ETRI-KAIST simultaneously validates “measurement-protection quantum key distribution”

Korean researchers have successfully established the world’s first “Measurement Protection (MP)” theory that enables stable Quantum Key Distribution (QKD) without need for measurement correction of quantum states, and experimentally verified it.

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

Quantum communication is a high level technology that transmits information in the quantum state of light. but in a free-space, mobile environment, communication is greatly affected by weather and changes in the surrounding environment, making it unstable. In particular, stable transmission of quantum states has been extremely challenging in dynamic environments that the status of channels change in real time, such as the sky, sea, or air.

This research overcomes those limitations, demonstrating for the first time the technical feasibility of stable quantum information exchange even in motion. It is expected that this achievement will lead to applications in various fields, such as secure satellite-to-ground communication, drone communications, and maritime communications.

Quantum Key Distribution (QKD) is a technology that distributes encryption keys based on the principles of quantum mechanics, making eavesdropping fundamentally impossible. Conventional QKD protocols required the repeated calibration of the receiver’s measurement devices whenever the 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 conducted by researchers at ETRI.

For generating 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 up to 30dB loss over a 10-meter path, introduced various polarization noises to simulate challenging free-space conditions, and verified successful quantum transmission and measurement. Additionally, three waveplates were installed on both the transmitter and receiver to implement local operations.

As a result, it was demonstrated that the Measurement Protection (MP)-based QKD system can increase the system’s maximum allowable Quantum Bit Error Rate (QBER), which represents the percentage of transmitted quantum bits that contain errors, by up to 20.7% compared to existing systems.

This means that stable quantum key distribution is possible without additional measurement compensation as long as the error rate among 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 findings were published on June 25th in the “Journal on Selected Areas in Communications,” an internationally prestigious academic journal in the field of communications published by the Institute of Electrical and Electronics Engineers (IEEE). ETRI researcher Ko Haesin and KAIST’s Dr. Spiros Kechrimparis participated as co-first authors.  

Meanwhile, ETRI also announced that it had presented an experimental compensation method for the “Polarization-Dependent Loss” problem, 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 in integrated chip-based QKD systems.

Integrated QKD is considered a next-generation technology to replace bulky and expensive bulk-optics systems, but polarization-dependent loss during integration can cause a major obstacle.

ETRI researchers have experimentally demonstrated that this loss can be compensated for 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 paper in “Advanced Quantum Technologies,” a leading international academic journal on quantum technology, in March 2025.

“Research on integrated photonic chips is indispensable for the expansion of the QKD market, and more research is needed because there are many technical challenges to be overcome in the implementation of QKD systems based on integrated photonic chips, including polarization-dependent losses” said Lim Kyong Chun, senior researcher of ETRI’s Quantum Communication Research Division.

Youn Chun Ju, Assistant Vice President of ETRI’s Quantum Technology Research Division, said, “The implementation of QKD independent of channel status variations significantly enhances the flexibility of quantum cryptography. We will expand this to long-distance free-space link technology to establish the foundation for a global quantum network.”

Professor, Bae Jun Woo of the Department of Electrical Engineering at KAIST also said, “This achievement will be a decisive turning point in making reliable quantum secure communication even in complex environments.”

Over the past decade, ETRI has achieved the following: ▲Development of integrated QKD chips and ultra-compact modules ▲100m transmission in real-world free-space QKD ▲Quantum computing compiler technology ▲Standardization of transmission systems with Korea’s the three major telecommunication companies ▲Key technologies for room-temperature quantum internet ▲Development of the world's first cryptographic quantum safety verification technology.

Today, ETRI researchers are one of the world’s leading quantum research groups, focusing on the development and commercialization of leading quantum technologies in quantum communications, quantum computing, and quantum sensors.

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This work was supported by the Institute of Information & communications Technology Planning & Evaluation (IITP) grant funded by the Korea government (MSIT), (“ETRI R&D Support Project,” “Core Technologies Project for Quantum Internet,” “QKD Industry Expansion and Next-Generation Technology Development Project,” “QKD Integration and Transmission Technology Advancement Project,” “Software Computing Industry Source Technology Development Project,” National Research Foundation (NRF)’s ‘Quantum Common Base Technology Development Project’ and ‘Mid-career Research’ and the Korea Aerospace Administration’s “Future Space Education Center.”