Highly efficient frequency downconversion at the single photon level

Abstract

Much recent progress has been achieved in the fabrication of single photon emitters based on color centers in diamond, e.g., silicon-vacancy (SiV) centers emitting at λ s = 738nm [1]. However, efficient single photon transmission in future quantum networks requires wavelengths in the telecom bands around λ 0 = 1310nm or λ c = 1550nm. In order to bridge this wavelength gap we investigate frequency downconversion according to λ s −1 − λ p −1 ≈ λ c −1, where λ p denotes the wavelength of a strong pump field. We here report on work achieving a conversion efficiency that is at least 40 times higher compared to similar experiments that have used optical powers at the single photon level [2,3]. Our setup is shown in Fig. 1(a). To emulate the single photon source at λ s an attenuated continuous wave Ti:Sapphire laser (Ti:Sa) together with a pulse picker is used. A home-built tunable continuous wave optical parametric oscillator generates the pump light at λ p = 1403nm. The two fields are coupled into a temperature controlled ridge waveguide (WG) made of periodically poled ZnO:LiNbO 3 . It is possible to excite only the fundamental WG mode at λ s (see inset in Fig. 1(a)) which in combination with the strong confinement in the WG provides a good spatial overlap between the interacting modes. Using a spectral filtering stage (prism, pinhole, bandpass filters) one of the three beams can be selected at the waveguide output and coupled into an optical fiber for further analysis using a single photon avalanche diode (SPAD) or a grating spectrometer, respectively.