Efficient fiber-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. Regarding TPI of distinct emitters, several experiments have been conducted, e.g., using trapped atoms, ions, NV-centers, SiV-centers, organic molecules and semiconductor quantum dots (QDs); however, the spectral range was far from the highly desirable telecom C-band. Here, we exploit quantum frequency conversion (QFC) to realize TPI at 1550 nm with single photons stemming from two remote QDs. We thereby prove QFC as bridging technology and precise and stable mechanism to erase the frequency difference between independent emitters. On resonance, a TPI-visibility of (29 ± 3) % has been observed, being only limited by spectral diffusion processes of the individual QDs. The used local fiber network covers several rooms between two floors of the building. Even the addition of up to 2-km of fiber channel shows no influence on the TPI-visibility, proving negligible photon wave-packet distortion. Our studies pave the way to establish long-distance entanglement distribution between remote solid-state emitters including interfaces with various quantum hybrid systems. (Image: Kolatschek, IHFG)
Publication: Two-photon interference in the telecom C-band after frequency conversion of photons from remote quantum emitters
Jonas H. Weber, Benjamin Kambs, Jan Kettler, Simon Kern, Julian Maisch, Hüseyin Vural, Michael Jetter, Simone L. Portalupi, Christoph Becher and Peter Michler 
Nature Nanotech (2018)
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Contact person: J. Weber