⚛️ quantum physics

Packet-wise multi-hop quantum routing of hybrid quantum-classical packets

This paper presents an experimentally demonstrated optical-fiber-based quantum router that enables transparent, multi-hop routing of hybrid quantum-classical packets without regenerating classical headers, successfully delivering BB84-based quantum key distribution to multiple receivers with nonzero secret key rates.

Original author: Tae Yeong Park, Youn Chang Jeong, Myeong Soo Kang, Kwang Yong Song, Paul G. Kwiat, Hee Su Park

Published 2026-11-11

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CC BY 4.0

Original author: Tae Yeong Park, Youn Chang Jeong, Myeong Soo Kang, Kwang Yong Song, Paul G. Kwiat, Hee Su Park

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the author. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a future where the internet does not just send emails and videos, but also carries the most delicate information in the universe: quantum data. This is the promise of quantum networks, systems designed to connect quantum computers and sensors so they can work together with a level of security and power that current technology cannot match. In these networks, information travels as packets, much like the digital letters we send today, but with a crucial difference. A standard internet packet is a solid block of data, but a quantum packet is a fragile hybrid. It contains a classical address, written in ordinary light, telling the network where to go, and a quantum payload, a stream of single photons carrying information that cannot be copied or observed without being destroyed. The great challenge for scientists has been building a router—a device that can read the address to decide the path, without touching or destroying the fragile quantum cargo inside. If the router looks too closely at the quantum part, the information vanishes. If it is too slow, the connection fails.

For years, researchers have tried to solve this by stopping the packet at every stop, reading the address, and then rebuilding the entire package before sending it on its way. This process, known as regeneration, is like a postal worker opening a sealed letter, copying the address, and then re-sealing a new envelope with a fresh stamp for the next leg of the journey. While this works, it introduces delays and requires complex equipment at every single node in the network. A team of researchers in South Korea, led by Tae Yeong Park and Hee Su Park, has now demonstrated a different approach. They built a quantum router that lets the packet pass through almost entirely untouched, reading only a tiny fraction of the address to make its decision, and then letting the rest of the packet continue on its way. This method, which they successfully tested over two hops to three different receivers, offers a much smoother and faster path for the future quantum internet.

The core of their invention is a device that treats the classical address and the quantum data as two separate travelers moving side by side on the same fiber-optic cable. The address is a pulse of light at one color, while the quantum data travels at a slightly different color. When the hybrid packet arrives at the router, a special mirror-like device called a fiber Bragg grating acts as a gentle filter. It reflects just ten percent of the classical address light toward a detector, allowing the system to read where the packet is supposed to go. The remaining ninety percent of the address light, along with the entire quantum payload, passes straight through without being disturbed. Once the detector reads the address, it sends a signal to an optical switch, a fast gate that opens the correct path for the rest of the packet to exit toward its destination. Because the quantum data never stops and is never measured, it remains intact, preserving the delicate quantum state needed for secure communication.

To prove this concept works in a real-world scenario, the team set up an experiment in their laboratory connecting one sender to three different receivers. They sent packets containing quantum keys, which are used to create unbreakable encryption codes, through two of these new routers in a chain. The first router directed the packet to either the first receiver or the second router. If it went to the second router, that device read the address again and sent the packet to either the second or the third receiver. The researchers used a specific method called the BB84 protocol, which relies on the timing of light pulses to encode information, and they included extra "decoy" signals to ensure no one was eavesdropping. They successfully delivered these quantum keys to all three receivers, even after the packets had passed through two routers. The system generated secret keys at a rate of up to 270 bits per second for the single-hop connection and slightly less for the double-hop routes, proving that the quantum information survived the journey.

The results showed that the new router design is highly efficient, introducing very little loss to the signal as it passed through. The total delay added by the system was minimal, and the error rate in the quantum data remained low enough to generate secure keys. The researchers noted that the small amount of noise that did appear was caused by the classical address light leaking slightly into the quantum channel, a problem they expect to solve with better equipment in the future. Unlike previous methods that required rebuilding the packet at every stop, this transparent approach allows the network to scale more easily. By simply cascading these routers, a network could potentially connect many more users without the heavy penalty of repeated signal regeneration. This work demonstrates that it is possible to build a dynamic, multi-user quantum network where information flows freely and securely, laying a practical foundation for the quantum internet of tomorrow.

Technical Summary: Packet-wise Multi-hop Quantum Routing of Hybrid Quantum-Classical Packets

Problem Statement
The realization of versatile and scalable quantum networks requires the ability to dynamically route quantum information across multiple nodes, moving beyond static point-to-point connections. Current implementations often rely on wavelength-division multiplexing (WDM) passive architectures or optical switch matrices that require global control and global scheduling of sender-receiver pairs. Furthermore, previous quantum router demonstrations typically involved measuring and regenerating classical headers at every node. This regeneration process necessitates laser sources at each node with precisely locked wavelengths and modulation frequencies, introducing complexity, potential noise, and latency. The challenge lies in developing a routing mechanism that can access classical address information to direct packets without disturbing the quantum payload or requiring active regeneration of the header at every hop.

