The coherent manipulation of the frequency of single photons is an important requirement for future quantum network technologies. It allows, for instance, quantum systems emitting in the visible range to be connected to the telecommunication wavelengths, thus extending the communication distances. Here we report on quantum frequency conversion of memory-compatible narrow-bandwidth photons at 606 nm to the telecom C-band at 1552 nm. The 200 ns long photons, compatible with praseodymium-based solid-state quantum memories, are frequency converted using a single-step difference frequency-generation process in a periodically poled lithium niobate waveguide. We characterize the noise processes involved in the conversion and, by applying strong spectral filtering of the noise, we demonstrate high signal-to-noise ratio conversion at the single-photon level (, for a mean input photon number per pulse of 1). We finally observe that a memory-compatible heralded single photon with a bandwidth of 1.8 MHz, obtained from a spontaneous parametric down-conversion pair source, still shows a strong non-classical behavior after conversion. We first demonstrate that correlations between heralding and converted heralded photons stay in the non-classical regime. Moreover, we measure the heralded autocorrelation function of the heralded photon using the converter device as a frequency-domain beam splitter, yielding a value of . The presented work represents a step towards the connection of several quantum memory systems emitting narrowband visible photons to the telecommunication wavelengths.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The ability to control the optical frequency of quantum state carriers (i.e., photons) is an important functionality for future quantum networks . It allows all matter of quantum systems (nodes of the network) to be compatible with the telecommunication C-band, therefore enabling long-distance fiber quantum communication between them. It also allows dissimilar nodes to be connected with each other, resulting in heterogeneous networks that can take advantage of the different capabilities offered by the diversity of its constituents . Quantum frequency conversion (QFC)  aims for the energy shift of an optical field, in a coherent and noise-free fashion, such that quantum properties are preserved to a high degree. Optical frequency conversion based on three- or four-wave mixing has been demonstrated using different platforms such as non-linear waveguides [4,5], non-linear crystals in cavities [6,7], microresonators [8,9], or atomic systems [10,11]. The high conversion efficiencies offered by these platforms has led to multiple experiments showing the increase of telecom detection efficiencies , generation of non-classical states of light at a specific wavelength [12,13], connections of quantum systems to the telecommunications band for long-distance communication applications [10,14–22], or the coupling of disparate quantum memory (QM) systems . The main challenge of QFC is to achieve high conversion efficiency together with low noise generation at the target wavelength, given that the required pump light often generates a high amount of uncorrelated noise photons.
Depending on the wavelengths of the input signal and converted photons, we can distinguish between three different cases of noise generation at the target wavelength in non-linear materials . The first case is one in which the pump field frequency is far below the frequency of signal and converted photons, thus making noise-free frequency conversion possible [13,25,26]. A second case is one in which one of the two photon frequencies are close to the pump, where Raman noise is generated [17,27]. The third case is one in which the pump field is in between the signal and converted photons, and also generates non-phase-matched parametric fluorescence noise  on top of Raman noise. Note that cascaded conversion using a long-wavelength pump, as demonstrated in Ref.  for up-conversion, could be a solution to avoid noise generation at the target wavelength.
In the context of quantum networks, several systems, known as emissive quantum nodes, combine the generation of light–matter entanglement with quantum storage capabilities . An important challenge is therefore the conversion of narrowband light emitted by these nodes to telecom wavelengths. The conversion of such narrowband photons indeed leads to stringent requirements for noise reduction, as temporal gating cannot be used due to their long temporal waveform. Several important quantum-node systems emit light around 600 nm, e.g., europium-doped QMs (580 nm) , praseodymium-doped QMs (606 nm) , and nitrogen-vacancy (NV) centers in diamonds (637 nm) . Single-step conversion requires a pump wavelength close to 1 μm, which generates non-phase-matched noise photons at the target wavelength. Several works have demonstrated QFC from the telecom to visible wavelengths of broadband photons [6,34,35]. In Ref. , we demonstrated QFC from 1570 to 606 nm of QM-compatible weak coherent states at the single-photon level, using the QM as an ultra-narrowband filter. First attempts have been made towards the QFC of NV center resonant light to telecom [28,36], but without demonstrating quantum light conversion.
