Researchers in China have achieved a major milestone in space-to-ground quantum key distribution (QKD) by demonstrating a functional QKD terminal with half the mass of a previous system. After sending the new terminal into space to orbit the Earth aboard the Tiangong-2 space laboratory, scientists at Hefei National Laboratory and the University of Science and Technology of China (USTC) conducted a series of 19 experiments between 23 October 2018 and 13 February 2019, successfully transmitting quantum keys between the satellite and four stations on the ground on 15 separate days.
Like other QKD terminals, the device in this study relies on the quantum behaviour of light to create the kinds of encryption keys needed to protect data. “QKD employs the fundamental unit of light – single photons – to encode information between two distant users,” explains Jian-Wei Pan, a physicist at USTC and a co-author of a paper on the research in Optica. “For example, the transmitter can randomly encode information on the polarization states of photons, such as horizontal, vertical, linear +45°, or linear –45°. At the receiver, similar polarization state decoding can be performed, and the raw keys can be obtained. After error correction and privacy amplification, the final secure keys can be extracted.”
The new slimmed-down QKD terminal is good news for users with high security requirements. Although traditional public-key cryptography is currently one of the best means of encryption, it relies on the fact that classical computers simply cannot solve certain problems in a reasonable amount of time. However, these intractable mathematical functions only work if the hacker is using a classical computer. As Pan points out, a quantum computer in the future could simply use Shor’s algorithm to crack even the best current cryptography methods.
If quantum computers can break classical encryption, one possible solution would be to use quantum encryption instead, when applicable. “QKD provides an information-secure solution to the key exchange problem,” says Pan. “The quantum no-cloning theorem dictates that an unknown quantum state cannot be cloned reliably. If the eavesdropper tries to eavesdrop in QKD, she unavoidably introduces disturbance to the quantum signals, which will then be detected by QKD users.”
Paul Kwiat, a physicist at the University of Illinois at Urbana-Champaign, US, who was not involved in the research, adds that any attacks on QKD must be made at the time of transmission. “In this sense, QKD is sometimes described as ‘future proof’ – it doesn’t matter what computation power some adversary develops 10 years from now (which would matter for public key cryptography); all that matters is the capabilities an eavesdropper has when the quantum key is initially distributed,” says Kwiat, who leads the quantum communications division at Q-NEXT, a research consortium focused on quantum information challenges.
While previous QKD work has been conducted with a different device on the Micius satellite, in the latest study the researchers were able to reduce the terminal’s mass by integrating the QKD payload with other systems such as control electronics, optics, and telescopes. This is a major step forward, but members of the Hefei–USTC team aren’t finished. One challenge they mention in their paper is that they cannot currently run the terminal during the day. This is because scattering of sunlight creates background noise that is five to six orders of magnitude more than what is seen in experiments conducted at night. That said, Pan and his colleagues are working on technologies like wavelength optimization, spectral filtering, and spatial filtering to enable daylight QKD operation.
Device-independent QKD brings unhackable quantum Internet closer
Pan states that the team has big plans, hopefully culminating in the creation of a global satellite-ground-integrated quantum network that can provide services to users worldwide. Following the success of this work, the team will begin constructing a quantum satellite constellation composed of several low-orbit satellites, a medium-to-high orbit satellite, and the ground-fibre QKD networks. “We think our work will contribute to an attractive area of research on how to construct the optimal satellite constellation,” says Pan.