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Correlating photons using the collective nonlinear response of atoms weakly coupled to an optical mode


Photons in a nonlinear medium can repel or attract each other, resulting in strongly correlated quantum many-body states1,2. Typically, such correlated states of light arise from the extreme nonlinearity granted by quantum emitters that are strongly coupled to a photonic mode2,3. However, unavoidable dissipation (such as photon loss) blurs nonlinear quantum effects when such approaches are used. Here, we generate strongly correlated photon states using only weak coupling and taking advantage of dissipation. An ensemble of non-interacting waveguide-coupled atoms induces correlations between simultaneously arriving photons through collectively enhanced nonlinear interactions. These correlated photons experience less dissipation than the uncorrelated ones. Depending on the number of atoms, we experimentally observe strong photon bunching or antibunching of the transmitted light. This realization of a collectively enhanced nonlinearity may turn out to be transformational for quantum information science and opens new avenues for generating non-classical light, covering frequencies from the microwave to the X-ray regime.

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Fig. 1: Schematics of the experimental process.
Fig. 2: Measured second-order correlation functions for four different mean atom numbers.
Fig. 3: Correlations at zero time delay versus the number of trapped atoms.

Data availability

The data supporting the findings of this study are available from J.V. ( or A.R. on reasonable request.

Code availability

The code used for modelling the data is available from S.M. on reasonable request.


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We acknowledge financial support by the European Commission under the projects ErBeStA (No. 800942) and the ERC grant NanoQuaNt, by the Austrian Science Fund (DK CoQuS Project No. W 1210-N16), by the DFG through CRC 1227 DQ-mat (project A06) and by the Danish National Research Foundation (Center of Excellence Hy-Q).

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Authors and Affiliations



K.H., S.M. and A.S.S. made the theory predictions and were responsible for modelling the data. J.H., A.S.P., A.R., S.R., P.S. and J.V. contributed to the design and the setting-up of the experiment. A.S.P. and J.H. performed the experiment. A.S.P. together with S.M. and J.V. was responsible for analysing the data. All authors contributed to the writing of the manuscript.

Corresponding authors

Correspondence to Sahand Mahmoodian or Arno Rauschenbeutel.

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The authors declare no competing interests.

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Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Second-order correlation functions for different atom numbers.

The measured correlation functions are shown in blue and the theoretical predictions (see main text) in orange.

Extended Data Fig. 2 Experimental sequence.

First, a MOT and a optical molasses phase are used to load atoms into the nanofibre-based trap (i). This is followed by a 10 ms ramp of the power of the red-detuned trapping field to increase β (ii) and a second molasses phase to re-cool the atoms (iii). To measure the OD of the ensemble, we then sweep the probe laser frequency across the atomic resonance and measure its transmission (iv). The main part of the experimental sequence consists of 350 repetitions of alternating probe and cooling pulses (v). Afterwards, we again measure the OD to check for atom losses (vi), remove the atoms from the trap (vii), and measure transmission through the bare fibre for calibration (viii).

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Prasad, A.S., Hinney, J., Mahmoodian, S. et al. Correlating photons using the collective nonlinear response of atoms weakly coupled to an optical mode. Nat. Photonics 14, 719–722 (2020).

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