⚛️ quantum physics

Telecom-compatible polarization-to-time-bin conversion of atom-photon entanglement for heterogeneous quantum networks

This paper demonstrates a key interface for heterogeneous quantum networks by successfully converting atom-photon entanglement from a single trapped calcium ion at 854 nm into telecom C-band time-bin qubits with 96.3% fidelity, thereby enabling robust long-distance transmission of quantum information.

Original authors: Christian Haen, Julian Groß-Funk, Max Bergerhoff, Pascal Baumgart, Jonas Meiers, Tobias Bauer, Christoph Becher, Jürgen Eschner

Published 2026-08-03

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CC BY 4.0

Original authors: Christian Haen, Julian Groß-Funk, Max Bergerhoff, Pascal Baumgart, Jonas Meiers, Tobias Bauer, Christoph Becher, Jürgen Eschner

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 the internet of the future, but instead of sending emails and cat videos, it's sending secrets that can never be hacked. This is the world of quantum networks, where information is carried by tiny particles of light called photons. But here's the catch: the devices that create this information, like trapped atoms, speak a different "language" of light than the fiber-optic cables that carry it across cities. It's like trying to send a message written in ancient Greek through a modern fiber-optic cable that only understands English. To make this work, scientists need translators that can change the "accent" of the light without losing the meaning of the message.

Even more tricky is the way the information is written. Some devices write secrets using the "tilt" of the light wave (polarization), while others write them using the "timing" of the light pulses (time-bins). Think of polarization like a spinning top that can spin left or right, and time-bins like a runner who can arrive early or late. If you try to send a message written by a spinning top through a cable that only understands runners, the message gets scrambled. This is a huge problem because the cables we already have underground are perfect for timing-based messages but terrible for spinning-top messages, which get confused by the heat and pressure of the cable. Scientists have been looking for a way to translate a spinning-top message into a timing message so it can survive the journey through our existing fiber-optic highways.

This paper tells the story of a team of scientists who built a successful translator for exactly this problem. They worked with a single trapped calcium ion, which is like a tiny, super-cooled atom held in place by invisible magnetic fields. This ion naturally creates a special link, called entanglement, between itself and a photon it spits out. In this experiment, the ion and the photon were "entangled" using the photon's polarization (its spin direction) at a wavelength of 854 nm. However, 854 nm light is too short to travel far through standard fiber-optic cables without getting lost. So, the team first used a "quantum frequency converter" to change the color of the light to 1550 nm, which is the standard "telecom" color that travels easily through long-distance cables.

But changing the color wasn't enough; they still had the spinning-top (polarization) problem. The team then built a clever device using fiber-optic cables that acted like a race track. They took the spinning-top message and split it into two paths: a short path and a long path. The light traveling the long path arrived later than the light on the short path. By carefully mixing these two paths, they erased the information about which path the light took, effectively converting the "spin" of the light into an "arrival time." Now, instead of a message defined by how the light spins, the message was defined by whether the light arrived early or late. This is the "time-bin" format, which is much tougher and doesn't get confused by the temperature changes in the cables.

The team didn't just build the machine; they proved it worked. They measured the final state of the light and the atom to see if the special link (entanglement) was still intact. They found that the process was incredibly successful, preserving the connection with a fidelity of 96.3(4.2)%. This means that out of 100 attempts, the translated message was almost perfectly identical to the original, proving that you can take a delicate quantum state, change its color, and rewrite its language, all without breaking the magic link. This achievement is a major step toward building a "heterogeneous" quantum network, where different types of quantum computers (like trapped ions and diamond-based sensors) can talk to each other over the existing internet infrastructure, paving the way for a truly global quantum internet.

Technical Summary: Telecom-Compatible Polarization-to-Time-Bin Conversion of Atom-Photon Entanglement

Problem Statement
The realization of large-scale, heterogeneous quantum networks requires interoperability between distinct physical platforms, such as trapped ions and solid-state systems, which often operate at different wavelengths and utilize different qubit encodings. While trapped ions naturally generate atom-photon entanglement in the polarization degree of freedom at visible wavelengths (e.g., 854 nm), long-distance quantum communication relies on the telecom C-band (1550 nm) to minimize transmission losses in optical fibers. Furthermore, polarization-encoded qubits are susceptible to environmental perturbations (temperature, stress) that alter fiber birefringence, necessitating active stabilization. In contrast, time-bin encoding offers robustness against slow polarization fluctuations. A critical gap exists in the ability to convert polarization-entangled photons from atomic quantum memories into time-bin qubits at telecom wavelengths without destroying the underlying entanglement.

