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Optica Quantum

High-fidelity distribution of triggered polarization-entangled telecom photons via a 36 km intra-city fiber network

Tim Strobel, Stefan Kazmaier, Tobias Bauer, Marlon Schäfer, Ankita Choudhary, Nand Lal Sharma, Raphael Joos, Cornelius Nawrath, Jonas H. Weber, Weijie Nie, Ghata Bhayani, Lukas Wagner, André Bisquerra, Marc Geitz, Ralf-Peter Braun, Caspar Hopfmann, Simone Luca Portalupi, Christoph Becher, Peter Michler

Published Jul 22, 2024• 12 citations
Publisher

Abstract

Fiber-based distribution of triggered, entangled, single-photon pairs is a key requirement for the future development of terrestrial quantum networks. In this context, semiconductor quantum dots (QDs) are promising candidates for deterministic sources of on-demand polarization-entangled photon pairs. So far, the best QD polarization-entangled-pair sources emit in the near-infrared wavelength regime, where the transmission distance in deployed fibers is limited. Here, to be compatible with existing fiber network infrastructures, bi-directional polarization-conserving quantum frequency conversion (QFC) is employed to convert the QD emission from 780 nm to telecom wavelengths. We show the preservation of polarization entanglement after QFC (fidelity to Bell state F ϕ + , c o n v =0.972±0.003) of the biexciton transition. As a step toward real-world applicability, high entanglement fidelities ( F ϕ + , l o o p =0.945±0.005) after the propagation of one photon of the entangled pair along a 35.8 km field-installed standard single mode fiber link are reported. Furthermore, we successfully demonstrate a second polarization-conserving QFC step back to 780 nm preserving entanglement ( F ϕ + , b a c k =0.903±0.005). This further prepares the way for interfacing quantum light to various quantum memories.

