Direct observation of photon bunching in thermal light in quantum Fourier transform spectroscopy

In quantum mechanics, photons are bosons -- there is no restriction on the number of them that occupy the same quantum state, so many of them can bunch together, and this is well known as photon bunching effect. However, photon bunching and multiphoton interference of bunched photons have never been directly observed from a broadband thermal chaotic light source. Nevertheless, it has been known that the interference intensity of a number N of bunched photons oscillates with a wavelength of N times shorter than the wavelength of the constituent single photons, which should be observable in Fourier transform spectroscopy. A quantum Fourier transform spectrometer was made based on a Michelson interferometer and a diffraction spectrometer to observe the photon bunching from multiphoton interference of the bunched photons in thermal light. The results demonstrates 2 to 12 bunched photons were observed simultaneously from thousands of wavelengths in the light of a tungsten halogen lamp. The experiment discloses some interesting properties of the bunched photons. The results will provide new insights for advancing the theories of quantum optics, and the new technology opens the way for new precise measurement of photon bunching and multiphoton interference of bunched photons that are in all kinds of light sources or that are generated with quantum entangled photons.