In this paper, we demonstrate efficient direct-frequency conversion of memory-compatible photons at 606 nm to the telecom C-band based on difference frequency generation in a non-linear waveguide. We characterize the noise processes in the device, limiting the single-photon-level operation of the frequency conversion, and apply strong filtering to obtain high signal-to-noise ratio (SNR) for weak coherent pulses compatible with a doped quantum memory. Finally, we employ an ultra-narrowband photon pair source to demonstrate quantum frequency conversion of a memory-compatible heralded single photon. To that end, we measure the cross-correlation between heralding and converted heralded photons. We also measure the heralded photon autocorrelation in a Hanbury–Brown–Twiss configuration using the converter device as a frequency-domain beam splitter.
2. FREQUENCY CONVERSION DEVICE
The concept of QFC can be described considering two optical modes and (signal and converted) at frequencies and , respectively. In the case of difference frequency generation, it satisfies , where is the frequency of the pump field. In the limit of an undepleted pump field, the Hamiltonian describing the three wave mixing process is [3,37]37] where the transmittance and reflection of the modes and depend on the pump amplitude applied.
In our experiment, the QFC process takes place in a 1.4 cm long periodically poled lithium niobate (PPLN) ridge waveguide (HC Photonics), temperature stabilized at 65°C. Both facets are anti-reflection coated for the three wavelengths involved in the experiment. The setup is depicted in Fig. 1(a). A pump laser at 994 nm is coupled with 55% efficiency, together with a 606 nm signal with 57% coupling efficiency inside the waveguide. At the output of the waveguide, the converted 1552 nm light field, 994 nm pump, and non-converted 606 nm signal are separated by means of dichroic mirrors and can be monitored independently. The converted signal passes through a filtering stage and is coupled into a single-mode fiber (79% coupling efficiency). The filtering stage is composed of a fiber Bragg grating (65% transmission, 2.5 GHz bandwidth) and an etalon [95% transmission, 210 MHz bandwidth, and free spectral range (FSR) of 4 GHz]. The unconverted signal is also filtered by means of a diffraction grating (75% reflection) and an etalon (90% transmission, 10 GHz linewidth, FSR of 60 GHz), and is coupled into a single mode fiber. At the input of the device, we can choose between different 606 nm light fields: bright laser light, weak coherent states at low average photon number per pulses, or heralded single photons generated by a photon-pair source.
C. QFC Performance
The frequency converter is first characterized using classical light as input. The conversion efficiency of the device is measured as a function of the coupled pump power inside the waveguide, using 1 mW of classical input light at 606 nm. For this measurement only, the pump power is swept using an acousto-optical modulator over a short time period of 100 μs. The coupled pump power and the difference-frequency converted light are then monitored at the output of the waveguide using photodiodes. This fast measurement offers the advantage of avoiding thermal effects (change of the quasi-phase-matching temperature due to local heating of the waveguide at high pump powers) sometimes encountered when changing pump power , and gives a more precise efficiency measurement. The blue trace in Fig. 2(a) shows the measured internal efficiency depending on the coupled pump power, inferred by correcting for all losses. At the maximum coupled pump power available of 530 mW, we measure an internal conversion efficiency of 62%. This value is in accordance with the measured depletion of the 606 nm signal. The device efficiency, also shown on the right axis, includes all optical losses: signal transmission (93%), coupling efficiency of the signal in the waveguide (57%), filtering efficiency (62%), and fiber coupling efficiency (79%) of the converted signal. The conversion efficiency is fitted with the model 
To assess the potential of our QFC device for converting light emitted by quantum memories, we also characterized it at the single-photon level, sending 606 nm weak coherent states with 200 ns FWHM Gaussian shape, mimicking long single photons compatible with praseodymium-doped quantum memories [40–42]. For this measurement and the following ones, the 994 nm pump runs continuously and the waveguide temperature is optimized for each pump power. The converted photons are then detected with an InGaAs single-photon detector D3 (ID230, ID Quantique, 10% detection efficiency, 10 Hz dark counts) and integrated over a 400 ns time window, containing more than 98% of the pulses. The device efficiency is also extracted from this measurement and plotted in Fig. 2(a), matching with the classical measurement. We also measured the SNR of the converted photons with different average input photon numbers per pulse, ranging from 0.04 to 1. From a linear fit of this measurement we extract the parameter , i.e., the number of photons per pulse at the input of the device, in order to obtain a SNR of 1 at the output . Figure 2(b) shows the measurement of as a function of the pump power. Interestingly, we observe a decrease in with increasing pump power, down to at 500 mW pump power. Although not intuitive, the decrease of (i.e., increase of SNR) with the pump power can be explained by characterizing the noise processes in the waveguide, as discussed in the next section. The green curve shows the expected values of the , calculated from the classical conversion efficiency measurement, and the noise level [Fig. 3(b)]. This measurement demonstrates the capability of our device to convert long photons with high signal to noise ratio, thanks to the high efficiency of the process and to the strong spectral filtering of the QFC noise.
