We report on double-resonant highly efficient sum-frequency generation in the blue range. The system consists of a 10-mm-long periodically poled KTP crystal placed in a double-resonant bowtie cavity and pumped by a fiber laser at 1064.5 nm and a Ti:sapphire laser at 849.2 nm. An optical power of 375 mW at 472.4 nm in a TEM00 mode was generated with pump powers of 250 mW at 849.2 nm and 200 mW at 1064.5 nm coupled into the double-resonant ring resonator with 88% mode-matching. The resulting internal conversion efficiency of 95(±3)% of the photons mode-matched to the cavity constitutes, to the best of our knowledge, the highest overall achieved quantum conversion efficiency using continuous-wave pumping. Very high conversion efficiency is rendered possible due to very low intracavity loss on the level of 0.3% and high nonlinear conversion coefficient up to 0.045(0.015) W −1 . Power stability measurements performed over one hour show a stability of 0.8%. The generated blue light can be tuned within 5 nm around the center wavelength of 472.4 nm, limited by the phase-matching of our nonlinear crystal. This can however be expanded to cover the entire blue spectrum (420 nm to 510 nm) by proper choice of nonlinear crystals and pump lasers. Our experimental results agree very well with analytical and numerical simulations taking into account cavity impedance matching and depletion of the pump fields.

We report a high-purity Einstein-Podolsky-Rosen (EPR) state between light modes with the wavelengths separated by more than 200 nm. We demonstrate highly efficient EPR-steering between the modes with the product of conditional variances $${{{{{{{{\mathcal{E}}}}}}}}}^{2}=0.11\pm 0.01\ll 1$$ E 2 = 0.11 ± 0.01 ≪ 1 . The modes display − 7.7 ± 0.5 dB of two-mode squeezing and an overall state purity of 0.63 ± 0.16. EPR-steering is observed over five octaves of sideband frequencies from RF down to audio-band. The demonstrated combination of high state purity, strong quantum correlations, and extended frequency range enables new matter-light quantum protocols.

We demonstrate double-resonant sum-frequency generation of coherent blue light in a bow-tie cavity with record high quantum conversion efficiency of 95% of mode-matched nearinfrared photons and a high output beam quality.

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We report continuous variable entanglement between light modes of different colors (852nm and 1064nm). Entanglement is demonstrated from radio-frequency down to the acoustic frequency range. The high-purity state violates the separability limit by -7.5 dB.