Chinese scientists unlock long-distance quantum communications network

Scientists at Peking University in China have cracked how to build long-distance quantum communication networks. In their recent research, the scientists demonstrated secure quantum communication over distances of 2,300 miles (3,700 km). 

Even as scientists and companies work on building error-proof quantum computers, the devices could end up being quite worthless if they cannot communicate with each other and transfer information over a quantum network. 

This is why researchers have also been working on developing secure quantum networks. Much of the effort has been on ensuring that the quantum networks can utilise existing fiber-optic networks. However, the development of such networks has been marred by prohibitive equipment costs and limited range. 

Shortcomings of QKD

Quantum Key Distribution (QKD) is regarded as the gold standard of quantum communication. The approach is known to be hack-proof, and even attempts to tap into the network are easily detected. These advantages of QKD also contribute to its shortcomings, as the network cannot serve multiple users simultaneously. 

Central to the QKD are the series of ‘trusted relay nodes’ which handle the quantum keys along the route. These nodes also introduce potential vulnerabilities into the quantum network. Researchers at Peking University decided to eliminate the relay nodes. 

QKD systems are also custom-built, making them prohibitively expensive. So, the researchers decided to base their approach on tech that can be easily manufactured at an industrial scale. 

How did they crack it? 

On the server side, the researchers introduced a super optical comb that generates ultra-stable laser lines at the same frequency. A low-frequency chip smaller than a fingernail, this comb enables devices to operate from an identical, non-wavering time base with a width as little as 40 hertz.  

On the client side, the team used 20 independent quantum transmitter chips with a complete suite of functions that enabled them to operate as telegraph operators at the quantum level.

The transmitters operated in pairs and received signals from the central comb. They then encoded the information into light pulses that could be sent over a fiber optic cable. 

The cable used in this setup was ~230 miles (370 km) long. In their experiments, the researchers found that the chip modulators achieved a 97.5 percent success rate. Since each pair could communicate over 230 miles, the aggregate networking capability across 20 chips was 2,300 miles (3,700 km). 

Both server and client-side chips demonstrated high performance and operational yield. Equally important, they were manufactured on industry wafers, making them highly scalable. 

The researchers are hopeful that their approach will help unlock intercity quantum networks that could support hundreds of users, without requiring any relays. More importantly, the use of standard fiber-optic cable in the setup shows that quantum networks will not require special cabling, making them viable in the long run. 

The team pointed out that their technology is not ready for deployment anytime soon and only works under carefully controlled laboratory conditions. To overcome this, the researchers have already begun the next phase of their work by incorporating single-photon detectors and optical frequency shifters onto the server chip and by expanding the number of microcomb channels, enabling them to serve more customers simultaneously.