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Singapore’s First Secure Quantum Network with Untrusted Devices

Image credits: news.nus.edu.sg

For the first time, a novel type of quantum key distribution (QKD) that is secure even if the users are not aware of the underlying quantum hardware has been successfully proven by a global team of researchers from the National University of Singapore (NUS) and LMU Munich (LMU). The research lays the way for a quantum internet that is more open and safer.

“The device independent QKD or DIQKD is the pinnacle of secure key exchange and could change the way we manage risks and trust in communication networks. For instance, because this method enables users to validate their quantum hardware without trusting the manufacturers, pertinent problems in cybersecurity like supply chain attacks and side-channels could be mitigated,” says Asst Prof Charles Lim, who proposed the project and initiated the collaboration with the team at LMU.

A crucial prerequisite for secure communication is the transmission of secret keys via a public channel. Today’s approaches for exchanging secret keys assume that certain mathematical problems are difficult to solve using cutting-edge computing technology. However, given the rapid development of next-generation computing capabilities such as quantum computing, this method may not be optimal for applications that place a premium on long-term security.

QKD provides a long-term and stable solution to the key exchange problem. A pair of identical and secret random keys are distributed between users via the measurement and exchange of single photons. The primary advantage of QKD is that its channel security is mathematically unbreakable, making it an ideal candidate for long-term sensitive data security. To realize this security promise, however, QKD implementation must be flawless, from the construction of quantum devices to side-channel attacks, which is a significant challenge in practice.

DIQKD is the foundation of the new experiment. Importantly, the security of DIQKD is independent of the quantum device specifications used in the protocol. However, there are two major challenges to implementing DIQKD. The underlying quantum noise must be very low, and the system must be extremely efficient in creating high-quality entanglement between the two users. Over long distances, achieving these two conditions at the same time has long been a problem.

The team used a new DIQKD protocol to tackle the first challenge. The protocol includes an additional set of key-generating measurements for users, whereas most protocols only have one. This makes the protocol more resistant to noise and loss, making it more difficult for an eavesdropper to steal information. Asst Prof Lim and colleagues at NUS developed this protocol.

To overcome the second obstacle, the researchers had to construct a high-quality entanglement between two quantum devices 400 meters away. Here, entanglement is achieved using quantum switching, in which separate photons from locally generated photon-atom entangled pairs are transferred over a 700-meter optical fibre and combined in a joint measurement system.

The researchers, on the other hand, faced a significant experimental problem in balancing entanglement quality, generation rate, and system noise, while undertaking security analysis to establish the setup’s capacity to generate DIQKD secret keys.

According to the researchers, they are working on methods to entangle distant quantum memories, which is an important step toward large-scale quantum networks. The experiment revealed an entanglement distribution between distant quantum memories, which serve as the foundation for long-distance quantum network links.

This study was carried out at the NUS College of Design and Engineering’s Department of Electrical and Computer Engineering and the NUS Centre for Quantum Technologies. The National Research Foundation (NRF) funded it through its NRF Fellowship programme.

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