New record for quantum key distribution
Taking the Inter-City Quantum Communication Network One Step Closer
◎Reporter Wu Changfeng
The primary factor affecting the transmission distance of quantum key distribution is the signal-to-noise ratio.
In principle, the transmission distance can be increased as long as the noise is sufficiently suppressed.
However, it cannot be said that the noise can be transmitted infinitely far. This is because the line has attenuation in addition to noise. Attenuation will reduce the key generation rate. If the key rate is too low, it cannot meet the needs of any practical application. Even if there is no noise, also lost its application value.
Han Zhengfu
Professor of University of Science and Technology of China
Recently, the well-known academic journal "Nature Photonics" published a research result online from the University of Science and Technology of China (hereinafter referred to as the University of Science and Technology of China).
Professor Han Zhengfu and his collaborators in the team of Academician Guo Guangcan of the University of Science and Technology of China have recently achieved 833 kilometers of optical fiber quantum key distribution, which has increased the world record for the safe transmission distance of quantum key distribution by more than 200 kilometers, and is a step toward realizing 1,000 kilometers of land-based quantum secure communication. an important step.
Quantum key distribution is based on the basic principles of quantum physics, and provides an eavesdropping-perceived key distribution method at the level of information security.
Light quantum is the natural carrier of quantum information, but the inevitable loss in the circuit limits the safe distance of quantum key distribution, which is also one of the key factors restricting the deployment and application of wide-area quantum secure communication networks.
Therefore, how to extend the safe distance of direct optical quantum key distribution is one of the most challenging difficulties and focuses at present.
So, how did the quantum key distribution on the 833-kilometer-long optical fiber achieve this world record?
Signal-to-noise ratio and attenuation determine transmission distance
"The safe distance of optical quantum key distribution and direct transmission is gradually approaching the level of 1,000 kilometers, laying a good foundation for the future wide-area quantum communication network." Han Zhengfu explained to reporters that the wide-area quantum communication network to be built in the future can be distributed in multiple cities with intercity backbone lines and metro subnets.
"A quantum communication network with such a layered structure can serve many users."
Han Zhengfu told reporters that the primary factor affecting the transmission distance of quantum key distribution is the signal-to-noise ratio.
Noise includes channel perturbations, detector dark counts, etc.
In principle, the transmission distance can be increased as long as the noise is sufficiently suppressed.
"But it doesn't mean that the noise can be transmitted infinitely far." Han Zhengfu further explained that this is because the line has attenuation in addition to noise. Attenuation will reduce the key generation rate. If the key rate is too low, it cannot satisfy any The practical application requires, even if there is no noise, it loses the application value.
Therefore, the main factors that determine the transmission distance are the signal-to-noise ratio and attenuation.
"Our team has been working on realizing long-distance quantum key distribution, setting a record of 206 kilometers in 2012. In 2018, we made some key breakthroughs in the theory and experiment of the two-field protocol, realizing a 300-kilometer fiber The double-field experiment of the channel is the first experiment in the world to break through the theoretical limit in the fiber channel." Han Zhengfu said that in these works, the research group developed a method for theoretical analysis of the protocol, which broke through the phase locking of the light source and the phase compensation of the channel. and several other key technologies, which laid the foundation for its realization of 833-kilometer optical fiber quantum key distribution.
"Quantum communication has always been a fiercely competitive international track." Han Zhengfu told reporters that the current internationally competitive team is the Toshiba Cambridge Research Institute in the United Kingdom. They are the originators of the original theory of the two-field protocol, which has also been realized experimentally. A 600-kilometer dual-field system.
In addition, the University of Leeds in the United Kingdom and the University of Tokyo in Japan also have attainments in theory.
Break through the two-field agreement theory
In 2018, the dual-field quantum key distribution protocol proposed by British scientists broke through the original theoretical limit, and its perfection of the theory and the development of experimental technology were extremely challenging.
"The previous protocol is generally a single-photon protocol. Although the coherent state is also required, the coherent state is locally prepared, and the phase compensation is relatively easy through unequal arm interference." Han Zhengfu said that the two-field protocol is different from the previous one. protocol, which is mainly based on coherent states that can be fabricated remotely and ex-situ that can interfere, which places very strict requirements on the light source.