D. Noise Processes
We now study in more detail the noise generation inside the converter. The noise processes are depicted in Fig. 3(a). A strong pump field generally generates Raman noise around its central frequency; in a PPLN waveguide, it has been estimated to have a width smaller than 30 THz () [24,43]. Raman noise therefore does not play a significant role in our experiment, since the frequency shift between pump and signal is 109 THz. A second type of noise is spontaneous parametric down-conversion (SPDC) noise as described in Ref. , generated at lower frequencies than the pump. In our case, we observe direct SPDC noise at the converted wavelength of 1552 nm (). Part of this noise, within the phase-matching bandwidth of the frequency converter, can be eventually converted to 606 nm by the pump field via sum frequency generation. In the case of , the expected behavior is therefore very different as the reconversion of the noise leads to a quadratic dependence on pump power. We measure the noise using only the pump as the input of the waveguide, and detecting the photons either at the 1552 nm output [Fig. 3(b)] or the 606 nm output [Fig. 3(c)]. The telecom noise saturates as a function of the pump power due to the just-explained back-conversion to 606 nm. This gives approximately a factor 2 reduction of the expected noise at the maximum pump power and explains why the SNR does not decrease with the pump power.
Taking the first three points to fit a linear slope, we find an internal SPDC noise-generation coefficient of normalized to a 1 THz bandwidth, similar to the one described in Ref. . The blue curve, matching our data, shows a model that takes into account the generation of noise along the waveguide at the position and its eventual back-conversion on the remaining length of the waveguide:3).
3. CONVERSION OF HERALDED SINGLE PHOTONS
To prove that our QFC device can operate with memory-compatible quantum light, we use a photon pair source, schematically depicted in Fig. 1(b).
A. Photon Pair Source
The source is a new generation of an earlier source [44,45] that has been used to demonstrate quantum storage of heralded single photons in a memory [41,42]. It is based on a 2 cm long PPLN crystal placed in a bow-tie cavity (FSR of 261 MHz). Pumped with 426 nm light in CW, it generates a signal photon at 606 nm and an idler photon at 1436 nm. The bi-photon linewidth is 1.8 MHz, making the 606 nm photon compatible for storage in a -based quantum memory [41,42]. The idler telecom photon is filtered with a Fabry–Perot cavity (linewidth of 80 MHz, ) in order to select a single frequency mode out of the eight modes at the output of the bow-tie cavity. It is then used to herald the signal 606 nm photon with an efficiency of 25% in single-mode fiber. In the 606 nm arm, there is no filtering of a single frequency mode, and the non-correlated modes contribute to accidental coincidences. Pumped with 1.65 mW of 426 nm light, the source generates about 280 heralded 606 nm photons per second, which are strongly non-classically correlated to the heralding photon. This number is limited by the transmission of the heralding photons (filtering cavity and fiber coupling) and their detection at D1 (10% efficiency). The correlation time of the photon pair is measured to be 120.9 ns. We measure the normalized cross-correlation function , where describes the probability for a coincidence detection of a signal and an idler photon, and () is the detection probabilities for single signal (idler) events. Using a detection window of 400 ns, we obtain , well above the classical threshold of 2, assuming thermal statistics for the signal and idler fields. The single-photon nature of the heralded 606 nm photon is verified measuring the heralded second-order autocorrelation function . This is done using a 50/50 fiber-beam splitter in the 606 nm arm and detecting the heralded correlations between the two outputs. The source exhibits , well below the classical threshold of 1 and below the threshold of 0.5 for a two-photon Fock state.
