Scientists Achieve Quantum Information Exchange on Commercial Telecommunications Networks Without Low-Temperature Cooling, Doubling Quantum Key Distribution Distance
Breakthrough in Quantum Security Communication
Recently, researchers from Toshiba Europe Ltd. have created a communication system that can prevent hacking attacks by using quantum mechanics principles.
The research team **implemented a coherence-based twin-field quantum key distribution protocol through a 254-kilometer commercial telecommunications network connecting Frankfurt and Kiel in Germany, achieving an encryption key distribution speed of 110 bits per second.**
Researchers state that this is the first time such a large-scale simplified quantum information exchange has been achieved on a commercial telecommunications network, marking an important step toward next-generation data security and representing significant progress in secure quantum communication deployment. The related paper was recently published in Nature.
China has also produced numerous Nature papers in the quantum field. When conducting quantum communication work on the ground and via satellite, China typically needs to deploy specialized adaptation equipment. In contrast, the Toshiba Europe team used a commercial telecommunications network and standard optical fibers to transmit quantum information without requiring ultra-low temperature cooling equipment.
The researchers believe that while quantum networks could be built using satellites as the Chinese team has done, utilizing existing fiber optic infrastructure is more cost-effective.
A practical immediate significance of this achievement is that **it means commercial components can be used to achieve higher-performance quantum key distribution, paving the way for national and even global deployment of quantum-secure communication infrastructure.**
This research matches the needs of coherence-based quantum communication with the capabilities of existing telecommunications infrastructure, potentially driving the development of high-performance quantum networks, including the implementation of advanced quantum communication protocols, quantum repeaters, quantum sensor networks, and distributed quantum computing.
Doubling the Practical Implementation Distance of Quantum Key Distribution
The researchers utilized quantum key distribution (QKD) encryption technology, enabling information transmission in traditional communication systems in a way that resists hacker attacks.
Quantum key distribution leverages a phenomenon called quantum entanglement, which refers to the potential correlation between the properties of subatomic particles even when separated by great distances.
By measuring the data of one particle, information about the other particle can be inferred. This makes these two particles usable as keys for exchanging encoded information, as external parties cannot read them.
This experiment constructed a 254-kilometer network between Frankfurt, Kirchfeld, and Kiel in Germany. The equipment used was very simple, avoiding reliance on expensive and energy-intensive equipment to control temperature and detect photon particles. Although using lower-precision equipment might reduce communication quality, it helps construct large-scale quantum information systems with multiple functions.
At the same time, this achievement benefited from a scalable optical coherence distribution method, supported by a system architecture and non-cryogenic single-photon detection. The non-cryogenic single-photon detection was assisted by out-of-band phase stabilization technology.
The experimental results show that the researchers achieved quantum communication similar to a repeater in a real network environment, doubling the implementation distance of practical quantum key distribution without using cryogenic cooling.
Repurposing Non-Ideal Avalanche Photodiodes
According to the introduction, implementing coherent or phase-based quantum communication faces many challenges, such as establishing a common phase reference framework between distant encoded users and the need to mitigate phase noise produced by lasers and transmission channels.
To address these challenges, the research team developed a practical architecture for optical frequency distribution and coordination between network nodes, and they used non-cryogenically cooled detectors to achieve active out-of-band phase stabilization.
In this system, the central node distributes two optical frequency reference signals to the transmitting node through service fibers, allowing them to lock their lasers to a common frequency reference, thereby achieving mutual phase locking.
Compared to using ultra-stable lasers and external cavities, this method is both practical and inexpensive for eliminating laser phase noise.
To mitigate phase noise produced by optical fibers, the research team adopted an out-of-band phase stabilization feedback system based on monitoring single-photon interference results using avalanche photodiodes (APDs).
Although avalanche photodiodes can provide semiconductor-based single-photon detection capabilities, their performance is not ideal compared to superconductive nanowire single-photon detectors (SNSPDs).
As shown in the table below, avalanche photodiodes have higher dark counts, lower detection efficiency, and are susceptible to afterpulse effects.
*(Source: Nature)*
However, avalanche photodiodes are one to two orders of magnitude cheaper than superconductive nanowire single-photon detectors, not only more practical but also able to work in temperature environments compatible with telecommunications infrastructure.
For this reason, the researchers combined out-of-band stabilization technology with avalanche photodiodes.
For long-distance coherent quantum links, establishing a common phase reference framework between users is essential. Distributing phase reference signals is more important than protocol encoding signals.
Traditional methods use time-division multiplexed phase reference pulses with the same optical frequency as the encoding signals, but such schemes are not compatible with avalanche photodiodes due to their afterpulse effect.
Using the same avalanche photodiodes to detect strong reference pulses and protocol encoding signals would introduce noise, which would mask the encoding signals.
In contrast, the research team’s out-of-band stabilization technology uses different optical frequencies for phase reference signals and protocol encoding signals, allowing each signal to be detected with independent detectors, thereby eliminating afterpulse crosstalk.
Alice, Bob, and Charlie: The Three Nodes of the Quantum Network
As mentioned earlier, this experiment was conducted in Germany using network infrastructure provided by GÉANT, Europe’s largest research and education network. GÉANT connects academic networks across European countries, aiming to provide high-speed, secure network infrastructure for research institutions and universities.
This communication link spanned a distance of 254 kilometers between Frankfurt and Kiel, Germany, with 56.0 dB of loss. A relay station was set up in Kirchfeld, Germany, located approximately three-fifths of the way through the entire route.
This setup formed a star-shaped quantum network with three nodes. Researchers named the two transmitters at the network edges Alice and Bob, and the central relay receiver Charlie. Charlie was connected to each transmitter via a fiber optic duplex cable.