Not only the light source, but the two-field protocol also requires a pair of coherent states to achieve near-perfect interference after transmission through different, long-range channels, which means that channel phase compensation is also challenging.
"In general, the two-field protocol puts forward high requirements for experimental technology. In theory, the security of this protocol is not described based on a single photon, so it is more complicated, so how to prove the security in theory and how to analyze each This imperfect characteristic is also a problem that must be solved." Han Zhengfu said.
After some serious research, Han Zhengfu and his team believe that the main purpose of this protocol is to break through the "key rate-channel loss" limit.
There are also theoretical physicists abroad who have proved that, in principle, any end-to-end protocol cannot break through a limit called the linear bound, so the transmission distance is very limited.
"The dual-field protocol moves the measurement device to the middle of the channel, and is based on single-photon interference, thereby bypassing the constraints of the linear bound and opening up new research directions, which is a highly innovative work," said Han Zhengfu.
"However, when the dual-field protocol was first proposed, its security proof was not perfect, and the experiment required global phase randomization, and after transmission, the user could filter out the phase matching to generate the key. These steps reduce the key Therefore, both theoretically and experimentally, the two-field protocol opens up new directions, but it also means that many problems need to be solved.” Han Zhengfu said that innovation must sometimes start from breaking through inherent theories.
In 2019, the research groups of Guo Guangcan and Han Zhengfu first proposed a two-field protocol with phase-free selection and verified the feasibility of such a protocol in a 300-kilometer fiber channel for the first time.
Create a new theory of four-phase modulation two-field protocol
"In 2018, our team theoretically proved for the first time that the encoding mode of the two-field protocol can be phase-randomized and phase-post-selection is not required. In this way, the key rate can be significantly improved and the experiment can be greatly simplified." Han Zheng Fu told reporters that their theoretical innovation surprised many colleagues at the time.
Because the academic community previously believed that global randomization is a necessary guarantee for security.
The simplicity of the phase-free post-selection protocol and the advantages of high key rate make this protocol one of the hot topics in the field of quantum communication.
"Of course, we also noticed that our phase-free post-selection protocol completely discarded the phase randomization during encoding, and the limit safe distance was somewhat reduced. Later, we thought that maybe a few phases can be appropriately added during encoding, which can further confuse eavesdropping. It can extend the safety distance." Han Zhengfu said that the two-field protocol with four-phase modulation they designed combines some features of the phase-free post-selection protocol and the post-selection protocol, and has special advantages in extreme cases.
After more than two years of exploration, the team of Guo Guangcan and Han Zhengfu proposed an improved four-phase modulation two-field protocol, and further improved the phase-locked frequency stabilization technology of independent light sources, high-bandwidth channel phase compensation technology, and single-photon with high signal-to-noise ratio. Key technologies such as detection signal discrimination technology have extended the secure transmission distance of optical fiber dual-field quantum key distribution to 833 kilometers.
Bold and rigorous theoretical innovation has once again gained an advantage for China's quantum communication in the fiercely competitive international track.
This achievement of Han Zhengfu's team not only greatly increased the optical fiber quantum key distribution distance from more than 500 kilometers to 833 kilometers, but also increased the security code rate by 50-1000 times, which is in the realization of thousands of kilometers of ground-based wide-area quantum secure communication networks. An important step on the road.
"There is no end to innovation, and it is difficult to predict the limit transmission distance of quantum key distribution in the future. However, based on the current development trend, it should be a good idea to reach about 1,000 kilometers." Han Zhengfu told reporters that based on the two-field protocol, Thousands of kilometers of quantum key distribution (QKD) backbone lines are possible.
For the vast China, it is of great value to realize a quantum communication network between large-scale national central cities.
Han Zhengfu also admitted that fundamentally, the two-field protocol cannot change the fact that the key rate exponentially decays with distance, and future intercontinental quantum communication still needs to rely on quantum relays.
"However, it may take a long time for quantum relay to be practical, and the two-field protocol can solve most of the current problems." Han Zhengfu said.