Efficient fiber-based long-distance quantum communication via quantum repeaters relies on deterministic single-photon sources at telecom wavelengths, with the potential to exploit 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. With respect to TPI of distinct emitters, several experiments have been conducted, e.g., using trapped atoms [1], ions [2], NV-centers [3, 4], SiV-centers [5], organic molecules[6] and semiconductor quantum dots (QDs) [7][8][9][10][11][12][13][14]; however, the spectral range was far from the highly desirable telecom C-band. Here, we report on TPI at 1550 nm between down-converted single photons from remote QDs [15], demonstrating quantum frequency conversion [16][17][18] as 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 [19,20]. Up to 2-km of additional fiber channel has been introduced in both or individual signal paths with no influence on TPI-visibility, proving negligible photon wave-packet distortion. The present experiment is conducted within a local fiber network covering several rooms between two floors of the building. Our studies pave the way to establish long-distance entanglement distribution between remote solid-state emitters including interfaces with various quantum hybrid systems [21][22][23][24].

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Multi-photon entangled states such as 'N00N states' have attracted a lot of attention because of their possible application in high-precision, quantum enhanced phase determination. So far, N00N states have been generated in spontaneous parametric down-conversion processes and by mixing quantum and classical light on a beam splitter. Here, in contrast, we demonstrate super-resolving phase measurements based on two-photon N00N states generated by quantum dot single-photon sources making use of the Hong-Ou-Mandel effect on a beam splitter. By means of pulsed resonance fluorescence of a charged exciton state, we achieve, in post selection, a quantum enhanced improvement of the precision in phase uncertainty, higher than prescribed by the standard quantum limit. An analytical description of the measurement scheme is provided, reflecting requirements, capability and restraints of single-photon emitters in optical quantum metrology. Our results point towards the realization of a real-world quantum sensor in the near future.Optical quantum metrology provides a route to enhance sensing applications by utilizing, e.g., non-classical states of light [1][2][3][4]. For many photonic sensing schemes, a general task is measuring a phase ϕ with a precision ∆ϕ. Here, the maximum achievable precision is subject to several limitations. The most fundamental boundary, based on a quantum mechanical uncertainty principle, is the so called Heisenberg limit (HL). It relates the error of phase estimation ∆ϕ with the photon number N used for the measurement to ∆ϕ HL = 1/N [1]. However, as a consequence of the central limit theorem of statistics, the phase determination of interferometric sensing schemes utilizing classical light states, is restricted to the so-called standard quantum limit (SQL), scaling with ∆ϕ SQL = 1/ √ N in absence of losses. On the contrary, a maximally path-entangled multi-photon state, a socalled N00N state(|N, 0 + |0, N ), acquires a phase at a rate N times as fast as classical light. As a consequence, the frequency of the obtained interference fringe pattern is increased by a factor of N , referred to as super-resolution [5,6]. If the contrast of the oscillations, exceeds the threshold C th = 1/ √ N , the regime of super-sensitivity [7,8] is reached. In this case, the entanglement allows for quantum enhanced phase measurements outperforming the SQL and approaching the fundamental Heisenberg limit. Practical imperfections, such as loss, decoherence, state preparation and detector inefficiency can degrade this quantum enhancement. Therefore, a careful resource accounting is necessary to judge real-world enhancement [9]. So far, various schemes for generating N00N states have been realized and phase super-resolution has been demonstrated in a number of studies [10][11][12][13][14]. Phase super-sensitivity, or beating the SQL has been demonstrated with four entangled photons using post selected state projection to study the N00N component of various initial N -photon states [15]. The largest N00N state generated to d...