Signatures of two-photon pulses from a quantum two-level system

An excited two-level system emits a single photon, but in special circumstances it can emit two. The reason for this unexpected two-photon emission lies with modified Rabi oscillations. A two-level atom can generate a strong many-body interaction with light under pulsed excitation 1 , 2 , 3 . The best known effect is single-photon generation, where a short Gaussian laser pulse is converted into a Lorentzian single-photon wavepacket 4 , 5 . However, recent studies suggested that scattering of intense laser fields off a two-level atom may generate oscillations in two-photon emission that come out of phase with the Rabi oscillations, as the power of the pulse increases 6 , 7 . Here, we provide an intuitive explanation for these oscillations using a quantum trajectory approach 8 and show how they may preferentially result in emission of two-photon pulses. Experimentally, we observe the signatures of these oscillations by measuring the bunching of photon pulses scattered off a two-level quantum system. Our theory and measurements provide insight into the re-excitation process that plagues 5 , 9 on-demand single-photon sources while suggesting the possibility of producing new multi-photon states.