Quantum secure direct communication (QSDC) is a branch of quantum cryptography offering the potential to send unhackable messages. However, a combination of error rates and limited distances have prevented its practical application. Now, the demonstration of a system with a much lower error rate over 102.2 kilometers (63.5 miles) of optical fiber demonstrates this may not be the case much longer.

Message confidentiality today relies on encryption – but with quantum computing progressing rapidly, this may not act as protection much longer, as quantum computers have the potential to break encryption based on prime number factors with staggering speed. However, what quantum physics takes away, it can also give back, and QSDC offers a theoretical path to sending and receiving messages securely.

The challenge is to make QSDC practical before quantum computers are powerful enough to leave us in a world without private communication, and researchers from China have taken two big steps towards that goal. In the journal Light Science and Applications, they announce that they have achieved reliable, long-distance QSDC.

“QSDC is different from quantum key distribution (QKD), which negotiates a secure key using quantum technology,” the paper notes. Avoiding the need to use private keys makes for faster transmission with even greater security.

“QSDC securely and reliably transmits information through a quantum channel with both noise and eavesdropping,” the paper adds. (It also notes QSDC has “great potential for 6G wireless communication" which will probably terrify the conspiracy theorists still railing against 5G.)

“If we replace parts of the internet today, where more eavesdropping attacks happen, with quantum channels, those parts will have the added ability to sense and prevent eavesdropping, making communication even safer.” study author Professor Gui-Lu Long of Tsinghua University told the South China Morning Post.

The paper describes a new physical design and protocol for QSDC which communicates the message using phase states and detects eavesdropping through time-bin states, but does not use quantum memory.

The result was an error rate of less than 0.1 percent – approximately ten times less than previous versions of QSDC. Moreover, where other forms of quantum communication often suffer rapid signal loss, preventing transmission over long distances, this system could demonstrate 0.54 bps over 100 kilometers (60 miles) of ultra-low loss fiber. At higher speeds, distances were shorter, but 30 kilometers (18.6 miles) of commercial fiber still demonstrated 22.4 kbps, enough for audio calls, let alone text messages.

Long is the inventor of QSDC, and the co-holder of the previous distance record, set in 2020 of just 18.5 kilometers (11.5 miles).