Long-distance and secure quantum key (QKD) distribution over a free-space channel

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A conceptual figure of the MDI-QKD experience in a city. Telescopes are located in high-rise buildings to transmit encoded photons. The turbulence of the atmosphere, which exists throughout the transmission channel, is the main challenge for photons to maintain spatial mode in the detection terminal. Credit: Yao Zheng / Micius Salon.

Quantum Key Distribution (QKD) is a technique that enables secure communications between devices using a cryptographic protocol based in part on quantum mechanics. This method of communication ultimately allows two parties to encrypt and decrypt the messages they send to each other using a unique key unknown to the other parties.

Device Independent Quantum Key Distribution (MDI-QKD) is a unique protocol that facilitates the creation of more secure QKD networks with untrusted devices. This protocol can enable QKD-based communication over longer distances, as well as higher key generation rates and more reliable network verification.

So far, MDI-QKD has only been successfully implemented using fiber optics. On the other hand, implementing the protocol over free-space channels has proven to be considerably difficult.

A research group led by Jian-Wei Pan of the China University of Science and Technology recently demonstrated long-distance and secure MDI-QKD over a free-space channel for the very first time. Their article, published in Physical examination letters, could pave the way for MDI-QKD satellite implementations.

“The ultimate goal of QKD is to achieve a secure quantum communication network on a global scale,” Qiang Zhang, one of the researchers who conducted the study, told Phys.org. “In order to achieve this ambitious goal, two main challenges must be met. One is to reduce the gap between QKD theory and practice, and the other is to extend the distance of QKD. of our recent work was to resolve these two difficulties. “

Theoretically, QKD offers greater security in communications by relying on the laws of physics. However, imperfections and vulnerabilities in real devices could cause deviations from the models used to perform security scans. The MDI-QKD protocol can help address this challenge by closing any loopholes in detection at once. Additionally, it can improve the performance and security of QKD implementations on real devices, including decoy states.

Satellite QKD implementations could extend the distance over which this secure communication can take place, as they would reduce transmission losses and negligible decoherence in space. By extending MDI-QKD from fiber optics to free space channels, the work of Pan and his colleagues could be a first step towards implementing MDI-QKD protocols at scale using satellites.

Long-distance and secure quantum key (QKD) distribution over a free-space channel

Possible configurations of MDI-QKD by satellite. (a) the satellite acts as a detection terminal, while two ground stations send photons via the uplink to the satellite. (b) A ground station acts as a detection terminal. The users of the terrestrial fiber-based network share secret keys with the satellite through the ground station. (c) MDI-QKD between three satellites. Credit: Cao et al.

“Although several fiber-based MDI-QKD experiments were performed prior to our study, none of them demonstrated the feasibility of the protocol with a free space channel,” said Zhang. “The main reason is that the amplitude and phase fluctuation induced by atmospheric turbulence makes it difficult to maintain indistinguishability in terms of spatial, temporal and spectral modes between independent photons.”

Since atmospheric turbulence typically destroys the spatial mode between independent photons, MDI-QKD implementations typically require the use of a single-mode fiber to perform spatial filtering before applying interferometric techniques. However, the use of single-mode fibers to couple photons generally leads to low coupling efficiency and intensity fluctuation. To solve this problem, the researchers developed a new adaptive optics system that improves the overall efficiency of the channel.

“As the rapid fluctuation of light intensity makes it difficult to share the time-frequency reference, we have developed new technologies to achieve high-precision time synchronization and frequency locking between independent photon sources located far apart. from others in order to maintain the indistinguishability of synchronization and spectral modes. Zhang said. “Thanks to these technical advances, we have accomplished a task that seemed impossible to accomplish before.”

The study is an important step on the way to implementing QKD on a large scale and using it to secure communications over longer distances. In addition, researchers were the first to perform photonic interference in long-distance atmospheric channels. This could open up interesting possibilities for the development of complex types of quantum information processing involving quantum interference, such as quantum entanglement exchange and quantum teleportation. It could also offer new ways to test the interface of quantum mechanics and gravity.

The researchers’ long-term goal is to demonstrate MDI-QKD by satellite and ultimately build a global quantum network. To achieve this, however, they will first need to overcome a number of additional challenges.

“One of these challenges is the high loss mainly induced by atmospheric fluctuation,” Zhang explained. “In the simplest configuration of the MDI-QKD via satellite, one satellite acts as the sensing terminal (ie two ground stations send photons through the ‘uplink’ to the satellite). The channel loss measured by the Micius satellite is approximately 41 ~ 52 dB from a ground station at an altitude of 5,100 miles. The loss is probably much higher from ground stations at a lower altitude. Another source of loss is single-mode fiber coupling efficiency, which is also very important with existing MDI-QKD systems. “

In order to enable efficient MDI-QKD satellite implementations, researchers will therefore first need to advance existing methods for making photons transit through channels in free space. To do this, they have so far developed an adaptive optics system and an algorithm that increases the efficiency of free space channels. In their next studies, they plan to create other algorithms and techniques to improve the overall transmission channel.

“The second challenge that we hope to overcome is associated with the movement of satellites,” Zhang added. “Since the signal pulses must overlap in the time domain at the detection terminal, a very precise prediction of the orbit of a satellite is required, and the transmission time of each coded pulse must also be timed precisely, so that they can finally overlap well in the sensing terminal. Doppler frequency shift, on the other hand, is a significant source of frequency mismatch which is troublesome for HOM interference. pulse code also needs to be accurately shifted for compensation. After solving all these technical challenges, we believe we will be able to achieve MDI-QKD by satellite. ”


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More information:
Distribution of quantum keys independent of the long-distance free space measurement device. Physical examination letters(2021). DOI: 10.1103 / PhysRevLett.125.260503.

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Quote: Long-distance and secure quantum key distribution (QKD) over a free space channel (2021, January 25) retrieved September 25, 2021 from https://phys.org/news/2021-01-long-distance-quantum- key -qkd-free-space.html

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