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The National Institute of Information and Communications Technology (NICT) has successfully demonstrated entanglement swapping (one of the key quantum communication protocols) using sum-frequency generation (SFG) between single photons for the first time.

Although nonlinear optical effects of single photons have long been theoretically recognized as powerful tools for advancing quantum communication protocols, such effects are extremely weak at the single-photon level and had never been applied to quantum operations.

By combining NICT's state-of-the-art technologies including high-speed-clocked entangled photon-pair sources, low-noise superconducting nanowire single-photon detectors, and a high-efficiency nonlinear optical crystal, the research team succeeded in observing SFG between single photons with an unprecedented signal-to-noise ratio.

Using this effect, they achieved the first experimental demonstration of entanglement swapping via single-photon SFG.

This achievement is expected to pave the way for miniaturized and efficient photonic quantum information processing circuits, as well as long-distance device independent quantum key distribution.

The results are in Nature Communications.

In the field of quantum information processing such as quantum communication and quantum computing, two-qubit gate operations are fundamental building blocks.

In optical implementations, two-photon interference has been used to realize such operations. While this method allows for a relatively simple experimental setup using only a standard beam splitter and photon detectors, it suffers from a major limitation: Unless the existence of a photon pair obtained through entanglement swapping is confirmed by a measurement (and thus destroyed), the fidelity becomes low, limiting the range of applications.

To overcome this limitation, a theoretical scheme based on entanglement swapping using sum-frequency generation (SFG) between single photons has been proposed.

In this approach, by detecting the photon generated via SFG between two single photons (the SFG photon), it becomes possible to perform high-fidelity entanglement swapping without destroying the resulting entangled photon pair. This feature offers significant advantages for loophole-free Bell tests and long-distance device independent quantum key distribution.

However, although SFG between single photons was , the detected signal at that time was extremely weak and buried in noise. Therefore, to apply this effect to entanglement swapping, it was essential to dramatically improve the signal-to-noise ratio (SNR) of the detected SFG signal.

World-first demonstration

In this study, the research team constructed an experimental setup by combining NICT's state-of-the-art technologies including high-speed- , low-noise superconducting single-photon (SNSPDs), and a high-efficiency .

As a result, the SFG photons were detected with a high SNR, achieving nearly an order of magnitude improvement compared with the previous study. Furthermore, the researchers confirmed the presence of strong entanglement in the final state, estimating a lower bound of the fidelity to the maximally entangled state as 0.770 ± 0.076.

These results represent the world's first experimental demonstration of entanglement swapping via sum-frequency generation between single photons. This achievement marks a significant step forward in photonic quantum information processing and is expected to serve as an important guideline for the development of next-generation nonlinear optical devices.

Future prospects

To apply the current system to the more advanced quantum information protocols beyond entanglement swapping, further improvement in the SNR will be required.

In the future, the research team aims to enhance nonlinear optical efficiency, leading to the miniaturization and efficiency improvement of photonic quantum information processing circuits and the extension of transmission distance in device independent quantum key distribution.