The aim of this project is to develop quantum optical communication technologies for distributed quantum computing systems where quantum computer nodes are connected via entangled photons.
Toward this goal, we employ two approaches: two-photon and single-photon interference. The former is more mature and suitable for near-term demonstrations of distributed quantum computing systems. The latter is technically more challenging, but has potential advantages, such as loss-tolerance and suitability for wavelength-division-multiplexed quantum memories. The project includes development of entangled photon sources, techniques for their evaluation, phase locking between distant entangled photon sources, entanglement swapping, and entangled photon routing. These technologies directly contribute to the development of scalable and robust integrated quantum communication systems.

So far, we have successfully developed various underlying technologies for quantum optical communication, such as quantum frequency converters, entangled photon sources that directly connect the quantum memory wavelength and the telecom wavelength, frequency stabilization, optical phase locking, loss-tolerant transmission techniques, and important components of multiparty entanglement distribution.
Figure 2 shows one of the entanglement distribution technologies we have developed, based on the novel “hybrid entanglement” of the photon-polarization degree of freedom and the photon-number degree of freedom, where the former is easier to handle for distributed quantum computing and the latter is useful for loss-tolerant entanglement transmission. Not only developing the hybrid-entanglement source, but we also demonstrated single-photon-interference entanglement swapping, which is a loss-tolerant entanglement distribution technique, by two hybrid-entanglement sources and were able to demonstrate the loss-tolerant distribution of polarization entangled twin-photons.