Methodology
The authors propose and experimentally demonstrate a novel optical-fiber-based quantum router designed for "transparent" operation. The system utilizes hybrid packets comprising a classical header and a quantum payload, separated by both wavelength and time.

Routing Mechanism: The router employs partially reflecting Fiber Bragg Gratings (FBGs) with 10% reflectivity. This allows the system to tap a portion of the classical header for address reading while transmitting the remaining 90% of the header and the undisturbed quantum payload forward.
Signal Processing: Reflected header signals are read by a photodiode (PD) via an optical circulator. A Field-Programmable Gate Array (FPGA) processes these signals to control an optical switch (OS), directing the packet to the appropriate output port.
Noise Mitigation: To suppress noise from Spontaneous Raman Scattering (SRS) caused by the intense classical signal on the quantum channel, the classical header and quantum payload are placed in adjacent, distinguished time slots with a 0.5 µs guard time. Additionally, the system uses a 5 µs fiber delay (1 km) to buffer the packet, allowing sufficient time for the FPGA to process the header and configure the switch before the quantum payload arrives.
Experimental Setup: The experiment involved a single sender (Alice) and three receivers (Bob1, Bob2, and Bob3) connected by two cascaded quantum routers. Optical fiber spools with lengths of 20 km, 10 km, and 5 km were inserted before Bob1, Bob2, and Bob3, respectively, to test mid-range communications. The classical header is generated by Alice using a laser diode (LD) with wavelength of 1530.33 nm (ITU-T channel 59) and an electro-optic modulator (EOM). In parallel, the quantum signal is generated using a time-bin-based QKD transmitter with wavelength of 1549.32 nm (ITU-T channel 35). These classical and quantum channels are combined by a dense WDM multiplexer into a common SMF to form a hybrid packet that propagates toward a first quantum router. One of the two output ports was labeled Bob1, and the other output port is connected to the input port of a second quantum router, whose two output ports are labeled Bob2 and Bob3. The classical header generation and the quantum router control are executed on the above-mentioned FPGA, and the quantum signal transmitter and receivers are controlled by an independent FPGA (NI-9149) sharing only the triggering signals. The routing process is initiated by arrival of the preamble signals by the continually monitoring classical-control FPGA. By arrangement of the optical components, each classical header is placed a guard time of 0.5 µs ahead of the corresponding quantum signal in the combined common optical fiber path.

Experimental Results

Routing Performance: The system successfully routed packets to three distinct receivers. The average Quantum Bit Error Rate (QBER) across the receivers was 3.4%. The difference in QBER between the classical channel being on and off was measured at 0.14%, attributed to Spontaneous Raman Scattering from leaked classical light.
Secret Key Rates (SKR): Using a finite-key regime analysis with a block size of $1.5 \times 10^6$ $1.5 \times 1 0^{6}$ bits, the experiment achieved non-zero secret key rates ranging from 35 to 270 bits per second (bps).
- Bob1 (one-hop): Higher SKR.
- Bob2 and Bob3 (two-hop): Lower SKR, with the authors attributing the variability and lower rates in Bob2 to experimental instabilities (fiber birefringence and ambient temperature) over the 1.6-hour collection period rather than fundamental routing limitations.
Loss and Latency: The average loss for quantum signals through a single router node was 2.8 dB. The switching time was approximately 500 ns, limited by the optical switch rise/fall times, with a total processing delay of roughly 5 µs.

Significance and Claims
The authors claim that these results demonstrate the feasibility of extensive quantum networks interconnected by multiple-step router nodes. The work serves as a groundwork for transitioning from point-to-point quantum links to scalable, multi-user quantum networks.

Scalability: The architecture allows for cascading $N$ nodes to reach up to $2^N$ end nodes without regenerating classical headers, provided the optical power remains above the discrimination threshold.
Future Potential: The paper suggests that the compatibility of this routing method with protocols requiring phase stability (such as mode-pairing QKD) and the use of classical headers for synchronization and birefringence stabilization are promising avenues for future research.
Limitations and Outlook: The authors modestly note that current error rates are relatively high due to experimental imperfections (e.g., finite extinction ratios and crosstalk) and that future improvements in extinction ratios and crosstalk mitigation could further reduce error rates. They also highlight that the performance is currently governed by the speed and efficiency of the optical switches, suggesting that advancements in low-loss, high-speed switching technology (e.g., Kerr-effect based switches) will be critical for expanding network coverage.

Technical Summary: Packet-wise Multi-hop Quantum Routing of Hybrid Quantum-Classical Packets

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