The equipment was placed in standard telecommunications racks in hosting data centers, operating alongside existing telecommunications equipment.
*(Source: Nature)*
The transmitting end prepared weak coherent pulse (WCP) qubits for the quantum communication protocol. The receiving end was responsible for distributing optical frequency reference signals to ensure coherence and performing interference operations on weak coherent pulses from the transmitting end.
Each node adopted a modular design, equipped with interconnected 19-inch rack boxes to enhance compatibility with telecom racks and system flexibility.
The subsystems were divided into three functional layers: service layer, management layer, and quantum layer, each spanning the three nodes. The quantum layer was responsible for executing the quantum communication protocol.
In the experiment, the specific protocol implemented was a sending-or-not-sending variant of twin-field quantum key distribution. This protocol enabled Alice and Bob to continuously generate shared secret bit strings, thereby achieving quantum-secure communication through symmetric key encryption.
The transmitters encoded information on the optical phase of weak coherent pulses, which were sent to Charlie where they interfered. Charlie used two single-photon detectors to monitor the interference results and published the results on a public channel, announcing which detector was triggered and when.
By combining the publicly announced interference results with their private knowledge of how they encoded the weak coherent pulses, the transmitters could initiate a filtering procedure that generated a shared secret bit string.
*(Source: Nature)*
Furthermore, the twin-field quantum key distribution protocol has two core advantages: first, it has measurement device independence, ensuring that the detection behavior of the third party (Charlie) does not leak key information; second, it has superior key rate scaling characteristics, meaning the secret key rate (SKR) improves at a square root proportion with channel loss.
Before conducting the field experiment, the research team conducted laboratory tests using a testbed that simulated actual field conditions. This testbed was equipped with single-mode fiber spools and fixed optical attenuators to simulate the distance and channel loss of installed fibers.
Through these tests, the researchers obtained data under controlled conditions for comparison with field results.
After deploying the system in Germany, the research team monitored polarization drift in the installed fibers by evaluating the light intensity of orthogonal polarization components recorded by the avalanche photodiodes.
Compared to the laboratory environment, the field environment showed higher polarization stability, with minimal intensity drift in orthogonal polarization components over 12 hours, mainly due to the natural temperature stability of underground fibers.
In contrast, the laboratory-installed fibers, although in a temperature-controlled environment, showed greater fluctuations due to less stable temperature conditions.
Through relevant tests, the researchers verified the polarization stabilization mechanism, finding that it consistently maintained a 21 dB intensity contrast between the preferred polarization axis and its orthogonal polarization axis.
Additionally, the research team addressed issues of signal attenuation caused by optical components, amplification noise from erbium-doped fiber amplifiers, optical injection locking (OIL), and crosstalk from classical signals in the service layer.
This allowed the system to implement the active odd-parity sending-or-not-sending (SNS) twin-field quantum key distribution protocol at a repetition rate of 1 GHz.
*(Source: Nature)*
Furthermore, the light pulse sequences used an interleaved arrangement with a 50% duty cycle, achieving precise phase stabilization through alternating transmission of key-generating weak coherent pulses and unmodulated pulses.
In the research, the team also optimized pulse intensity and distribution to maximize key generation under asymmetric link conditions and to meet the critical security conditions for asymmetric protocol use. The resulting key rates are shown in the figure below.
*(Source: Nature)*
This proves that the system effectively implemented an untrusted quantum repeater on commercial telecommunications infrastructure.
Without using quantum memory or photon cluster states, and with detectors of comparable performance, it is only possible to break this limitation if optical coherence is maintained between signals that interfere at the central relay station.
This experiment demonstrated the advantage of utilizing optical coherence in extending the maximum distance of quantum communication. Compared to previous experiments, this experiment effectively doubled the communication distance and increased the tolerable loss budget by approximately three orders of magnitude.
The key generation rate obtained from this experiment is not only sufficient to support the low-rate one-time pad encryption required for critical data transmission but also meets the requirement of refreshing an AES-256 key every few seconds, a performance indicator that completely exceeds the operational requirements of commercially available off-the-shelf AES encryptors. (Note: AES is a symmetric encryption algorithm certified by the National Institute of Standards and Technology, supporting 128/192/256-bit key lengths, such as AES-256.)
By using thermoelectric coolers to cool the avalanche photodiodes to below -30°C, the detection range of the system can be significantly expanded.
Previously, the global record for point-to-point quantum key distribution systems with cryogenic cooling was a link loss of 71.9 dB, and this advance is expected to surpass that record.
At the same time, this system adopts a star topology with detectors set at the central node. The advantage of this structure is that the network can be easily expanded by simply adding transmitters connected to this central hub.
In general, **this work demonstrates the compatibility of coherence-based quantum communication with existing network infrastructure and the possibility of deploying effective quantum repeaters on commercial networks.**
The researchers also achieved the longest transmission distance for quantum key distribution using non-refrigerated detection technology and built a star-shaped quantum key distribution network over long distances.
This achievement proves that environmental conditions in real-world telecommunications centers are comparable to those simulated in laboratories, and sometimes even better. This provides favorable conditions for further commercialization and prototyping of coherent quantum communication equipment.
At the same time, this achievement, verified through highly asymmetric national-scale links, lays the technical foundation for achieving high-performance, practical quantum communication and quantum networks.
**References:**
Pittaluga, M., Lo, Y.S., Brzosko, A. et al. Long-distance coherent quantum communications in deployed telecom networks. Nature 640, 911–917 (2025). https://doi.org/10.1038/s41586-025-08801-w
