100 km, the current Longest Distance of Quantum Secure Direct Communication

Confidentiality of communication is essential in modern societies. Traditional way of secure communication is to use encryption, which is based on the computational difficulty of certain mathematical problems such as factorizing large integers. In such schemes, the two parties first distribute a key using an asymmetric cryptographic algorithm such as RSA, which is based on the difficulty of integer factorization. Then they use the distributed key as the key in the symmetric cryptographic algorithm such as AES to transfer the message. However, Peter Shor designed an algorithm in 1994 that factorizes integers easily in a quantum computer, therefore cryptographic scheme such as RSA will become obsolete in the quantum computer era.  Rapid progress in quantum computing hardware is posing serious threats to asymmetric encryption schemes. To meet this challenge, one can use either post quantum cryptography (PQC), classical cryptographic algorithms that can resist quantum computing attack, or quantum key distribution (QKD) that negotiates secure key using quantum states.  

Is it possible to securely transmit information directly without using explicit encryption? The answer is yes, and the technology is quantum secure direct communication (QSDC), invented in the start of the new millennium by Gui-Lu Long and Xiaoshu Liu. QSDC transmits information directly using quantum states, and it does not require a pre-shared key. Of course, QSDC can also distribute secure key like QKD, and then used in classical communication with symmetric encryption.

In a recent paper published in Light Science & Application, a team of scientists from Gui-Lu Long’s group and Jianhua Lu’s group, of Tsinghua University and Beijing Academy of Quantum Information Sciences, China, designed and implemented an elaborate physical system with much enhanced performance. The proposed scheme uses photonic time-bin states for monitoring, and phase states for communication respectively. This design has several advantages. First, the system is robust against both polarization and phase errors. It does not use active feedback and the precise matching of the pair of interferometers. Second, the newly designed system greatly increases the reliability of the system and leads to ultra-low quantum bit error rate (QBER) of less than 0.1% at normal conditions, one order of magnitude better than existing systems.  Because of this, the transmission distance of this new QSDC system has been increased from the previous 18.5 km to a new record of 100 km in fiber.

The transmission rate of the new QSDC system is 0.54 bps at 100 km.  Transmission rate strongly depends on the transmission distance. At shorter distance, the transmission rate is much higher. It is 22.4 kbps at 30 km of fiber, which will satisfy the rate requirement of many practical applications.  Currently, the system is operating at 50 MHz repetition rate, and it can easily be upgraded to 1 GHz using off-the-shelf technology, and the transmission rate will also be increased consequently. Moreover, by combining QSDC with PQC, one can construct secure-repeater quantum network, which can extend the transmission distance endlessly by using classical repeaters at nodes between 30 to 50 kilometers apart.

      The scientists summarize their work in the following. “The primary contributions of this work are: (1) We proposed a novel design of physical system with a new protocol. We use both photonic time-bin and phase states and choose the time-bin states for eavesdropping detection and use the phase states for communicating the message; (2) We designed a quantum-memory-free QSDC scheme based on low-density Bose-Chaudhuri-Hocquenghem error correction codes; (3) We implemented the system and tested it with a clock-rate of 50 MHz through fiber at different distances.”  

“The system is free from phase and polarization drift, does not use the complicated active compensation subsystem. This enables an ultra-low QBER and the long-term stability against environmental noises. The new optical design uses a two-way structure, and it allows the returned pulses to bypass the modulators, which supports high clock-rate modulation up to 1 GHz, hence giving a high transmission rate.” they added.