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Conference on Lasers and Electro-Optics

Single photon quantum frequency conversion as tool for quantum networks

Christoph Becher

Published Jan 1, 2018• 1 citation
Publisher

Abstract

We demonstrate that quantum frequency down-conversion of single photons can be used to interface stationary quantum bits with telecom photons. The preservation of polarization-entanglement and indistinguishability allows for connecting quantum bits across a quantum network.
DOI
10.1364/cleo_qels.2018.fw4e.3

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Establishing a network of quantum systems will be based on single photons as flying qubits. For a long range transmission, wavelengths in the low-loss telecom bands will be necessary. As such a network may consist of dissimilar systems emitting at different wavelengths, an interface will be required converting the photons to the telecom band without disturbing the single photon quantum properties. We here demonstrate frequency down-conversion of single photons from a quantum dot and prove the preservation of the single photon character [1].
Single-photon frequency conversion for quantum interface plays an important role in quantum communications and networks, which is crucial for the realization of quantum memory, faithful entanglement swapping and quantum teleportation. In this chapter, we will present our recent experiments about single-photon frequency conversion based on quadratic nonlinear processes. Firstly, we demonstrated spectrum compression of broadband single photons at the telecom wavelength to the near-visible window, marking a critical step towards coherent photonic interface. Secondly, we demonstrated the nonlinear interaction between two chirped broadband single-photon-level coherent states, which may be utilized to achieve heralding entanglement at a distance. Finally, we theoretically introduced and experimentally demonstrated single-photon frequency conversion in the telecom band, enabling switching of single photons between dense wavelength-division multiplexing channels. Moreover, quantum entanglement between the photon pair is maintained after the frequency conversion. Our researches have realized three significant quantum interfaces via single-photon frequency conversion, which hold great promise for the development of quantum communications and networks.
In a typical quantum repeater scenario, remote quantum memories have to be interconnected by flying qubits transmitted in quantum channels. e.g. single photons in photonic fiber links. To establish long-distance communication, photons in the low-loss telecom wavlength regime are desirable. However, the majority of the candidates for quantum memories such as trapped atoms or ions, rare earth ensembles, color centers in diamond, atomic ensembles etc. possess optical transitions merely in the visible or near-infrared spectral regime below 1000 nm. One possible solution is the transduction of these wavelengths to the telecom region via quantum frequency conversion (QFC) based on three-wave-mixing processes in a nonlinear χ(2)-medium. Technical achievements in recent years enabled high conversion efficiencies and low conversion-induced noise contributions resulting in signal-to-background ratios high enough to demonstrate the preservation of several classical and quantum properties of the photons, e.g. first- and second-order coherence [1], time-bin entanglement [2], polarization entanglement [3] or indistinguishability [4]. QFC thus provides a powerful toolbox to interface quantum systems to the telecom bands and beyond that yet to interconnect dissimilar quantum systems.
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.
Realising a global quantum network requires combining individual strengths of different quantum systems to perform universal tasks, notably using flying and stationary qubits. However, transferring coherently quantum information between different systems is challenging as they usually feature different properties, notably in terms of operation wavelength and wavepacket. To circumvent this problem for quantum photonics systems, we demonstrate a polarisation-preserving quantum frequency conversion device in which telecom wavelength photons are converted to the near infrared, at which a variety of quantum memories operate. Our device is essentially free of noise, which we demonstrate through near perfect single photon state transfer tomography and observation of high-fidelity entanglement after conversion. In addition, our guided-wave setup is robust, compact, and easily adaptable to other wavelengths. This approach therefore represents a major building block towards advantageously connecting quantum information systems based on light and matter.