Finally, we connect the photon pair source to the quantum frequency converter. The correlations between the herald and the non-converted signal (detectors D1 and D2) and between the herald and the converted signal (detectors D1 and D3) are measured as a function of the QFC pump power, and are shown in Fig. 4(a). At 0 mW pump power, the normalized cross-correlation function of for the non-converted light (gray open squares) corresponds to the source without any effect of the QFC (except for the additional transmission losses). The value for the non-converted photons then rapidly drops with increasing pump power due to the drop of SNR induced by the high amount of quadratic noise generated through the 10 GHz etalon filter. In contrast, when looking at the 1552 nm converted signal (blue dots), we observe that the value of slightly increases with pump power up to at 440 mW. It is worth mentioning that the filtering stage of the converted signal used to obtain a high SNR of the converted photon also filters a single-frequency mode from the bow-tie cavity. It then reduces the number of accidental coincidences from the source as the other modes, non-correlated with the single frequency idler mode, are not detected. On the contrary, the QFC itself adds noise to the channel, thus reducing the cross-correlation function. Thus, it is difficult to directly compare the converted cross-correlation function with the non-converted one shown in Fig. 4(a). Instead, we measured the cross-correlation of the source before the QFC, sending the 606 nm heralded single photon through a 12 MHz transparency window of a doped crystal  that filters a single-frequency mode of the bow-tie cavity and measures . We now have an estimate of the performance of the source in fully single-mode operation that can be compared with the converted one measured [blue dots in Fig. 4(a)]. The effect of the noise of the QFC device on correlations can be estimated using the same approach as in Ref. :4(a)] using the model shown previously [green curve of Fig. 2(b)]. We can then observe that, up to the pump power of 1.45 W for which the maximum conversion efficiency should occur, the correlations, although degraded by the QFC-induced noise, remain well above the classical limit of 2. The model suggests that the converted light can exhibit non-classical correlations at any pump power. The small discrepancy between the measured data and the model is probably due to an overestimation of the non-converted correlations, as the filtering is much narrower in that case.
In order to show the preservation of the single-photon nature of the converted heralded photon, we measure, for the first time to our knowledge, the heralded autocorrelation function of the signal photon using the QFC as a frequency-domain beam splitter [46,47]. The pump power of the QFC could in principle be tuned in such a way that the photon entering the waveguide has the same probability of being converted or staying in its original state [Eq. (1)]. In our case, we equalize the photon-detection rates between the two outputs of the frequency beam splitter by fixing the QFC pump power at 250 mW, leading to a conversion efficiency of 35%. This also dramatically reduces the amount of quadratic noise detected at D2. We can then record the triple coincidences between the heralding photon at D1, the unconverted 606 photon at D2, and the converted 1552 nm photon at D3 to measure the heralded autocorrelation function in a 400 ns window. The histogram of the triple coincidence is shown in Fig. 4(b), sorted by the number of heralding events between succeeding detections at signal or converted photon detectors . The value at bin 0 corresponds to of , and is well below the classical threshold and proves the single-photon nature of the converted light. Note that this value is an upper bound, as a high amount of uncorrelated noise is added to the non-converted output of the beam splitter. This measurement also highlights the potential of a quantum frequency converter as a beam splitter device which, for instance, could be used to perform Bell measurements between modes of different wavelengths .
In conclusion, we have developed a memory-compatible quantum frequency conversion device bridging the gap between visible light around 600 nm and the telecom C-band. The device shows high intrinsic conversion efficiency and low noise at the target wavelength. The device conversion efficiency could be greatly improved using a longer waveguide as well as more efficient filtering or coupling techniques. Note that despite a measured device efficiency of 15%, this conversion process becomes advantageous after only 1 km of fiber transmission of 606 nm photons. We showed conversion of quantum memory compatible photons, at the single-photon level, with high signal-to-noise ratio. We finally demonstrated that single photons compatible with a quantum memory still exhibit strong quantum correlations after the conversion. The degradation of quantum properties is only due to the noise generated by the QFC. This issue could be tackled with stronger filtering. Our work opens the route towards connecting different solid-state quantum memory systems emitting in the visible range (e.g., europium and praseodymium quantum memories or NV centers) to the telecom C-band.
Note: After submission of our paper, we became aware of a related publication demonstrating quantum frequency conversion from NV centers .
European Regional Development Fund (FEDER); Ministerio de Economía y Competitividad (MINECO) (FIS2015-69535-R); MINECO Severo Ochoa program (SEV-2015-0522); Fundación Cellex; Generalitat de Catalunya (CERCA); FP7 Ideas: European Research Council (IDEAS-ERC) (279967, QuLIMA).
We thank Margherita Mazzera and Daniel Rieländer for their helpful discussions and for their contributions at the early stage of the experiment. G. H. acknowledges support by the ICFOnest international postdoctoral fellowship program.
