A two-level atom can generate a strong many-body interaction with light under pulsed excitation1,2,3. The best known effect is single-photon generation, where a short Gaussian laser pulse is converted into a Lorentzian single-photon wavepacket4,5. 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 increases6,7. Here, we provide an intuitive explanation for these oscillations using a quantum trajectory approach8 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 plagues5,9 on-demand single-photon sources while suggesting the possibility of producing new multi-photon states.

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The authors thank A. Rasmussen for productive and helpful discussions in framing the context of this work. We gratefully acknowledge financial support from the National Science Foundation (Division of Materials Research—Grant No. 1503759), the DFG via the Nanosystems Initiative Munich, the BMBF via Q.com (Project No. 16KIS0110) and BaCaTeC. K.A.F. acknowledges support from the Lu Stanford Graduate Fellowship and the National Defense Science and Engineering Graduate Fellowship. J.W. acknowledges support from the PhD programme ExQM of the Elite Network of Bavaria. C.D. acknowledges support from the Andreas Bechtolsheim Stanford Graduate Fellowship. J.V. gratefully acknowledges support from the TUM Institute of Advanced Study.

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    • Kevin A. Fischer
    •  & Lukas Hanschke

    These authors contributed equally to this work.


  1. E. L. Ginzton Laboratory, Stanford University, Stanford, California 94305, USA

    • Kevin A. Fischer
    • , Constantin Dory
    •  & Jelena Vučković
  2. Walter Schottky Institut and Physik Department, Technische Universität München, 85748 Garching, Germany

    • Lukas Hanschke
    • , Jakob Wierzbowski
    • , Tobias Simmet
    • , Jonathan J. Finley
    •  & Kai Müller


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K.A.F. performed the theoretical work and modelling. L.H., T.S., J.W. and K.M. performed the experiments. C.D. performed trial experiments. J.V. and J.J.F. provided expertise. K.M. organized the collaboration and supervised the experiments. All authors participated in the discussion and understanding of the results.

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The authors declare no competing financial interests.

Corresponding authors

Correspondence to Kevin A. Fischer or Kai Müller.

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