Few-photon coherent nonlinear optics with a single molecule

Photons are efficiently funnelled into a single molecule if they are nearly resonant with the sharp molecular transition. In this condition, the coherent nonlinear optical effect can be induced with only a few photons without high-finesse cavities. The pioneering experiments in linear spectroscopy were performed using flames in the 1800s, but nonlinear optical measurements had to wait until lasers became available in the twentieth century. Because the nonlinear cross-section of materials is very small1,2, macroscopic bulk samples and pulsed lasers are usually used. Numerous efforts have explored coherent nonlinear signal generation from individual nanoparticles3,4,5 or small atomic ensembles6,7,8 with millions of atoms. Experiments on a single semiconductor quantum dot have also been reported, albeit with a very small yield9. Here, we report the coherent nonlinear spectroscopy of a single molecule under continuous-wave single-pass illumination and the switching of a laser beam by on the order of ten pump photons. The sharp molecular transitions and efficient photon–molecule coupling at a tight focus10 allow for optical switching with less than a handful of pump photons and are thus promising for applications in quantum engineering11.