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Implementation and Security Analysis of Practical Quantum Secure Direct Communication
Ruoyang Qi, Zhen Sun, Zaisheng Lin, Penghao Niu, Wentao Hao, Liyuan Song, Qin Huang, Jiancun Gao, Liuguo Yin, Gui-Lu Long
TL;DR
Quantum secure direct communication could address threats to conventional cryptography, but practical implementation requires security analysis and coding suited to noisy, lossy channels. This paper implements a DL04-based QSDC system using Wyner wiretap analysis and concatenated LDPC codes. It achieves 50 bps secure communication at 1 MHz over 1.5 kilometers, supporting text and reasonably sized image or sound files.
Problem
Practical application of QSDC requires an implemented system that addresses security analysis and operation under realistic high-noise, high-loss conditions.
Method
The system analyzes security with Wyner’s wiretap-channel theory and uses concatenated LDPC codes designed for high loss and high error rates.
Results
50 bps secure information transmission is achieved at a 1 MHz repetition rate over 1.5 kilometers.
Takeaways & Limitations
The implementation can transmit text messages and image or sound files of reasonable size in a realistic quantum-communication environment.
Takeaways & Limitations
Traditional error-correcting coding cannot be used because the system operates with high channel loss, high error rates, and low detector efficiency.
Abstract
from arXiv · showhide
Fast development of supercomputer and perspective quantum computer is posing increasing serious threats to communication security. Based on the laws of quantum mechanics, quantum communication offers provable security of communication, and is a promising solution to counter such threats. Quantum secure direct communication (QSDC) is one of the important branches of quantum communication. Different from other branches of quantum communication, it transmits secret information directly. Recently, remarkable progress has been made in the proof-of-principle experimental demonstrations of QSDC. However, it remains a technical feast to march QSDC into practical application. Here, we report an implementation of practical quantum secure communication system. The security is analyzed in the Wyner wiretap channel theory. The system uses a coding scheme based on concatenation of low density parity check (LDPC) codes, which works in a regime with realistic environment of high noise and high loss. The present system operates with a repetition rate of 1 MHz, and at a distance of 1.5 kilometers. The secure communication rate is 50 bps, which can effectively send text message and files such as image and sounds with a reasonable size.
INTRODUCTION
QSDC is presented as a direct-information alternative to key-distribution-based security, motivated by threats from computational uncertainty and quantum computers. The paper reports a practical implementation using wiretap-channel security analysis and LDPC coding for realistic noisy, lossy conditions.
- QSDC transfers information directly without key distribution, avoiding security loopholes associated with key storage and ciphertext attacks.
- The work reports an experimental implementation of a practical quantum secure communication system based on the DL04 protocol.
- Practical QSDC requires security analysis of information transmission and operation below the channel’s secrecy capacity.The system estimates secrecy capacity from sampling-check error rates before selecting a lower-rate coding scheme.
- The implementation uses concatenated LDPC codes designed for the high-loss and high-error regime characteristic of quantum communication.Traditional coding is not suited to the system’s realistic operating conditions.
- 1 MHz repetition, 1.5 kilometers, and 50 bps secure information transmission demonstrate a practical operating system.The achieved rate can transmit text messages and image or sound files of reasonable size.
RESULTS
The PDL04-QSDC protocol uses four sequential steps to check the channel, encode secret information, test for eavesdropping, and decode successfully received messages.
- Protocol steps: Bob prepares qubits randomly in four states and sends them to Alice, who samples photons in randomly chosen Z- or X-bases.The states 0 and 1 are Z-basis eigenstates, while + and − are X-basis eigenstates.
- Protocol steps: Alice publishes sample positions, bases, and results so Bob can compare them with his preparations and estimate the Bob-to-Alice error rate.
- Protocol steps: Alice selects a rate no greater than the estimated secrecy capacity and encodes message blocks with LDPC codes.Random numbers are inserted into the encoded sequence to check eavesdropping on the Alice-to-Bob channel.
- Protocol steps: Bob deterministically decodes the message in the preparation basis when the error rate remains within the LDPC code’s correcting capability.Transmission stops if the error rate exceeds that capability.
2) Security analysis
The security analysis models QSDC as a Wyner wiretap channel, bounding Eve’s information and Alice–Bob mutual information under collective attacks, loss, and detector inefficiency. These bounds provide a lower bound on secrecy capacity for coding-rate selection.
- The secrecy capacity is defined as the difference between maximum Alice–Bob and Alice–Eve mutual information.
- The analysis assumes a complete mixed state prepared by Bob and considers collective attacks represented by joint operations on qubits and Eve’s ancilla.
