Phys. Rev. X 7, 031012 (2017)

Young-Sik Ra and co-workers from Collège de France have developed a mode-tunable coherent single-photon subtractor based on sum-frequency generation. A femtosecond laser (central wavelength of 795 nm) was divided into two as input and gate beams with Hermite–Gaussian (HG) time–frequency modes. The two beams were focused onto a BiBO crystal to generate a frequency up-converted photon via sum-frequency generation. When the up-converted photon was detected by a single-photon detector, subtraction of a single photon from the input beam was heralded. The choice of the gate-beam mode determined the time–frequency modes of single-photon subtraction. To characterize single-photon subtraction with various choice of the gate-beam modes, the subtraction matrix of each single-photon subtraction was measured by employing coherent-state quantum process tomography. The subtraction matrices in the HG modes directly showed the coherence between different HG modes. A high mode selectivity (typically larger than 0.9) and low imperfections (dark count contribution around 1% and optical loss around 2%) of the single-photon subtractor showed its direct applicability to generate multimode non-Gaussian states.