Hybrid quantum networks will be based on nodes that operate at different wavelengths, requiring quantum channel standardization via quantum frequency conversion (QFC). QFC is typically based on highly efficient sum- or difference-frequency generation in second-order nonlinear materials, such as periodically poled lithium niobate waveguides. The presence of the strong pump beam in such a nonlinear medium leads to unwanted nonlinear processes that produce noise. One of these noise processes is spontaneous Raman scattering (SRS). Typically, the pump is chosen to be the longest wavelength in the second-order nonlinear mixing process so that noise photons at the signal wavelength are produced by the less efficient anti-Stokes Raman scattering process rather than the Stokes scattering process. Since SRS is a temperature-dependent process, lowering the temperature reduces the Raman-scattered photons. We discuss the theory of temperature-dependent Raman scattering and present experimental results of the temperature dependence of dark count rates in a guided-wave QFC device.
Hybrid quantum networks will be based on nodes that operate at different wavelengths, requiring quantum channel standardization via quantum frequency conversion (QFC). QFC is typically based on highly efficient sum- or difference-frequency generation in second-order nonlinear materials, such as periodically poled lithium niobate waveguides. The presence of the strong pump beam in such a nonlinear medium leads to unwanted nonlinear processes that produce noise. One of these noise processes is spontaneous Raman scattering (SRS). Typically, the pump is chosen to be the longest wavelength in the second-order nonlinear mixing process so that noise photons at the signal wavelength are produced by the less efficient anti-Stokes Raman scattering process rather than the Stokes scattering process. Since SRS is a temperature-dependent process, lowering the temperature reduces the Raman-scattered photons. We discuss the theory of temperature-dependent Raman scattering and present experimental results of the temperature dependence of dark count rates in a guided-wave QFC device.
Quantum frequency conversion is important in quantum networks to interface nodes operating at different wavelengths and to enable long-distance quantum communication using telecommunications wavelengths. Unfortunately, frequency conversion in actual devices is not a noise-free process. One main source of noise is spontaneous Raman scattering, which can be reduced by lowering the device operating temperature. We explore frequency conversion of 1554 nm photons to 837 nm using a 1813 nm pump in a periodically poled lithium niobate waveguide device. By reducing the temperature from 85°C to 40°C, we show a three-fold reduction in dark count rates, which is in good agreement with theory.
High-efficiency quantum frequency conversion (QFC) for group-IV color centers, such as silicon vacancies (SiV), is required for long-distance quantum networking protocols. Performance of QFC depends on the choices of network and pump wavelengths and the choice between bulk crystal and waveguide, since these directly determine the dominant noise processes. These furthermore lead to fiber-based or free-space, single- or multi-pass setups with different complexities, power requirements, losses, and hence, external efficiencies. Here, we present a QFC device design for SiV QFC with simultaneous state-of-the-art performance and simple setup. Namely, by choosing the C-band pump wavelength of λ p = 1561 nm, which leads to an E-band network wavelength of λ n = 1398 nm, we remain in the anti-Stokes noise regime in a one-stage, single-pass conversion scheme. The high-quality erbium-doped fiber amplifiers in this band aid in maximizing the internal nonlinear process. We measure an external (internal) photon conversion efficiency of 44.5 ± 1.8 % (8.5 ± 1.8 %) at an external (internal) pump power of 1.51 W (0.97 W), limited by fiber coupling efficiencies. This is an increase in efficiency by 9 % (25 %) compared to prior demonstrations for SiV QFC, which were limited by factors such as internal device efficiencies, limited available pump power, and losses. We measure a pump-generated noise spectral density of 4913 counts/s/nm, which can be attributed to the anti-Stokes Raman scattering, in line with other one-stage demonstrations, though higher by an order of magnitude than that obtained in two-stage conversion. Without extra filtering, we expect this to lead to a networking photon signal-to-noise ratio of above 7000 for cavity-integrated SiVs in emission-based schemes.