- Eve’s information is bounded by h(ξ), where ξ depends on the X- and Z-basis error rates measured during error checking.The binary Shannon entropy h is used in the bound.
- Detector inefficiency and channel loss modify Eve’s bound through Q_Eve, the maximum rate at which Eve can access qubits.
- The legitimate channel is modeled as cascaded binary symmetric and binary erasure channels, with mutual information determined by Q_Bob and the Alice–Bob bit error rate.
3) Experimental results
The experiment evaluates system stability and secure transmission over a 1.5-kilometer fiber link. At 25.1 dB total loss, the system achieves a secure information rate of 50 bps within the information-theoretic secure region.
- System stability: 0.8% error rates were observed for Alice’s X-basis and Z-basis measurements under normal working conditions.Error rates were estimated block by block, with each block containing 1312 × 830 pulses.
- System stability: 0.3% of pulses sent by Bob were received as photon counts, while Bob’s error rate was lower than Alice’s because of the protocol’s intrinsic robustness.This corresponds to approximately 3 photon counts per 1000 pulses.
- Secure coding performance: 0.00184 was the estimated secrecy capacity at 25.1 dB loss, while the selected LDPC coding scheme achieved a transmission rate of 0.00096 at bit error rate 10^-6.The pseudo-random sequence length was optimized to N = 830.
- Secure communication: 50 bps was achieved as the secure information rate, and this rate was well within the secure area between Alice–Bob and Alice–Eve information curves.The area between the curves represents rates that guarantee security reliably.
- System conditions: 1.5 kilometers of fiber produced 25.1 dB total system loss, including 14.5 dB quantum-channel loss and 0.6 dB fiber loss.The detector efficiency was about 70%, and the optical elements contributed about 13 dB loss.
DISCUSSION
The discussion addresses photon loss and the mismatch between conventional error-correcting codes and practical QSDC conditions. It presents a concatenated LDPC, pseudo-random-sequence, and universal-hashing scheme designed for reliable and secure transmission under high noise and loss.
- Practical challenges: High photon loss, inefficient photon sources, high channel loss, and low-efficiency detectors make practical quantum communication challenging.These conditions motivate specialized coding rather than conventional low-loss, low-noise designs.
- Practical challenges: Traditional error-correcting codes are designed for low-loss and low-noise regimes and cannot be used for this QSDC purpose.The paper identifies this regime mismatch as a limitation of conventional coding approaches.
- Coding scheme: The proposed coding scheme concatenates LDPC codes with a pseudo-random sequence and universal-hashing preprocessing to provide reliable and secure transmission.For each message block, random bits are hashed with the message, LDPC-encoded, and expanded into transmitted sequences.
- Coding scheme: Alice selects a coding rate not exceeding the secrecy capacity, while Bob decodes received coded bits using log-likelihood ratios and iterative LDPC belief propagation.The security condition requires the eavesdropper’s information to remain bounded relative to the coding parameters.
MATERIALS AND METHODS
The system implements practical QSDC over a 1.5-kilometer fiber using single-photon pulse encoding, active control, LDPC coding, and wiretap-channel security analysis. It achieves secure transmission despite high noise and loss, while the check mode has a higher error rate and the demonstrated rate remains improvable.
- Experimental setup: 1 MHz repetition rate and a 1.5-kilometer fiber support the practical QSDC experiment.Bob sends attenuated single-photon pulses in two time bins, while Alice randomly performs error-checking or coding.
- Optical design: Forward-backward photon routing automatically compensates polarization drift and enhances interference visibility.The check module cannot use this retrace-light circuit and instead requires active polarization control.
- Error control: The check-mode error rate is usually higher than the communication-mode error rate.Active polarization restoration is used because the retrace-light circuit is unavailable in the check module.
- Error control: LDPC coding and pseudo-random sequences are applied to combat error and loss in the high-noise, high-loss environment.If the measured error rate is below threshold, encoded photons are sent back to Bob.
- Performance: 50 bps secure information rate is achieved over 1.5 kilometers.The authors characterize the rate using secrecy capacity: an information rate below that capacity provides security and reliability.
CONTRIBUTONS
The authors divide contributions across protocol and optical-circuit design, LDPC and pseudo-random-sequence development, security analysis, supervision, and manuscript preparation.
- Contributions: RYQ, ZSL, PHN, JCG, and GLL designed the protocol, optical circuits, and physical layout.
- Contributions: ZS, WTH, LYS, QH, and LGY developed the LDPC coding and pseudo-random sequences.
- Contributions: RYQ, ZSL, LGY, and GLL completed the security analysis.
- Contributions: LGY and GLL supervised the project, GLL led it, and all authors contributed to writing.