⚛️ quantum physics

Bichromatic Quantum Teleportation of Weak Coherent Polarization States on a Metropolitan Fiber

This paper demonstrates the successful teleportation of weak coherent polarization states with 90% fidelity over a 30-km metropolitan fiber loop in Berlin using commercial components, proving the compatibility of quantum networking protocols with existing live telecom infrastructure carrying co-propagating classical data.

Original authors: Zofia A. Borowska, Shane Andrewski, Giorgio De Pascalis, Olivia Brasher, Mael Flament, Alexander N. Craddock, Niccolò Bigagli, Ronny Döring, Michaela Ritter, Ralf-Peter Braun, Klaus Jons, Marc Geitz

Published 2026-02-19

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Zofia A. Borowska, Shane Andrewski, Giorgio De Pascalis, Olivia Brasher, Mael Flament, Alexander N. Craddock, Niccolò Bigagli, Ronny Döring, Michaela Ritter, Ralf-Peter Braun, Klaus Jons, Marc Geitz, Oliver Holschke, Matheus Sena, Mehdi Namazi

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 authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a very delicate, fragile glass sculpture (a piece of quantum information) that you want to send to a friend in another city. The problem is, the sculpture is so fragile that if you try to put it in a truck and drive it down a bumpy highway, it will shatter.

Quantum Teleportation is the solution. Instead of moving the sculpture itself, you scan it, destroy the original, and use a special "magic link" to instantly rebuild an identical copy at your friend's house. The original is gone, but the copy is perfect.

This paper is about a team of scientists and engineers who successfully tested this "magic link" in the real world, using the actual fiber-optic cables that carry your internet and phone calls right now.

Here is the story of how they did it, broken down into simple parts:

1. The Setting: The Busy Highway

Usually, scientists test these things in a quiet, empty lab. But for this to work in the real world, they had to test it on Deutsche Telekom's actual city fiber network in Berlin.

Think of this fiber network as a busy 6-lane highway.

The Classical Traffic: The lanes are full of heavy trucks and cars carrying your Netflix streams, emails, and Zoom calls (these are the "classical" signals).
The Quantum Signal: The scientists wanted to send their delicate "glass sculpture" (the quantum data) on this same highway, in a tiny, invisible lane right next to the heavy trucks.

The challenge? The heavy trucks create vibrations and noise that could easily break the delicate quantum sculpture. Also, the highway twists and turns, which can rotate the sculpture's orientation (a problem called "polarization drift").

2. The Magic Trick: The "Bichromatic" Bridge

The scientists faced a second problem: Language barriers.

The "sender" (a quantum computer or sensor) speaks a language of light at 795 nanometers (a specific color of near-infrared light).
The "highway" (the fiber optic cable) is built to carry light at 1324 nanometers (a different color, in the telecom "O-band") because it travels further with less loss.

If you try to put the 795nm light directly into the 1324nm cable, it gets lost.

The Solution: They used a "Bichromatic" (two-color) entangled source. Imagine a machine that creates pairs of twins:

Twin A stays at the sender's location (795nm).
Twin B is born ready to travel on the highway (1324nm).

These twins are "entangled," meaning they are magically connected. If you do something to Twin A, Twin B reacts instantly, no matter how far apart they are.

3. The Experiment: The Swap

Here is the step-by-step process they performed:

The Input: They prepared a "weak" quantum message (a single photon) at 795nm. This is the message they want to send.
The Meeting: They brought this message to meet Twin A (the 795nm half of the entangled pair).
The Bell-State Measurement (The "Scan"): They made these two 795nm photons crash into each other in a specific way. This is like scanning the message and Twin A together.
- The Catch: When they scan them, the original message and Twin A are destroyed.
- The Magic: Because Twin A was entangled with Twin B (the one waiting at the highway entrance), the act of destroying Twin A instantly imprints the message onto Twin B.
The Journey: Twin B, now carrying the message, is converted to the 1324nm color. It hops onto the 30-kilometer fiber loop in Berlin.
The Ride: It travels through the real city cables, passing right next to live 10 Gbps internet traffic (the "trucks").
The Arrival: The signal arrives at the destination. The scientists check to see if the sculpture was rebuilt correctly.

4. The Results: Did it Work?

They tested two scenarios:

Scenario A (Quiet Highway): They sent the message without any internet traffic next to it.
- Result: The message arrived with 90% fidelity (90% perfect).
Scenario B (Busy Highway): They sent the message while 10 Gbps of internet traffic was screaming past it in the same cable.
- Result: The message still arrived with 86% fidelity.

Why is this a big deal?
In the past, quantum experiments were like performing a magic show in a soundproof room. This experiment proved you can perform the same magic show in a noisy, crowded stadium (a real city network) without the audience (the internet traffic) ruining the trick.

5. The "Why" and The Future

Why do we need this?

The Future Internet: We are building a "Quantum Internet" to connect super-secure quantum computers and sensors.
The Problem: Quantum computers often use specific atoms (like Rubidium) that only talk to light at 795nm. But our global internet cables only speak 1324nm.
The Solution: This experiment proves we can build "bridges" (teleportation) that translate between the two languages, allowing quantum devices to talk to each other over long distances using existing infrastructure.

In a nutshell:
The team proved that you can teleport a quantum state from one place to another using a "magic twin" system, sending it through a 30km city fiber cable that was simultaneously carrying live internet traffic. They did this with commercial equipment, proving that the Quantum Internet is not just a lab dream—it's ready to be built on top of the cables we already have.

