Researchers validate measurement-protection quantum key distribution

Korean researchers have successfully established a measurement protection (MP) theory that enables stable quantum key distribution (QKD) without the need for measurement correction of quantum states, and verified it experimentally.

The Electronics and Telecommunications Research Institute (ETRI), through joint research with KAIST, has developed for the first time in the world a new technology capable of implementing stable quantum communication even in moving environments such as satellites, ships and drones.

Quantum communication is a high-level technology that transmits information in the quantum state of light. But in a mobile and open space environment, communication is greatly affected by weather conditions and changes in the environment, making it unstable. In particular, stable transmission of quantum states proves extremely difficult in dynamic environments where channel states change in real time, such as the sky, sea or air.

This research, published in the IEEE Journal on Selected Areas of Communicationsovercomes these limitations, demonstrating for the first time the technical feasibility of stable quantum information exchange, even in motion.

This achievement is expected to lead to applications in various fields, such as secure satellite-to-ground communications, drone communications, and maritime communications.

QKD is a technology that distributes encryption keys based on the principles of quantum mechanics, making eavesdropping fundamentally impossible. Conventional QKD protocols required repeated calibration of receiver meters whenever channel conditions changed.

However, this research proved that stable key distribution can be achieved regardless of channel conditions with only simple local operations. The theory was developed by Professor Bae Jun-woo’s team at KAIST and the experiments were carried out by researchers at ETRI.

To generate single-photon pulses, the researchers used a 100 MHz light source, a vertical cavity surface emitting laser (VCSEL), a type of semiconductor laser that emits vertically from the top surface of the chip.

They implemented a long-distance free-space transmission environment with a loss of up to 30 dB over a 10-meter path, introduced various polarization noises to simulate harsh free-space conditions, and verified the success of transmission and quantum measurements. Additionally, three waveplates were installed on the transmitter and receiver to implement local operations.

As a result, it has been demonstrated that the measurement protection (MP)-based QKD system can increase the system’s maximum allowed quantum bit error rate (QBER), which represents the percentage of transmitted qubits containing errors, by up to 20.7% compared to existing systems.

This means that a stable distribution of quantum keys is possible without additional measurement compensation as long as the error rate among the received quantum bits is less than 20.7%. This paves the way for reliable quantum communication by achieving stable key generation in various noisy channel environments without calibration. The researchers expect these results to be applicable in environments similar to satellite-to-ground links.

The results were published in June in the Journal on selected areas of communications. Researcher Ko Haesin from ETRI and Dr. Spiros Kechrimparis from KAIST participated as co-first authors.

At the same time, ETRI also presented an experimental compensation method for the problem of “polarization dependent loss”, a key challenge for the practical application of QKD. This research is expected to contribute to the commercialization of compact and lightweight QKD equipment by addressing the performance degradation of chip-based integrated QKD systems.

Integrated QKD is considered a next-generation technology to replace bulky and expensive optical systems, but polarization-related loss during integration can be a major obstacle.

ETRI researchers have demonstrated experimentally that this loss can be compensated with simple optical components and applied to polarization-based QKD systems to achieve stable key distribution without performance degradation. This achievement was also published as a cover story in “Advanced quantum technologies“.

“Research on integrated photonic chips is indispensable for the expansion of the QKD market, and further research is needed because many technical challenges need to be overcome when implementing QKD systems based on integrated photonic chips, including polarization-dependent losses,” said Lim Kyong Chun, principal researcher at ETRI’s Quantum Communication Research Division.

Youn Chun Ju, Assistant Vice President of ETRI’s Quantum Technologies Research Division, said: “Implementing QKD regardless of channel state variations significantly improves the flexibility of quantum cryptography. We will extend this to free-space long-distance backhaul technology to establish the foundations of a global quantum network.

Professor Bae Jun Woo from the Department of Electrical Engineering at KAIST also said: “This achievement will be a turning point in establishing secure and reliable quantum communication even in complex environments. »