100 km, the current longest distance of Quant


image: Laser: 1550 nm with 50 MHz pulse repetition frequency; FPGA Field Programmable Gate Array, ATT Attenuator, PC Polarization Controller, ILP Inline Polarizer, CIR Optical Circulator, PBS Polarization Beam Splitter, FC 90:10 Filter Coupler, PMFC Polarization Maintaining Filter Coupler, PM phase modulator, IM intensity modulator with 45.1 dB extinction ratio, ISO isolator, FR 90 degree Faraday rotator, SPD superconducting nanowire single photon detector.
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Credit: by Haoran Zhang, Zhen Sun, Ruoyang Qi, Liuguo Yin, Gui-Lu Long and Jianhua Lu

Confidentiality of communications is essential in modern societies. The traditional way of secure communication is to use encryption, which is based on the computational difficulty of certain mathematical problems such as factoring large integers. In such schemes, both 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 devised an algorithm in 1994 that easily factors integers into a quantum computer. Therefore, a cryptographic scheme such as RSA will become obsolete in the age of quantum computing. Rapid advances in quantum computing hardware pose serious threats to asymmetric encryption schemes. To address this challenge, one can use either Post-Quantum Cryptography (PQC), classical cryptographic algorithms that can withstand quantum computing attacks, or Quantum Key Distribution (QKD), which negotiates a 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 at the start of the new millennium by Gui-Lu Long and Xiaoshu Liu. QSDC transmits information directly using quantum states and does not require a pre-shared key. Of course, QSDC can also distribute a secure key like QKD, then used in classic 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, Tsinghua University and Beijing Academy of Quantum Information Science, in China, designed and implemented an elaborate physical system with significantly improved performance. . The proposed scheme uses temporal photonic states for monitoring and phase states for communication respectively. This design has several advantages. First, the system is robust against bias and phase errors. It does not use active feedback and precise interferometer pair matching. Second, the newly designed system dramatically increases system reliability and leads to an ultra-low quantum bit error rate (QBER) of less than 0.1% under normal conditions, an order of magnitude higher than existing systems. . For this reason, the transmission distance of this new QSDC system has been increased from the previous 18.5 km to a new fiber record of 100 km.

The transmission rate of the new QSDC system is 0.54 bps at 100 km. The transmission rate is highly dependent 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 throughput requirements of many practical applications. Currently, the system operates at a repetition rate of 50 MHz, and it can easily be upgraded to 1 GHz using standard technology, and the transmission rate will also be increased accordingly. Moreover, by combining QSDC with PQC, one can build a secure repeater quantum network, which can extend the transmission distance infinitely by using classical repeaters to nodes 30 to 50 kilometers apart.

The scientists summarize their work in the following. “The main contributions of this work are: (1) We proposed a new physical system design with a new protocol. We use both time and phase photonic states and choose the time states for eavesdropping detection and use the phase states to communicate the message; (2) We designed a QSDC scheme without quantum memory based on low density Bose-Chaudhuri-Hocquenghem error-correction codes; (3) We implemented the system and tested it with a clock frequency of 50 MHz via fiber at different distances.

“The system is free of phase and polarization drift, does not use the complex active compensation subsystem. This enables ultra-low QBER and long-term stability against environmental noise. The new optical design uses a bi-directional structure and allows the returned pulses to bypass the modulators, which supports high clock rate modulation up to 1 GHz, thus providing a high transmission rate, they added.

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