Slimmed-down terminal transmits quantum keys from space


Chinese researchers have shown a working quantum key distribution (QKD) terminal with half the mass of an earlier system, marking a significant advancement in the field of space-to-ground QKD. Scientists from Hefei National Laboratory and the University of Science and Technology of China (USTC) carried out a series of 19 experiments between October 23, 2018, and February 13, 2019, successfully transmitting quantum keys between the satellite and four stations on the ground on 15 different days. This was done after the new terminal was launched into orbit around the Earth aboard the Tiangong-2 space laboratory.

The gadget used in this study, like other QKD terminals, generates the kinds of encryption keys required to safeguard data by exploiting the quantum behavior of light. Jian-Wei Pan, a physicist at USTC and a co-author of an article on the subject in Optica, says that QKD uses single photons, the fundamental unit of light, to encode information between two remote users. As an illustration, the transmitter can encrypt information about the polarization states of photons, such as horizontal, vertical, linear +45°, or linear -45°, at random. Similar polarization state decoding can be done at the receiver to acquire the raw keys. The final safe keys can be recovered after mistake rectification and privacy amplification.

Scalable security

For users with strict security needs, the new, slimmer QKD terminal is welcome news. Traditional public-key cryptography relies on the fact that traditional computers can't always solve certain issues in an acceptable period of time, despite the fact that it is now one of the greatest methods of encryption. These difficult mathematical operations, though, only function when a conventional computer is being used by the hacker. As Pan notes, even the finest existing cryptography techniques could be broken by a quantum computer in the future using just Shor's algorithm.

When quantum encryption is appropriate, one potential defense against classical encryption being cracked by quantum computers is to utilize it instead. Pan claims that QKD offers an information-secure solution to the key exchange problem. According to the quantum no-cloning theorem, it is impossible to reliably copy an unknown quantum state. The quantum signals will unavoidably be disturbed if the eavesdropper tries to eavesdrop in QKD, which will then be noticed by QKD users.

Any attacks against QKD must be launched at the time of transmission, says Paul Kwiat, a physicist at the University of Illinois from Urbana-Champaign in the US who was not involved in the study. According to Kwiat, who oversees the quantum communications division at Q-NEXT, a research consortium focusing on quantum information challenges, "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.

Daylight saving time

However, in the most recent study, the researchers were able to minimize the terminal's mass by combining the QKD payload with other components like control electronics, optics, and telescopes. Previous QKD work had been carried out on the Micius satellite with a different device. Although this is a significant advance, the Hefei-USTC team is not yet done. The fact that they are now unable to operate the terminal throughout the day is one issue they discuss in their report. This is due to the fact that background noise produced by sunlight scattering is five to six orders of magnitude greater than that observed in trials carried out at night. To enable daytime QKD functioning, Pan and his colleagues are developing technologies including wavelength optimization, spectral filtering, and spatial filtering.

Reference: Research in Optica

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