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arXiv:1903.01171 · Applied Optics

Experimental investigation of quantum key distribution over a water channel

Shicheng Zhao, Wendong Li, Yuan Shen, YongHe Yu, XinHong Han, Hao Zeng, Maoqi Cai, Tian Qian, Shuo Wang, Zhaoming Wang, Ya Xiao, Yongjian Gu

Published May 8, 2019• 65 citations
arXivHTMLPDF

Abstract

Quantum key distribution (QKD) has undergone significant development in recent decades, particularly with respect to free-space (air) and optical fiber channels. Here, we report a proof-of-principle experiment for the BB84 protocol QKD over a water channel. First, we demonstrate again the polarization preservation properties of the water channel in optical transmission according to the measured Mueller matrix, which is close to the unit matrix. The reason for the polarization preservation, revealed by Monte Carlo simulation, is that almost all the received photons are unscattered. Then, we performed the first polarization encoding BB84 protocol QKD over a 2.37 m water channel. The results show that QKD can be performed with a low quantum bit error rate, less than 3.5%, with different attenuation coefficients.

DOI
10.1364/ao.58.003902

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Quantum key distribution (QKD) has undergone significant development in recent decades, particularly with respect to free-space (air) and optical fiber channels. Here, we report a proof-of-principle experiment for the BB84 protocol QKD over a water channel. First, we demonstrate again the polarization preservation properties of the water channel in optical transmission according to the measured Mueller matrix, which is close to the unit matrix. The reason for the polarization preservation, revealed by Monte Carlo simulation, is that almost all the received photons are unscattered. Then, we performed the first polarization encoding BB84 protocol QKD over a 2.37 m water channel. The results show that QKD can be performed with a low quantum bit error rate, less than 3.5%, with different attenuation coefficients.
In recent years, the feasibility of quantum key distribution (QKD) in a water channel has been verified by theory and experiment. Here, we present an experimental investigation of QKD and decoy-state QKD based on the BB84 protocol. The experiment was carried out in a 10 m water tank. The attenuation coefficient of tap water is 0.08/m, which is close to Jerlov Type II seawater. We measured the probability-of-detection matrix of polarization states, and the average fidelity of the four polarization states is up to 98.39%. For the 10 m underwater QKD experiment, 20 MHz optical pulses are generated by modulating the laser diode (LD) and attenuated to an average of 0.1 photons per pulse. The security key rate can reach 563.41 kbits/s and the quantum bit error rate (QBER) is 0.36%. Two decoy states (one of which is the vacuum state) was used in the 10 m underwater decoy-state QKD experiment, and the average QBER of signal state is 0.95%, the security key rate reaches 711.29 kbits/s. According to the parameters of the decoy-state experiment, the maximum secure transmission distance of the underwater decoy-state QKD is predicted to be 19.2 m, while it can be increased to 237.1 m in Jerlov Type I seawater with a lower dark count single photon detector (SPD).
Polarization encoding in quantum key distribution systems is widely used for distributing secret keys between a sender and a receiver. As the polarization-encoded photons pass through the quantum channel, their polarization states might be changed. A prior verification of the channel characteristic is required in order to investigate the effect of the channel on the polarization of the photons passing through it. In this work, an experimental investigation of polarization preservation of a pure water channel is presented. The investigation relies on the verification of the Mueller matrix in order to check the polarization preservation of the channel. The measurements were carried out using a single photon detection module. The results showed that the polarization of the optical pulses is preserved when passing through the pure water channel and it can be used as a quantum channel in the quantum key distribution systems.
Polarization encoding quantum key distribution has been proven to be a reliable method to build a secure communication system. It has already been used in an inter-city fiber channel and near-Earth atmosphere channel, leaving an underwater channel the last barrier to conquer. Here we demonstrate a decoy-state BB84 quantum key distribution system over a water channel with a compact system design for future experiments in the ocean. In the system, a multiple-intensity modulated laser module is designed to produce the light pulses of quantum states, including signal state, decoy state, and vacuum state. Classical communication and synchronization are realized by wireless optical transmission. Multiple filtering techniques and wavelength division multiplexing are further used to avoid cross talk of different lights. We test the performance of the system and obtain a final key rate of 245.6 bps with an average quantum bit error rate of 1.91% over a 2.4 m water channel, in which the channel attenuation is 16.35 dB. Numerical simulation shows that the system can tolerate up to 21.7 dB total channel loss and can still generate secure keys in 277.9 m Jerlov type I ocean channel.
In this work, we implement a proof-of-concept underwater free-space Quantum Key Distribution (QKD) system and analyze its performance in a controlled laboratory test environment. We implement a BB84 protocol with time-bin encoding operating at 520 nm. The quantum channel was composed of a five-meter-long tank equipped with the possibility of actively controlling the water turbulence. Finally, we measure the Quantum Bit Error Rate (QBER) in the various scenarios and we report the results together with those relating to the parameters of the considered channel.
Quantum key distribution is considered as one of the safe ways of distributing keys among the frames employed in communication. In 1984, BB84 appeared first as a protocol for QKD. In the present study paper, we proposed a new way of encoding a stream of bits into polarized photons using Legendre Symbol (a/p), where both of the sender and the receiver negotiate of using the function of Legendre symbol. Hence, in this case the proposed protocol uses only quantum channel instead of the classical channel, this leads to less time spent and less quantum error rate occurring comparing with the previous BB84 protocol. The experimental outcome confirms that the proposed protocol delivers efficient results comparing with the original BB84 protocol.
This paper mainly solves the photon polarization encoding problem of quantum key distribution (QKD) between different media. The influence of the transmission rate of different photon component on quantum bit error rate (QBER) has been quantitatively analyzed, with a practical analysis of QBER of QKD between air and water. Furthermore, we have put forward a single-photon compensation scheme for eliminating such non-ideal BB84 protocol, as well as a more robust and practical dual-photon encoding scheme to offset such interfacial non-unitary noise. This takes an important step towards the air-sea-ground wide area quantum communication in the future.