Implementation and security analysis of practical quantum secure direct communication

Rapid development of supercomputers and the prospect of quantum computers are posing increasingly serious threats to the security of communication.Using the principles 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 important branch of quantum communication.In contrast to other branches of quantum communication,it transmits secret information directly.Recently,remarkable progress has been made in proof-of-principle experimental demonstrations of QSDC.However,it remains a technical feat to bring QSDC into a practical application.Here,we report the implementation of a practical quantum secure communication system.The security is analyzed in the Wyner wiretap channel theory.The system uses a coding scheme of concatenation of lowdensity parity-check(LDPC)codes and works in a regime with a realistic environment of high noise and high loss.The present system operates with a repetition rate of 1 MHz at a distance of 1.5 kilometers.The secure communication rate is 50 bps,sufficient to effectively send text messages and reasonably sized files of images and sounds.

Experimental free-space quantum secure direct communication and its security analysis

We report an experimental implementation of free-space quantum secure direct communication based on single photons.The quantum communication scheme uses phase encoding,and the asymmetric Mach–Zehnder interferometer is optimized so as to automatically compensate phase drift of the photons during their transitions over the free-space medium.At a 16 MHz pulse repetition frequency,an information transmission rate of 500 bps over a 10 m free space with a mean quantum bit error rate of 0.49%±0.27%is achieved.The security is analyzed under the scenario that Eve performs the collective attack for single-photon state and the photon number splitting attack for multi-photon state in the depolarizing channel.Our results show that quantum secure direct communication is feasible in free space.

Quantum secure direct communication with entanglement source and single-photon measurement

Quantum secure direct communication(QSDC)transmits information directly over a quantum channel.In addition to security in transmission,it avoids loopholes of key loss and prevents the eavesdropper from getting ciphertext.In this article,we propose a QSDC protocol using entangled photon pairs.This protocol differs from existing entanglement-based QSDC protocols because it does not perform Bell-state measurement,and one photon of the entangled pair is measured after the entanglement distribution.It has the advantage of high signal-to-noise ratio due to the heralding function of entanglement pairs,and it also has the relative ease in performing single-photon measurement.The protocol can use a practical entanglement source from spontaneous parametric down-conversion(SPDC);Gottesman-Lo-Lu¨tkenhaus-Preskill theory and the decoy state method give a better estimate of the error rate.Security analysis is completed with Wyner’s wiretap channel theory,and the lower bound of the secrecy capacity is estimated.Numerical simulations were carried out to study the performance of the protocol.These simulations demonstrated that the protocol with a practical SPDC entanglement source performed well and was close to the case with an ideal entanglement source.