Researchers at the University of Science and Technology of China and other Chinese labs, with the collaboration of a lab in the US, have implemented a secure quantum protocol known as Measurement-Device-Independent Quantum Key Distribution (MDIQKD), suitable for practical networks and devices, over a distance of 404 km. The breakthrough, which doubles the previous MDIQKD record, opens the door to secure wide area quantum communication networks.

The research was published earlier this week (November 2) in Physical Review Letterswith the title “Measurement-Device-Independent Quantum Key Distribution Over a 404 km Optical Fiber.” A companion “Synopsis: Quantum Cryptography Goes a Long Way” was published in APS Physics.

The researchers note that, while the potential of Quantum Key Distribution (QKD) to provide unconditional secure communication between two distant parties is undisputed, its feasibility has been questioned due to certain limitations in the practical application of real-life QKD systems. The new method is seen as an important step toward practical, operational QKD networks.

Quantum Key Distribution permits securely sharing keys for one-time pad (OTP) cryptography. OTP encryption is mathematically guaranteed to be unbreakable, but only if the keys are not compromised. Therefore, secure key transmission and storage is the main challenge for ultra-secure OTP cryptography. But quantum entanglement – long-range instant correlations between photons – can be used to establish a shared key in such a way as to permit detecting any attempt to eavesdrop on the key. Therefore, quantum encryption offers complete, invulnerable security based on the laws of fundamental physics.

Quantum computing, a complementary quantum technology, first proposed by Nobel laureate Richard P. Feynman in 1982 (see also Feynman’s Lectures on Computation), could in the future permit cracking all traditional encryption schemes with sophisticated algorithms and superior computing power. But quantum encryption is invulnerable to quantum computing attacks.

Toward fast, Secure, Unbreakable Wide Area Quantum Encryption Networks

MDIQKD – a quantum cryptography protocol first proposed in 2012 – tolerates relatively inefficient photon detectors and permits improving the security of quantum communication by using “decoy” photons to detect eavesdropping attacks. The researchers sent infrared photons through optical fibers with lengths between 102 and 404 km, and demonstrated security up to the longest distance.

“Measurement-device-independent quantum key distribution (MDIQKD) with the decoy-state method negates security threats of both the imperfect single-photon source and detection losses,” reads the paper’s abstract. “Lengthening the distance and improving the key rate of quantum key distribution (QKD) are vital issues in practical applications of QKD. Herein, we report the results of MDIQKD over 404 km of ultralow-loss optical fiber and 311 km of a standard optical fiber while employing an optimized four-intensity decoy-state method.”

MDIQKD had proven very slow so far, compared to the requirements for practical QKD. “The best demonstration so far sent information over a distance of 200 kilometers at a data rate of just 0.018 bits per second,” noted a MIT Technology Review commentary to a draft version of the paper. “At this rate, perfectly secure quantum cryptography would never be practical.” But now the Chinese scientists have achieved a very significant increase in speed compared to previous results.

In addition to the long transmission distances, our system generates a 1.38 kbits per second secure finite key at 102 km, therefore constituting a strong candidate for a metropolitan quantum network with an unreliable relay.

The scientists added that system performance could be further improved by increasing the system clock rate and the efficiency of the photon detectors.

Physicists at Corning Inc., a research company headquartered in Corning, New York, and specialized in materials science and optical physics, have participated in the research. Among them, fiber optics and quantum communication specialist Daniel Nolan.

But it appears that China is pursuing a government-supported, well-funded quantum technology development effort for both civilian and military applications, ranging from unbreakable encrypted communication networks to combat support operations, more aggressively than the US. Early results of China’s quantum technology program are the world’s first quantum satellite and a “quantum radar” able to detect stealth planes 100km away, both recently covered by Hacked.

Images from and Wikimedia Commons.