1. Problem Statement

As quantum technologies mature, there is a critical need to integrate quantum networking protocols into existing telecommunications infrastructure. However, several challenges hinder the practical deployment of quantum networks:

Real-World Conditions: Quantum signals must operate over existing fiber networks that are subject to environmental perturbations (e.g., temperature changes causing polarization drift) and co-propagate with high-power classical data traffic.
Wavelength Mismatch: Many quantum devices (e.g., trapped ions, neutral atoms, Rb-based systems) operate at visible or near-infrared wavelengths (780–795 nm), while low-loss transmission in deployed fiber is optimal in the telecom O-band (1310 nm) or C-band (1550 nm).
Noise and Crosstalk: Co-propagating classical traffic introduces inter-channel crosstalk and excess noise, which can degrade entanglement fidelity and teleportation success.
Scalability: Demonstrating quantum teleportation requires not just point-to-point links but robust interfaces between device-compatible photons and fiber-compatible photons under carrier-grade conditions.

2. Methodology and Experimental Setup

The authors conducted a field trial on Deutsche Telekom's metropolitan fiber testbed in Berlin, utilizing a "bichromatic" approach to bridge the wavelength gap.

Source Components:
- Input State: A weak coherent state (WCS) at 795 nm generated by an attenuated, frequency-locked DBR laser. This represents the qubit to be teleported.
- Entanglement Source: A bichromatic warm-atom entangled photon source (Qunnect's Qu-SRC) based on spontaneous four-wave mixing in a warm $^{87}$ Rb vapor cell. It generates non-degenerate pairs: an idler at 795 nm and a signal at 1324 nm (O-band). The state approximates $|\Phi^+\rangle = (|HH\rangle + |VV\rangle)/\sqrt{2}$ with 95% fidelity.
Bell-State Measurement (BSM):
- The 795 nm WCS photon and the 795 nm idler photon interfere at a local 50:50 non-polarizing beam splitter.
- Successful detection of the $|\Psi^-\rangle$ Bell state (via Superconducting Nanowire Single-Photon Detectors, SNSPDs) heralds the conditional transfer of the input state onto the 1324 nm signal photon.
Transmission and Stabilization:
- The 1324 nm signal photon is transmitted through a 30-km field-deployed fiber loop.
- Polarization Compensation: To counteract time-varying fiber birefringence, the team used Qu-APC (Automatic Polarization Compensation) modules. These allow for local calibration on short fibers and automatic compensation over the long 30-km span.
- Multiplexing: A Mux/Demux system combined the O-band quantum signal with C-band (1561.42 nm) classical traffic (10 Gbit/s DWDM) to test coexistence.
Analysis:
- State tomography was performed at the receiver end using motorized waveplates (QWP/HWP) and PBS to reconstruct the density matrix of the teleported state.

3. Key Contributions

First Field-Deployed Bichromatic Teleportation: This work demonstrates quantum teleportation over a live metropolitan fiber network using a bichromatic source to map a device-compatible 795 nm qubit to a telecom-compatible 1324 nm photon.
Coexistence with Classical Traffic: The experiment successfully demonstrated teleportation in the presence of co-propagating 10 Gbit/s C-band classical data, proving compatibility with wavelength-division multiplexed (WDM) infrastructure.
Robust Polarization Management: The implementation of automated polarization compensation (Qu-APC) enabled stable operation over 30 km despite environmental drifts, a critical requirement for real-world quantum networks.
Carrier-Grade Validation: The trial utilized commercial, field-deployable components within a telecom operator's datacenter and fiber network, moving beyond laboratory "dark fiber" demonstrations.

4. Results

The study evaluated teleportation fidelity under three conditions: local (bypassing fiber), 30 km fiber without classical traffic, and 30 km fiber with classical traffic.

Fidelity Performance:
- Local (Lab): Average fidelity of 92.3 ± 2.2%.
- 30 km (Dark Fiber): Average fidelity of 90.1 ± 3.3%. The slight decrease was attributed to reduced HOM (Hong-Ou-Mandel) visibility due to the higher source brightness required to compensate for fiber loss.
- 30 km (With Classical Traffic): Average fidelity of 85.9 ± 4.3%. The reduction is attributed to residual crosstalk from the classical channel reaching the detectors despite bandpass filtering.
State Reconstruction:
- Input states $|H\rangle, |D\rangle, |R\rangle$ were successfully teleported to output states $|V\rangle, |A\rangle, |R\rangle$ (consistent with the $|\Psi^-\rangle$ BSM outcome without unitary correction).
- Density matrix reconstructions confirmed high purity in all configurations.
HOM Visibility:
- Local visibility: $72.8 \pm 5.3\%$ .
- 30 km visibility: $68.6 \pm 5.6\%$ .
- Both values exceeded the classical limit, confirming quantum interference.

5. Significance and Impact

Scalable Quantum Networking: This work validates the "end-node" function of a quantum internet: the interface between quantum processors (often operating at 780–795 nm) and the fiber backbone (O/C-bands).
Operator-Run Quantum Internet: By utilizing Deutsche Telekom's infrastructure and commercial components, the paper provides a blueprint for telecommunications operators to host and scale quantum services.
Hybrid Network Viability: The successful demonstration of high-fidelity teleportation alongside live classical data proves that quantum networks can share existing fiber infrastructure without requiring dedicated "dark fiber" for every link, significantly lowering deployment costs.
Foundation for Repeaters: While this experiment focused on state transfer, the bichromatic entanglement source and teleportation protocol are essential precursors to entanglement swapping and quantum repeaters, which are necessary for long-distance quantum communication.

In conclusion, this paper represents a major milestone in transitioning quantum teleportation from a laboratory curiosity to a deployable technology compatible with the global telecommunications ecosystem.