Quantum frequency conversion (QFC) will be an indispensable ingredient in future quantum technologies. For example, large-scale fibre-based quantum networks will require QFC to interconnect heterogeneous building blocks like emitters, channels, memories and detectors. The performance of existing QFC devices - typically realised in periodically-poled nonlinear crystals - is often severely limited by parasitic noise that arises when the pump wavelength lies between the inter-converted wavelengths. Here we comprehensively investigate the noise spectrum of a QFC device pumped by a CW 1064 nm laser. The converter was realised as a bulk periodically-poled potassium titanyl phosphate (ppKTP) crystal quasi-phase-matched for conversion between 637 nm and 1587 nm, which was also polished and coated to resonantly enhance the pump field by a factor of 50. While Raman scattering dominates the noise background from 1140 nm to 1330 nm, at larger energy shifts (beyond 60 THz), parasitic spontaneous parametric down-conversion (SPDC) is the strongest noise source. Further, the noise spectrum was contaminated by a regular succession of narrow-band peaks, which we attribute to a heretofore unidentified higher-order counter-propagating SPDC processes - with quasi-phase-matching orders up to 44 evident in our measurements. This work provides a comprehensive overview of the limiting noise sources in QFC devices that use quasi-phase-matched crystals and will prove an invaluable resource in guiding their future development.
Quantum frequency conversion (QFC) will be an indispensable ingredient in future quantum technologies. For example, large-scale fibre-based quantum networks will require QFC to interconnect heterogeneous building blocks like emitters, channels, memories, and detectors. The performance of existing QFC devices – typically realized in periodically poled nonlinear crystals – is often severely limited by parasitic noise that arises when the pump wavelength lies between the inter-converted wavelengths. Here we comprehensively investigate the noise spectrum of a QFC device pumped by a CW 1064 nm laser. The converter was realized as a bulk periodically poled potassium titanyl phosphate (ppKTP) crystal quasi-phase-matched for conversion between 637 nm and 1587 nm, which was also polished and coated to resonantly enhance the pump field by a factor of 50. While Raman scattering dominates the noise background from 1140 nm to 1330 nm, at larger energy shifts (beyond 60 THz), parasitic spontaneous parametric down-conversion (SPDC) is the strongest noise source. Further, the noise spectrum was contaminated by a regular succession of narrow-band peaks, which we attribute to a heretofore unidentified higher-order counter-propagating SPDC processes – with quasi-phase-matching orders up to 44 evident in our measurements. This work provides a comprehensive overview of the limiting noise sources in QFC devices that use quasi-phase-matched crystals and will prove an invaluable resource in guiding their future development.
Quantum frequency conversion (QFC) enables optical interfaces between dissimilar quantum systems, but suppressing noise while maintaining high conversion efficiency remains challenging. Here, we demonstrate that adapted poling in thin-film lithium niobate (TFLN) waveguides reduces the pump-power requirement while shifting the dominant noise mechanism and its power scaling. In 2-cm-long waveguides, adapted poling enables maximum conversion efficiency at an on-chip pump power of 19±0.8~mW, reducing the required pump power by 8~dB and the total conversion noise by 14~dB relative to conventional periodic poling at their respective maximum-conversion operating points. Controlled measurements with different pump conditions and device configurations reveal a transition from predominantly quadratic noise scaling in periodically poled devices to predominantly linear scaling in adapted-poling devices. This transition accompanies a shift from noise dominated by cascaded second-harmonic generation and Stokes spontaneous Raman scattering to direct anti-Stokes spontaneous Raman scattering. These results establish adapted poling as an effective approach to suppressing nonlinear noise, enabling low-noise integrated frequency converters for quantum networks.
Quantum frequency conversion (QFC) is essential for bridging the spectral gap between stationary qubits and low-loss optical communication channels. Practicable QFC interfaces connecting ultraviolet and telecommunication bands are of great significance but have remained elusive, constrained by the dual challenges of a high-performance frequency converter and effective noise suppression in short-wavelength pumping configurations. We address this by demonstrating the phase-matching mechanism with domain defects and differential-tuning behavior of the spontaneous parametric down-conversion noises. Based on a quantitative analysis of performance degradation induced by domain defects, we propose specific fabrication guidelines and experimentally achieve an ultrahigh normalized conversion efficiency of 839%/(W cm^{2}) for the fundamental guided mode in thin-film lithium niobate waveguides. Meanwhile, we reveal a new noise control mechanism for short-wavelength pumping by utilizing the differential-tuning behaviors of difference-frequency generation and spontaneous parametric down-conversion processes. Then a single-stage QFC from 393 nm to the telecom C band with a record-high external efficiency of 28.8% and an ultralow noise level of 35 counts per second is achieved experimentally. The proposed mechanisms would enable other QFCs with higher efficiency and lower noise, advancing the development of scalable quantum networks. Furthermore, it holds significant potential for driving progress in both classical and integrated nonlinear photonics.