" working at the single-photon level.
Photons, the elementary particles of light, are considered important as an information conveyance medium in quantum computers and communications. However, it has been difficult to induce interaction between two photons. To overcome this issue, however, a research group in the USA and Australia (Knill, Laflamme, and Milburn) showed that "nonlinear switches" working at the single-photon level could be realized using the quantum interference generated on a half mirror, while an integration of these switches could form an optical quantum computer. This proposal received considerable attention, but it has taken 10 years to realize these switches since they were suggested, due to technical difficulties, including the necessity for high-quality quantum interference between the photons on the half mirror, and the overlap of the different optical paths with a precision of one millionth of a millimeter.
We realized a compact, very stable implementation by improving the photon sources, by using special half mirrors that we developed ourselves, and by devising an ultra-stable optical interferometer.
As a result, we became the first to successfully develop a fundamental optical quantum circuit for an optical quantum computer as had been suggested by Knill et al. The average gate fidelity, as attained by the circuit, was 0.82, or a sufficiently high quantum nature.
This outcome of our research is expected to be applied to quantum cipher communications that realize safe communication, to quantum computers that will be able to solve problems that are currently beyond our reach, and to quantum information communications using less energy.
This research was conducted jointly by Dr. Jeremy O’Brien, Professor, University of Bristol, UK, and Dr. Holger Hofmann, Associate Professor, Hiroshima University.