1. H. J. Kimble, “The quantum internet,” Nature 453, 1023–1030 (2008). [CrossRef]
2. I. A. Walmsley and J. Nunn, “Editorial: building quantum networks,” Phys. Rev. Appl. 6, 040001 (2016). [CrossRef]
3. P. Kumar, “Quantum frequency conversion,” Opt. Lett. 15, 1476–1478 (1990). [CrossRef]
4. C. Langrock, E. Diamanti, R. V. Roussev, Y. Yamamoto, M. M. Fejer, and H. Takesue, “Highly efficient single-photon detection at communication wavelengths by use of upconversion in reverse-proton-exchanged periodically poled LiNbO3 waveguides,” Opt. Lett. 30, 1725–1727 (2005). [CrossRef]
5. S. Tanzilli, W. Tittel, M. Halder, O. Alibart, P. Baldi, N. Gisin, and H. Zbinden, “A photonic quantum information interface,” Nature 437, 116–120 (2005). [CrossRef]
6. M. A. Albota and F. N. C. Wong, “Efficient single-photon counting at 155 μm by means of frequency upconversion,” Opt. Lett. 29, 1449–1451 (2004). [CrossRef]
7. A. Samblowski, C. E. Vollmer, C. Baune, J. Fiurášek, and R. Schnabel, “Weak-signal conversion from 1550 to 532 nm with 84% efficiency,” Opt. Lett. 39, 2979–2981 (2014). [CrossRef]
8. Q. Li, M. Davanço, and K. Srinivasan, “Efficient and low-noise single-photon-level frequency conversion interfaces using silicon nanophotonics,” Nat. Photonics 10, 406–414 (2016). [CrossRef]
9. X. Guo, C.-L. Zou, H. Jung, and H. X. Tang, “On-chip strong coupling and efficient frequency conversion between telecom and visible optical modes,” Phys. Rev. Lett. 117, 123902 (2016). [CrossRef]
10. A. G. Radnaev, Y. O. Dudin, R. Zhao, H. H. Jen, S. D. Jenkins, A. Kuzmich, and T. A. B. Kennedy, “A quantum memory with telecom-wavelength conversion,” Nat. Phys. 6, 894–899 (2010). [CrossRef]
11. P. J. Bustard, D. G. England, K. Heshami, C. Kupchak, and B. J. Sussman, “Quantum frequency conversion with ultra-broadband tuning in a Raman memory,” Phys. Rev. A 95, 053816 (2017). [CrossRef]
12. C. E. Vollmer, C. Baune, A. Samblowski, T. Eberle, V. Händchen, J. Fiurášek, and R. Schnabel, “Quantum up-conversion of squeezed vacuum states from 1550 to 532 nm,” Phys. Rev. Lett. 112, 073602 (2014). [CrossRef]
13. D. Kong, Z. Li, S. Wang, X. Wang, and Y. Li, “Quantum frequency down-conversion of bright amplitude-squeezed states,” Opt. Express 22, 24192–24201 (2014). [CrossRef]
14. H. Takesue, “Single-photon frequency down-conversion experiment,” Phys. Rev. A 82, 013833 (2010). [CrossRef]
15. N. Curtz, R. Thew, C. Simon, N. Gisin, and H. Zbinden, “Coherent frequency-down-conversion interface for quantum repeaters,” Opt. Express 18, 22099–22104 (2010). [CrossRef]
16. B. Albrecht, P. Farrera, X. Fernandez-Gonzalvo, M. Cristiani, and H. de Riedmatten, “A waveguide frequency converter connecting rubidium based quantum memories to the telecom C-band,” Nat. Commun. 5, 3376 (2014). [CrossRef]
17. S. Zaske, A. Lenhard, C. A. Keßler, J. Kettler, C. Hepp, C. Arend, R. Albrecht, W. M. Schulz, M. Jetter, P. Michler, and C. Becher, “Visible-to-telecom quantum frequency conversion of light from a single quantum emitter,” Phys. Rev. Lett. 109, 147404 (2012). [CrossRef]
18. N. Maring, K. Kutluer, J. Cohen, M. Cristiani, M. Mazzera, P. M. Ledingham, and H. de Riedmatten, “Storage of up-converted telecom photons in a doped crystal,” New J. Phys. 16, 113021 (2014). [CrossRef]
19. P. Farrera, N. Maring, B. Albrecht, G. Heinze, and H. de Riedmatten, “Nonclassical correlations between a C-band telecom photon and a stored spin-wave,” Optica 3, 1019–1024 (2016). [CrossRef]
20. R. Ikuta, T. Kobayashi, K. Matsuki, S. Miki, T. Yamashita, H. Terai, T. Yamamoto, M. Koashi, T. Mukai, and N. Imoto, “Heralded single excitation of atomic ensemble via solid-state-based telecom photon detection,” Optica 3, 1279–1284 (2016). [CrossRef]
21. M. Bock, P. Eich, S. Kucera, M. Kreis, A. Lenhard, C. Becher, and J. Eschner, “High-fidelity entanglement between a trapped ion and a telecom photon via quantum frequency conversion,” arXiv: 1710.04866 (2017).