DOI
10.1364/opticaq.530838

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•Nov 11, 2022•1 citation
Fiber-based distribution of triggered, entangled, single-photon pairs is a key requirement for the future development of terrestrial quantum networks. In this context, semiconductor quantum dots (QDs) are promising candidates for deterministic sources of on-demand polarization-entangled photon pairs. So far, the best QD polarization-entangled-pair sources emit in the near-infrared wavelength regime, where the transmission distance in deployed fibers is limited. Here, to be compatible with existing fiber network infrastructures, bi-directional polarization-conserving quantum frequency conversion (QFC) is employed to convert the QD emission from 780 nm to telecom wavelengths. We show the preservation of polarization entanglement after QFC (fidelity to Bell state F ϕ + , c o n v =0.972±0.003) of the biexciton transition. As a step toward real-world applicability, high entanglement fidelities ( F ϕ + , l o o p =0.945±0.005) after the propagation of one photon of the entangled pair along a 35.8 km field-installed standard single mode fiber link are reported. Furthermore, we successfully demonstrate a second polarization-conserving QFC step back to 780 nm preserving entanglement ( F ϕ + , b a c k =0.903±0.005). This further prepares the way for interfacing quantum light to various quantum memories.
Fiber-based distribution of triggered, entangled, single-photon pairs is a key requirement for the future development of terrestrial quantum networks. In this context, semiconductor quantum dots (QDs) are promising candidates for deterministic sources of on-demand polarization-entangled photon pairs. So far, the best QD polarization-entangled-pair sources emit in the near-infrared wavelength regime, where the transmission distance in deployed fibers is limited. Here, to be compatible with existing fiber network infrastructures, bi-directional polarization-conserving quantum frequency conversion (QFC) is employed to convert the QD emission from \unit[780]{nm} to telecom wavelengths. We show the preservation of polarization entanglement after QFC (fidelity to Bell state Fϕ+,conv=0.972±0.003F_{ϕ^+, conv}=0.972\pm0.003Fϕ+,conv​=0.972±0.003) of the biexciton transition. As a step towards real-world applicability, high entanglement fidelities (Fϕ+,loop=0.945±0.005F_{ϕ^+, loop}=0.945\pm0.005Fϕ+,loop​=0.945±0.005) after the propagation of one photon of the entangled pair along a \unit[35.8]{km} field installed standard single mode fiber link are reported. Furthermore, we successfully demonstrate a second polarization-conversing QFC step back to \unit[780]{nm} preserving entanglement (Fϕ+,back=0.903±0.005F_{ϕ^+, back}=0.903\pm0.005Fϕ+,back​=0.903±0.005). This further prepares the way for interfacing quantum light to various quantum memories.
A global quantum internet is based on scalable networks, which require reliable quantum hardware. Among them are quantum light sources providing deterministic, high-brightness, high-fidelity entangled photons and quantum memories with coherence times exceeding the millisecond range. Long-distance operation demands quantum light sources emitting at telecommunication wavelengths. A cornerstone for such networks is the demonstration of quantum teleportation. Here, we realize full-photonic quantum teleportation employing semiconductor quantum dots, which can fulfill all the aforementioned requirements. Two remote GaAs quantum dots, emitting in the near-infrared, are used: one as an entangled-photon pair source and the other as a single-photon source. During the experiment, the single photon is prepared in conjugate polarization states and interfaced with the biexciton emission of the entangled pair employing a polarization-selective Bell state measurement. This process teleports the respective polarization state onto the exciton emission of the entangled pair. The frequency mismatch between the triggered sources is erased using two polarization-preserving quantum frequency converters, enabling remote two-photon interference at telecommunication wavelengths, yielding a visibility of 30(1)%. A post-selected teleportation fidelity up to 0.721(33), significantly above the classical limit, demonstrates successful quantum teleportation between light from distinct sources. These results mark an important development for semiconductor-based quantum light sources.
Entangled light sources are considered as core technology for multiple quantum network architectures. Of particular interest are sources that are based on a single quantum system as these offer intrinsic security due to the sub-Poissonian nature of the photon emission process. This is important for applications in quantum communication where multi-pair emission generally compromises performance. A large variety of sources has been developed, but the generated photons remained far from being utilized in established standard fiber networks, mainly due to lack of compatibility with telecommunication wavelengths. In this regard, single semiconductor quantum dots are highly promising photon pair sources as they can be engineered for direct emission at telecom wavelengths. In this work we demonstrate the feasibility of this approach. We report a week-long transmission of polarization-entangled photons from a single InAs/GaAs quantum dot over a metropolitan network fiber. The photons are in the telecommunication O-band, favored for fiber optical communication. We employ a polarization stabilization system overcoming changes of birefringence introduced by 18.23 km of installed fiber. Stable transmission of polarization-encoded entanglement with a high fidelity of 91% is achieved, facilitating the operation of sub-Poissonian quantum light sources over existing fiber networks.
On-demand indistinguishable single-photon sources are essential for quantum networking and communication. Semiconductor quantum dots are among the most promising candidates, but their typical emission wavelength renders them unsuitable for use in fiber networks. Here, we present quantum frequency conversion of near-infrared photons from a bright quantum dot to the telecommunication C-band, allowing integration with existing fiber architectures. We use a custom-built, tunable 2400 nm seed laser to convert single photons from 942 nm to 1550 nm in a difference-frequency generation process. We achieve an end-to-end conversion efficiency of ≃35%, demonstrate count rates approaching 1 MHz at 1550 nm with g(2)(0)=0.043(1), and achieve Hong-Ou-Mandel (HOM) visibilities of 60%. We expect this scheme to be preferable to quantum dot sources directly emitting at telecom wavelengths for fiber-based quantum networking.
Entanglement between stationary quantum memories and photonic channels is the essential resource for future quantum networks. Together with entanglement distillation, it will enable efficient distribution of quantum states. We report on the generation and observation of entanglement between a ^{87}Rb atom and a photon at telecom wavelength transmitted through up to 20 km of optical fiber. For this purpose, we use polarization-preserving quantum frequency conversion to transform the wavelength of a photon entangled with the atomic spin state from 780 nm to the telecom S band at 1522 nm. We achieve an unprecedented external device conversion efficiency of 57% and observe an entanglement fidelity between the atom and telecom photon of ≥78.5±0.9% after transmission through 20 km of optical fiber, mainly limited by decoherence of the atomic state. This result is an important milestone on the road to distribute quantum information on a large scale.
Entanglement-based quantum networks require quantum photonic interfaces between station- ary quantum memories and photons, enabling entanglement distribution. Here we present such a photonic interface, designed for connecting a 40Ca+ single-ion quantum memory to the tele- com C-band. The interface combines a memory-resonant, cavity-enhanced spontaneous parametric down-conversion (SPDC) photon pair source with bi-directional polarization-conserving quantum frequency conversion (QFC). We demonstrate preservation of high-fidelity entanglement during con- version, fiber transmission over up to 40 km and back-conversion to the memory wavelength. Even for the longest distance and bi-directional conversion the entanglement fidelity remains larger than 95% (98%) without (with) background correction.