In telecom networks connected by fiber optic cable, transmitted photons tend to be absorbed by the material that comprises the cable after traveling just a few kilometers. To prevent the signal from deteriorating, repeaters are placed at regular intervals of the cable to amplify the signal. Efforts to transmit photons over a quantum internet will face a similar challenge. Professor Tomoyuki Horikiri and colleagues at Yokohama National University are tackling this issue by developing a new source of entangled photons. While entangled photons could be used to transfer information, long-distance quantum communication could suffer from optical fiber losses, with entangled photons becoming disentangled due to interaction with their surroundings. Quantum repeaters, where quantum memories would be loaded, would be necessary for prolonging the distance of the quantum communication. The quantum repeater would store the quantum state of the photon that is being transmitted and would work through a repeated exchange of quantum states between light and matter. This would require a source of entangled photons that is compatible with quantum memory. Wavelength converter for the Yokohama National University researchers’ two-photon comb. Courtesy of Tomoyuki Horikiri, Yokohama National University. Efforts to develop sources of entangled photons have so far struggled to meet all the requirements for repeater-quantum memory compatibility and real-world application — that is, a high number of photons for large amounts of traffic, narrow linewidth, and high entanglement fidelity. Spontaneous parametric down-conversion (SPDC), a common way to produce entangled photons, uses crystals to convert single high-energy photons into pairs of entangled photons with half the original energy. “This has been great for quantum information experiments,” Horikiri said. “But for broadband quantum communications, SPDC is not very compatible with the very narrow energy transitions involved in production of the quantum memory needed for quantum repeaters.” The Yokohama team improved upon this technique, providing proof of concept for a new source of quantum-memory-compatible entangled photons that could be deployed through fiber optic cable with low losses. The researchers used the two-photon comb technique to realize a large number of frequency multimodes and entangled photon pairs with a narrow sub-MHz linewidth even in a telecom band. They achieved two-photon, 10-km transmission in an optical fiber, or overall 20-km distribution, and subsequent wavelength conversion. They observed two-photon correlation with a normalized correlation coefficient, despite the limited bandwidth of the wavelength converter. They demonstrated entanglement fidelity of more than 95% and Bell-state generation even with frequency multimode. This approach could lead to a method for entanglement distribution that is compatible with quantum memory and frequency-multiplexed, long-distance quantum communication applications. It could be applied to experimental optics, in addition to quantum information science. The researchers next plan to deploy their technique through multiple repeater nodes for realizing much longer distances. The research was published in Communications Physics (www.doi.org/10.1038/s42005-020-00406-1).