The elementary particles of light, photons, are considered important as an information conveyance medium for quantum computers and communications. The realization of an “optical quantum circuit” combining fundamental “quantum gates” is desired. The “quantum entanglement filter” is an optical quantum circuit that can extract only pairs of photons that exist in a state of specific quantum entanglement. However, the quantum entanglement filter is one of the largest optical quantum circuits in the world, both in terms of the number of quantum gates and the number of classical interferences. Consequently, it was technically very difficult to develop such a filter. We finally produced a quantum entanglement filter by combining a special optical element called a partially polarizing beam splitter and a displaced Sagnac interferometer. This outcome will lead to the realization of future quantum computers and ultra-long-distance quantum ciphers.
