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Experimental quantum secure direct communication with single photons
Jianyong Hu, Bo Yu, Mingyong Jing, Liantuan Xiao, Suotang Jia, Guoqing Qin, Guilu Long
TL;DR
Secure communication is important, but channel noise can let an adversary gain information. This paper presents a single-photon QSDC protocol and experimentally demonstrates frequency-coded transmission, while noting that it does not provide a rigorous unconditional security proof.
Problem
Channel noise can allow an adversary to gain information, making secure communication important.
Method
The paper presents a practical QSDC protocol based on single photons and the SICO-DL04 protocol.
Results
16 frequency peaks were observed, with noise and loss broadening peaks but leaving their central frequencies unchanged; the experiment achieved a communication rate of 4 kbps.
Takeaways & Limitations
The results support practical single-photon quantum secure direct communication under channel noise and loss.
Takeaways & Limitations
A rigorous unconditional security proof of the protocol is not presented.
Abstract
from arXiv · showhide
Quantum communication holds promise for absolutely security in secret message transmission. Quantum secure direct communication is an important mode of the quantum communication in which secret messages are securely communicated over a quantum channel directly. It has become one of the hot research areas in the last decade, and offers both high security and instantaneousness in communication. It is also a basic cryptographic primitive for constructing other quantum communication tasks such as quantum authentication, quantum dialogue and so on. Here we report the first experimental demonstration of quantum secure direct communication with single photons. The experiment is based on the DL04 protocol, equipped with a simple frequency coding. It has the advantage of being robust against channel noise and loss. The experiment demonstrated explicitly the block data transmission technique, which is essential for quantum secure direct communication. In the experiment, a block transmission of 80 single photons was demonstrated over fiber, and it provides effectively 16 different values, which is equivalent to 4 bits of direct transmission in one block. The experiment has firmly demonstrated the feasibility of quantum secure direct communication in the presence of noise and loss.
INTRODUCTION
Quantum secure direct communication (QSDC) sends secret information directly over a quantum channel without first establishing a key. This paper introduces and experimentally demonstrates a single-photon protocol designed for noisy and lossy channels.
- QSDC transmits secret information directly through a quantum channel without first establishing a key.
- Block transmission sends quantum carriers in groups and checks each block for eavesdropping before continuing.This avoids information leakage before Eve’s detection and enables direct secure communication.
- The DL04 protocol is the first QSDC protocol based on single photons and is easier to implement than protocols using entanglement sources.
- QSDC is also used as a cryptographic primitive for constructing quantum signatures, quantum dialogues, and direct secret sharing.
- Channel noise can let Eve gain information, while privacy amplification is complex and disrupts the direct-communication picture by merging and reshuffling photons.
- SICO-DL04 introduces simple frequency coding into DL04, encoding information in the spectrum of a sequence of single photons.The protocol is reported to work efficiently with channel loss and noise and was demonstrated experimentally.
MATERIALS AND METHODS
The protocol encodes information by applying periodic state flips to sequences of single photons and recovers it from the resulting modulation frequency. Frequency components increase the information capacity of each block.
- Alice prepares a sequence of randomly chosen single-photon states and records each measured photon’s bit value and arrival time.The protocol uses the four states |0⟩, |1⟩, |+⟩, and |−⟩.
- Bob reserves a fraction C of received photons for random X- or Z-basis eavesdropping checks before encoding the secret information.If the measured error rate exceeds the threshold, communication is aborted.
- Bob encodes information by applying periodic state-flip operations to photon sequences, with modulation frequency f=1/T representing the information.A random phase offset δT prevents Eve from guessing the period from the modulation signal alone.
- Photon loss from optical-fibre attenuation and detector inefficiency produces missing records, but the frequency-coding scheme is described as robust against error and loss.
- Alice estimates the coding frequency from the measured values and arrival times using a discrete time Fourier transform, then reads the encoded information from the spectral peak.
- Using r frequency components yields Nmax different combinations and b=log2 Nmax bits per photon sequence.The transmission rate is expressed as I=b/Tspan.
RESULTS AND DISCUSSION
The experiment demonstrates frequency-coded single-photon QSDC that recovers encoded frequencies despite channel noise and loss, while transmitting 4 bits per 80-photon block. The security analysis is explicitly approximate because no rigorous unconditional proof is presented.
- Experimental results: 16 spectral peaks remain identifiable because channel noise and loss broaden and lower peaks but preserve their central frequencies.Modulation-frequency identification requires a signal-to-noise ratio higher than 1.
- Experimental results: Increasing the mean photon number per pulse raises the modulation signal above background noise while the background remains low.The experiment examines signal and background noise across different mean photon numbers.
- Transmission performance: 80 single photons form each spectrum-calculation block, with one responding frequency channel and 16 possible values encoding 4 bits per block.The reported experimental communication rate is 4 kbps.
- Limitations: The security discussion provides a rough estimate, and the paper does not present a rigorous unconditional security proof for SICO-DL04.The experimental setup uses a field-programmable gate array and an optical-fiber delay line in the detection procedure.
- Security analysis: Security requires RAlice / REve > N / b, because obtaining fewer than b qubits does not imply that Eve obtains any information bits.The modulation frequency also requires enough collected data for spectrum calculation.
- Operating range: For weak laser pulses, the secure communication distance is about 10 kilometers, with 16 frequency channels within a 400 kHz bandwidth.The frequency spacing is 25 kHz.
CONCLUSIONS
The paper presents SICO-DL04, a single-photon QSDC protocol using frequency coding and experimentally demonstrates block transmission under channel noise and loss.
- CONCLUSIONS: SICO-DL04 encodes information in modulation frequencies that periodically flip photon states rather than in individual photons.The information is encoded on the statistical properties of photon sequences.
- CONCLUSIONS: The protocol is reported to be robust against channel loss and noise and does not require privacy amplification.Its security was analyzed against major eavesdropping strategies and channel noise and loss.
- CONCLUSIONS: The frequency-coding scheme can use several modulation frequencies simultaneously, increasing the information carried by a photon block and simplifying the protocol.The authors contrast it with protocols using complicated quantum error-correction codes.
- CONCLUSIONS: 80 photons per block provided 16 frequency values, equivalent to 4 bits of information per single-photon sequence.The experiment used a 400 kHz modulation-frequency range with 25 kHz spacing.
- CONCLUSIONS: 4 kbps was demonstrated experimentally, providing a first experimental demonstration of block transmission for QSDC with single photons.The experiment used existing technology and demonstrated QSDC in practical channel noise and loss.