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Two-photon interference in the telecom C-band after frequency conversion of photons from remote quantum emitters


Efficient fibre-based long-distance quantum communication via quantum repeaters relies on deterministic single-photon sources at telecom wavelengths, potentially exploiting the existing world-wide infrastructures. For upscaling the experimental complexity in quantum networking, two-photon interference (TPI) of remote non-classical emitters in the low-loss telecom bands is of utmost importance. Several experiments have been conducted regarding TPI of distinct emitters, for example, using trapped atoms1, ions2, nitrogen vacancy centres3,4, silicon vacancy centres5, organic molecules6 and semiconductor quantum dots7,8. However, the spectral range was far from the highly desirable telecom C-band. Here, we exploit quantum frequency conversion to realize TPI at 1,550 nm with single photons stemming from two remote quantum dots. We thereby prove quantum frequency conversion9,10,11 as a bridging technology and a precise and stable mechanism to erase the frequency difference between independent emitters. On resonance, a TPI visibility of 29  ± 3% has been observed, limited only by the spectral diffusion processes of the individual quantum dots12,13. The local fibre network used covers several rooms between two floors of the building. Even the addition of up to 2 km of fibre channel shows no influence on the TPI visibility, proving the photon wavepacket distortion to be negligible. Our studies pave the way to establish long-distance entanglement distribution between remote solid-state emitters including interfaces with various quantum hybrid systems14,15,16.

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Fig. 1: Frequency conversion of single photons from distinct quantum dots for remote TPI in the telecom C-band.
Fig. 2: Remote TPI measurement with QD1 and QD2.
Fig. 3: Demonstration of a local fibre network via additional fibre path length before TPI is carried out.
Fig. 4: Simulated degradation of remote TPI visibility after single-photon wavepacket distortion in telecommunication fibre.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.


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The authors thank T. Herzog for the installation of the telecom fibre links. The authors also thank M. Bock for discussions and advice during preparation of the QFC set-ups. This work was financially supported by the Deutsche Forschungsgemeinschaft (DFG) via projects MI 500/26-1 and BE 2306/6-1, as well as by the German Federal Ministry of Science and Education (Bundesministerium für Bildung und Forschung, BMBF) within project (contract nos. 16KIS0115 and 16KIS0127). The research of IQST is supported financially by the Ministry of Science, Research and Arts Baden-Württemberg.

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Authors and Affiliations



J.H.W., B.K., S.K. and S.L.P. performed the experiment with the support of J.K. B.K. built the frequency converters. M.J. provided the samples. J.H.W. and B.K. analysed the data. B.K., H.V. and J.M. set up the theoretical model. J.M. and H.V. conducted the numerical simulations. J.H.W., S.L.P. and B.K. wrote the manuscript with support from P.M. and input from all authors. P.M. and C.B. coordinated the project. All authors actively took part in all scientific discussions.

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Correspondence to Simone L. Portalupi or Peter Michler.

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Weber, J.H., Kambs, B., Kettler, J. et al. Two-photon interference in the telecom C-band after frequency conversion of photons from remote quantum emitters. Nature Nanotech 14, 23–26 (2019).

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