Methodology
The authors demonstrate a complete interface converting atom-photon entanglement from polarization encoding at 854 nm to time-bin encoding at 1550 nm. The experimental setup involves a single trapped $^{40}\text{Ca}^+$ ion in a linear Paul trap.

Entanglement Generation: The ion is excited by a 393-nm laser pulse, emitting an 854-nm photon. Through atomic state projection, the photon becomes polarization-entangled with the ion's Zeeman sub-levels ( $|D_{5/2}, +1/2\rangle$ and $|D_{5/2}, -3/2\rangle$ ), creating a maximally entangled state $|\psi\rangle \propto |+ \rangle |L\rangle + e^{i\omega_L t} |- \rangle |R\rangle$ .
Frequency Conversion: The 854-nm photons undergo quantum frequency conversion (QFC) to 1550 nm, preserving the photonic polarization and the atom-photon entanglement.
Polarization-to-Time-Bin Conversion: The 1550-nm photons pass through a fiber-based, unbalanced Mach-Zehnder interferometer (Time-Bin Encoder, TBE) with a path length difference of $\Delta L \approx 18$ m ( $\sim 90$ ns). A polarizer set to diagonal polarization erases "which-path" information, mapping the horizontal ( $|H\rangle$ ) and vertical ( $|V\rangle$ ) polarization components onto early ( $|1\rangle$ ) and late ( $|2\rangle$ ) time-bin states, respectively.
Analysis and Stabilization: The converted state is analyzed using a second unbalanced Michelson-type interferometer. To ensure stability, the system employs automated polarization drift compensation (APC) for the fiber links and active/passive phase stabilization for the interferometers.
Verification: The final state is verified via full quantum state tomography. The photons are frequency-converted back to 854 nm for detection by a superconducting nanowire single-photon detector (SNSPD).

Key Contributions

First Demonstration with Atomic Memory: Alongside independent work by Ferrari et al., this study presents the first demonstration of polarization-to-time-bin conversion for photons entangled with a single atomic quantum memory.
Telecom Compatibility: The interface successfully bridges the gap between the atomic transition wavelength (854 nm) and the telecom C-band (1550 nm) while converting the qubit encoding.
Full Tomography: Unlike previous partial measurements, the authors perform full quantum state tomography to reconstruct the complete density matrix of the converted state.
Robustness: The system demonstrates the feasibility of interfacing trapped-ion memories with time-bin-based networks, a prerequisite for heterogeneous quantum repeaters combining ions with solid-state platforms (e.g., diamond color centers).

Results
The authors quantified the performance of the interface through several metrics:

Process Fidelity: The combined process fidelity of the time-bin encoder and analyzer, measured using attenuated laser pulses, is $F_P = 97.3(1.1)\%$ .
Reference Fidelity: The atom-photon entanglement fidelity after double frequency conversion (854 nm $\to$ 1550 nm $\to$ 854 nm) but without time-bin conversion is $F_{pol} = 94.8(0.8)\%$ (background-corrected).
Conversion Fidelity: After the full conversion process (including time-bin encoding and analysis), the background-corrected fidelity of the atom-photon entanglement is $F_{TB} = 91.3(4.1)\%$ .
Preservation of Entanglement: The ratio $F_{TB}/F_{pol} = 96.3(4.2)\%$ indicates that the polarization-to-time-bin conversion process preserves the entanglement with high fidelity. The reduction in raw fidelity is primarily attributed to significant losses in the analyzer interferometer, which lowers the signal-to-background ratio (SBR), rather than decoherence of the quantum state.

Significance
The paper claims that this work establishes a key enabling technology for heterogeneous quantum networking architectures. By providing a telecom-compatible interface that converts polarization qubits to the more robust time-bin encoding, the system facilitates the interconnection of diverse quantum memories (specifically trapped ions and diamond color centers) over existing metropolitan fiber infrastructure. The authors note that this interface is compatible with the Saarbrücken urban fiber network and represents a standard element for future quantum networks, complementing quantum frequency conversion. The successful preservation of entanglement fidelity through the conversion process validates the approach for long-distance, distributed quantum computing and quantum repeater applications.

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