There is a growing number of software and algorithm developers trying to maximize the computational capabilities of NISQ devices at hand. However, in real and practical algorithms, they often encounter multi-controlled gates e.g. Toffoli gates, whose cost becomes the bottleneck in circuit transpilation and significantly restricts the application of such algorithms.
To answer this critical problem, we are now developing a new technique that realizes multi-controlled gates in higher-energy states of transmons, for instance by using the lowest three energy levels, called “qutrits”. As gate fidelity and coherence time improve, qutrit-based extension of quantum information processing has now become an interesting research topic, with a broad impact on other quantum systems since most of them naturally have access to higher energy levels.
As the application of this research, we are also studying more efficient schemes of quantum error correction (QEC) (arxiv: 2109.00086) as well as material properties and fabrication techniques best suited to such higher-level-based quantum processors.