22. R. Ikuta, T. Kobayashi, T. Kawakami, S. Miki, M. Yabuno, T. Yamashita, H. Terai, M. Koashi, T. Mukai, T. Yamamoto, and N. Imoto, “Polarization insensitive frequency conversion for an atom-photon entanglement distribution via a telecom network,” arXiv: 1710.09150 (2017).
23. N. Maring, P. Farrera, K. Kutluer, M. Mazzera, G. Heinze, and H. de Riedmatten, “Photonic quantum state transfer between a cold atomic gas and a crystal,” Nature 551, 485–488 (2017). [CrossRef]
24. J. S. Pelc, L. Ma, C. R. Phillips, Q. Zhang, C. Langrock, O. Slattery, X. Tang, and M. M. Fejer, “Long-wavelength-pumped upconversion single-photon detector at 1550 nm: performance and noise analysis,” Opt. Express 19, 21445–21456 (2011). [CrossRef]
25. S. Ates, I. Agha, A. Gulinatti, I. Rech, M. T. Rakher, A. Badolato, and K. Srinivasan, “Two-photon interference using background-free quantum frequency conversion of single photons emitted by an InAs quantum dot,” Phys. Rev. Lett. 109, 147405 (2012). [CrossRef]
26. A. Lenhard, J. Brito, M. Bock, C. Becher, and J. Eschner, “Coherence and entanglement preservation of frequency-converted heralded single photons,” Opt. Express 25, 11187–11199 (2017). [CrossRef]
27. X. Fernandez-Gonzalvo, G. Corrielli, B. Albrecht, M. Grimau, M. Cristiani, and H. de Riedmatten, “Quantum frequency conversion of quantum memory compatible photons to telecommunication wavelengths,” Opt. Express 21, 19473–19487 (2013). [CrossRef]
28. J. S. Pelc, C. Langrock, Q. Zhang, and M. M. Fejer, “Influence of domain disorder on parametric noise in quasi-phase-matched quantum frequency converters,” Opt. Lett. 35, 2804–2806 (2010). [CrossRef]
29. J. S. Pelc, Q. Zhang, C. R. Phillips, L. Yu, Y. Yamamoto, and M. M. Fejer, “Cascaded frequency upconversion for high-speed single-photon detection at 1550 nm,” Opt. Lett. 37, 476–478 (2012). [CrossRef]
30. M. Afzelius, N. Gisin, and H. de Riedmatten, “Quantum memory for photons,” Phys. Today 68(12), 42–47 (2015). [CrossRef]
31. C. Laplane, P. Jobez, J. Etesse, N. Gisin, and M. Afzelius, “Multimode and long-lived quantum correlations between photons and spins in a crystal,” Phys. Rev. Lett. 118, 210501 (2017). [CrossRef]
32. K. Kutluer, M. Mazzera, and H. de Riedmatten, “Solid-state source of nonclassical photon pairs with embedded multimode quantum memory,” Phys. Rev. Lett. 118, 210502 (2017). [CrossRef]
33. B. Hensen, H. Bernien, A. E. Dréau, A. Reiserer, N. Kalb, M. S. Blok, J. Ruitenberg, R. F. L. Vermeulen, R. N. Schouten, C. Abellán, W. Amaya, V. Pruneri, M. W. Mitchell, M. Markham, D. J. Twitchen, D. Elkouss, S. Wehner, T. H. Taminiau, and R. Hanson, “Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres,” Nature 526, 682–686 (2015). [CrossRef]
34. H. Rütz, K. H. Luo, H. Suche, and C. Silberhorn, “Quantum frequency conversion between infrared and ultraviolet,” Phys. Rev. Appl. 7, 024021 (2017). [CrossRef]
35. M. Allgaier, V. Ansari, L. Sansoni, V. Quiring, R. Ricken, G. Harder, B. Brecht, and C. Silberhorn, “Highly efficient frequency conversion with bandwidth compression of quantum light,” Nat. Commun. 8, 14288 (2017). [CrossRef]
