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Laser Physics Letters

Network converter of photonic qubits between phase-frequency and polarization encodings

K. S. Melnik, Mansur M. Minnegaliev, Sergey A. Moiseev

Published Jul 1, 2026• 0 citations
Publisher

Abstract

Abstract The conversion of photonic qubits between different physical encodings is a necessary condition for the creation of hybrid quantum networks. We propose and demonstrate a network scheme for a photonic qubit state converter from phase-frequency to polarization encoding, implemented in a Sagnac loop topology over a distance of 25 km. The obtained results of photonic qubit conversion with a fidelity of ( 0.99 ± 0.01 ) open up the possibility of creating photonic qubit converters that are more resistant to external noise and can be integrated into a subcarrier wave quantum communication network with independent detection.

DOI
10.1088/1612-202x/ae816a

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Abstract The conversion of photonic qubits between different physical encodings is a necessary condition for the creation of hybrid quantum networks. We propose and demonstrate a network scheme for a photonic qubit state converter from phase-frequency to polarization encoding, implemented in a Sagnac loop topology over a distance of 25 km. The obtained results of photonic qubit conversion with a fidelity of ( 0.99 ± 0.01 ) open up the possibility of creating photonic qubit converters that are more resistant to external noise and can be integrated into a subcarrier wave quantum communication network with independent detection.
Quantum information processing, communication, and sensing networks are being developed with various qubit platforms that use different encoding schemes. Connecting quantum network nodes to distribute entanglement requires matching photon qubit basis encoding. In this work, we implement an interconversion protocol which converts photon qubit encoding from the polarization basis to the time-bin basis, transmits the photons through a transport fiber with large fluctuations in polarization, and converts back to polarization encoding for ease of measurement. This interconversion scheme faithfully transmits a polarization Bell state across the transport fiber by converting sources of infidelity to changes in transmission rate. These results illustrate a practical approach for interfacing distinct qubit platforms to enable modular and flexible operation in heterogeneous quantum networks.
The encoding of quantum information in photonic time-bin qubits is apt for long-distance quantum communication schemes. In practice, due to technical constraints such as detector response time, or the speed with which copolarized time-bins can be switched, other encodings, e.g., polarization, are often preferred for operations like state detection. Here, we present the conversion of qubits between polarization and time-bin encodings by using a method that is based on an ultrafast optical Kerr shutter and attain efficiencies of 97% and an average fidelity of $0.827\ifmmode\pm\else\textpm\fi{}0.003$ with shutter speeds near 1 ps. Our demonstration delineates an essential requirement for the development of hybrid and high-rate optical quantum networks.
Time-bin photonic qubits are well-suited for quantum network applications due to their robustness to polarization instability in fiber links and potential for heterogeneous networks. In this work, we implement the first entanglement-preserving polarization-to-time-bin conversion of a photon qubit in an entangled state with a matter qubit, together with the independent work of Haen \textit{et al.} \cite{haen2026telecom}. Photons initially generated with polarization encoding are converted to the time-bin basis through a polarization-discriminating asymmetric Mach--Zehnder interferometer. The photonic qubits are generated via the 109210921092~nm transition of a 88^{88}88Sr+^{+}+ ion. We measure state fidelity bounds of 0.906±0.011≤F≤0.934±0.0110.906 \pm 0.011 \le \mathcal{F} \le 0.934\pm 0.0110.906±0.011≤F≤0.934±0.011, with conversion error <0.028< 0.028<0.028, and find this fidelity shows no statistically significant reduction under depolarizing noise, even at full depolarization strength.
A key enabling feature of future quantum networks is interoperability between platforms that operate at different wavelengths and with different qubit encodings. We demonstrate an interface that converts atom-photon entanglement from polarization encoding at an atomic wavelength to time-bin encoding in the telecom C-band. Atom-entangled photons at 854 nm are generated from a single 40^{40}40Ca+^++ ion. After quantum frequency conversion to 1550 nm, the photonic polarization qubit is converted into a time-bin qubit using a fiber-based Mach--Zehnder-like encoder. Full quantum tomography of the final state verifies that the process preserves entanglement with 96.3(4.2)% fidelity. Together with the independent work of Ferrari et al. [arXiv:2607.07805 (2026)], this is the first demonstration of polarization-to-time-bin conversion of photons entangled with a single atomic quantum memory. The telecom-compatible interface enables robust qubit transmission over optical fibers and provides a key building block for heterogeneous quantum networking architectures.