Researchers have successfully demonstrated a secure transmission using measurement-device-independent quantum key distribution (MDI-QKD) protocol, sending information over 170 kilometers.

Qin Wang, Ph.D., from the College of Telecommunication & Information Engineering at the Nanjing University of Posts and Telecommunications, is set to talk about their proof of principle at the inaugural Quantum 2.0 conference, sponsored by The Optical Society (OSA). The all-virtual event will co-locate the MDI-QKD demonstration with OSA Frontiers in Optics and Laser Science APS/FLS (FiO + LS) conference, slated on September 14 to 17.

Difference Between Ideal and Practical QKD Systems
(Photo: QCrypt Conference YouTube Channel)
Xing-Yu Zhou (Nanjing University of Posts & Telecommunications); Hua-Jian Ding (Nanjing University of Posts & Telecommunications); Chun-Hui Zhang (Nanjing University of Posts & Telecommunications); Qin Wang (Nanjing University of Posts & Telecommunications) present MDI-QKD with Uncharacterized Sources, which helped support the development of a novel MDI-QKD protocol.

Maintaining Security Throughout a 170-Kilometer Transmission

QKD is one of the methods to attain a virtually unhackable encryption key, using concepts from quantum mechanics. It employs the quantum properties of light to create secure and randomized keys, allowing its users to either encrypt or decrypt information.

Furthermore, a measurement-device-independent QKD protocol takes this a step further by keeping the QKD transmitted data to be immune to attempts to access the data through the detection devices - these setups are responsible for observing and measuring individual photons. It solves one of the persisting problems in quantum cryptography, attacks on the detector side channels. A previous study has demonstrated that MDI-QKD eliminates all detector side channels and offers a significantly higher key generation rate compared to a full device-independent QKD.

RELATED: Simple Mod Can Keep Quantum States 10,000 Times Longer

Wang explains their methodology in an OSA press release dated August 19. "We investigate the three-state MDI-QKD protocol with uncharacterized sources and conduct an experimental demonstration, where it allows imperfect state-preparation and the only assumption is the prepared states are in a two-dimensional Hilbert space," Wang said. She adds that their work marks a significant improvement in both the security and practicability of employing QKD using existing technology.

To work around the existing problem, researchers developed a novel approach that uses three characterized quantum states of photons for encoding data. Existing MDI-QKD protocols are supposedly capable of resisting detection loopholes. However, using this protocol maintains a perfect state preparation, making practical applications difficult.

Building on Previous Studies and Existing Tech

One previous study proposes an MDI-QKD protocol supposedly secure against detection attacks. The work cited in the OSA article, led by Yin Zhen-Qiang, employs uncharacterized sources, provided qubit sources are used. The proof for their proposed protocol suggests that the transmission can be successfully made even without knowledge of the encoding states. This approach to the MDI-QKD relies on often discarded mismatched-basis data from phase-error rare calculations.

RELATED: Yale Creates Error-Correcting Cat for Quantum Computers

Through an experimental setup for both encoding and detection, Wang's team demonstrated that their approach to the MKI-QKD could transmit keys with higher key generation rates (10^-7 /pulse key rate), over longer distances. It far surpasses other currently existing MDI-QKD protocols, with their computations promising secure transmissions for up to 200 kilometers.

The OSA press release notes that the work by Wang's team can be further developed by using the decoy-state method or the twin-field QKD protocols. Decoy state QKD is the most widely used quantum key distribution scheme, able to work around the fundamental weakness of basic QKD systems. It uses varying intensity levels from the transmitter side, allowing qubits to be transmitted with randomly-chosen intensity levels, having one signal state and a couple of decoy states, hence the name.

Meanwhile, twin-field QKD is an experimental protocol that, theoretically, can allow 550km QKD without quantum repeaters. Twin-field protocols use two light sources, creating pulses for each user. These pulses are phase-randomized and phase-encoded. Its inclusion of a secret bit makes it difficult to penetrate, leaving an attacker unable to determine the absolute values of both secret bits.


Check out more news and information on Quantum Theory in Science Times.