36. R. Ikuta, T. Kobayashi, S. Yasui, S. Miki, T. Yamashita, H. Terai, M. Fujiwara, T. Yamamoto, M. Koashi, M. Sasaki, Z. Wang, and N. Imoto, “Frequency down-conversion of 637 nm light to the telecommunication band for non-classical light emitted from NV centers in diamond,” Opt. Express 22, 11205–11214 (2014). [CrossRef]
37. R. Ikuta, Y. Kusaka, T. Kitano, H. Kato, T. Yamamoto, M. Koashi, and N. Imoto, “Wide-band quantum interface for visible-to-telecommunication wavelength conversion,” Nat. Commun. 2, 537 (2011). [CrossRef]
38. H. Rütz, K. H. Luo, H. Suche, and C. Silberhorn, “Towards a quantum interface between telecommunication and UV wavelengths: design and classical performance,” Appl. Phys. B 122, 1–8 (2016). [CrossRef]
39. R. V. Roussev, C. Langrock, J. R. Kurz, and M. M. Fejer, “Periodically poled lithium niobate waveguide sum-frequency generator for efficient single-photon detection at communication wavelengths,” Opt. Lett. 29, 1518–1520 (2004). [CrossRef]
40. M. Gündoğan, P. M. Ledingham, K. Kutluer, M. Mazzera, and H. de Riedmatten, “Solid state spin-wave quantum memory for time-bin qubits,” Phys. Rev. Lett. 114, 230501 (2015). [CrossRef]
41. D. Rieländer, K. Kutluer, P. M. Ledingham, M. Gündoğan, J. Fekete, M. Mazzera, and H. de Riedmatten, “Quantum storage of heralded single photons in a praseodymium-doped crystal,” Phys. Rev. Lett. 112, 1–5 (2014). [CrossRef]
42. A. Seri, A. Lenhard, D. Rieländer, M. Gündoğan, P. M. Ledingham, M. Mazzera, and H. de Riedmatten, “Quantum correlations between single telecom photons and a multimode on-demand solid-state quantum memory,” Phys. Rev. X 7, 021028 (2017). [CrossRef]
43. S. Zaske, A. Lenhard, and C. Becher, “Efficient frequency downconversion at the single photon level from the red spectral range to the telecommunications C-band,” Opt. Express 19, 12825–12836 (2011). [CrossRef]
44. D. Rieländer, A. Lenhard, M. Mazzera, and H. de Riedmatten, “Cavity enhanced telecom heralded single photons for spin-wave solid state quantum memories,” New J. Phys. 18, 123013 (2016). [CrossRef]
45. J. Fekete, D. Rieländer, M. Cristiani, and H. de Riedmatten, “Ultranarrow-band photon-pair source compatible with solid state quantum memories and telecommunication networks,” Phys. Rev. Lett. 110, 1–5 (2013). [CrossRef]
46. T. Kobayashi, R. Ikuta, S. Yasui, S. Miki, T. Yamashita, H. Terai, T. Yamamoto, M. Koashi, and N. Imoto, “Frequency-domain Hong–Ou–Mandel interference,” Nat. Photonics 10, 441–444 (2016). [CrossRef]
47. S. Clemmen, A. Farsi, S. Ramelow, and A. Gaeta, “Ramsey interference with single photons,” Phys. Rev. Lett. 117, 1–6 (2016). [CrossRef]
48. S. Fasel, O. Alibart, S. Tanzilli, P. Baldi, A. Beveratos, N. Gisin, and H. Zbinden, “High-quality asynchronous heralded single-photon source at telecom wavelength,” New J. Phys. 6, 163 (2004). [CrossRef]
49. M. Raymer, S. van Enk, C. McKinstrie, and H. McGuinness, “Interference of two photons of different color,” Opt. Commun. 283, 747–752 (2010). [CrossRef]
50. A. Dréau, A. Tcheborateva, A. Mahdaoui, C. Bonato, and R. Hanson, “Quantum frequency conversion to telecom of single photons from a nitrogen-vacancy center in diamond,” arXiv: 1801.03304 (2018).