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A topological source of quantum light


Quantum light is characterized by distinctive statistical distributions that are possible only because of quantum mechanical effects. For example, single photons and correlated photon pairs exhibit photon number distributions with variance lower than classically allowed limits. This enables high-fidelity transmission of quantum information and sensing with lower noise than possible with classical light sources1,2. Most quantum light sources rely on spontaneous parametric processes such as down-conversion and four-wave mixing2. These processes are mediated by vacuum fluctuations of the electromagnetic field. Therefore, by manipulating the electromagnetic mode structure, for example with dispersion-engineered nanophotonic systems, the spectrum of generated photons can be controlled3,4,5,6,7. However, disorder, which is ubiquitous in nanophotonic fabrication, causes device-to-device spectral variations8,9,10,11. Here we realize topologically robust electromagnetic modes and use their vacuum fluctuations to create a quantum light source in which the spectrum of generated photons is much less affected by fabrication-induced disorder. Specifically, we use the topological edge states realized in a two-dimensional array of ring resonators to generate correlated photon pairs by spontaneous four-wave mixing and show that they outperform their topologically trivial one-dimensional counterparts in terms of spectral robustness. We demonstrate the non-classical nature of the generated light and the realization of a robust source of heralded single photons by measuring the conditional antibunching of photons, that is, the reduced likelihood of photons arriving together compared to thermal or laser light. Such topological effects, which are unique to bosonic systems, could pave the way for the development of robust quantum photonic devices.

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Fig. 1: Schematic of the experimental set-up.
Fig. 2: Spectral distribution of the generated photons.
Fig. 3: Source characterization.
Fig. 4: Robustness of spectral correlations between pump and signal photons.
Fig. 5: Similarity scaling as a function of device size.

Data availability

The data that support the findings of this study are available from the corresponding author on reasonable request.


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This research was supported by AFOSR-MURI FA9550-14-1-0267, YIP-ONR, Sloan Foundation and the Physics Frontier Center at the Joint Quantum Institute. We thank V. V. Orre for help with the experimental set-up, A. Karasahin for help with the SEM, T. Huber and D. Englund for discussions and Q. Quraishi for providing the nanowire detectors.

Reviewer information

Nature thanks V. Peano and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information




S.M. and M.H. conceived the idea. S.M. performed the numerical simulations and the experimental measurements. E.A.G. contributed to source characterization. M.H. supervised the project. All authors contributed to analysing the data and writing the manuscript.

Corresponding author

Correspondence to Sunil Mittal.

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The authors declare no competing interests.

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Extended data figures and tables

Extended Data Fig. 1 Spectral correlations.

ac, Measured Γ(ωsωp) on three different 2D devices; dg, measured Γ(ωsωp) on four different 1D devices, in addition to those presented in Fig. 3. Clockwise edge bands for the 2D devices and mid-band for the 1D devices are highlighted.

Extended Data Fig. 2 Transmission spectra of 2D and 1D devices.

a, b, Measured transmission spectra for (a) different 2D and (b) different 1D devices. The shaded regions highlight the edge and the bulk bands for the 2D system and the mid-band for the 1D system. For the 2D devices, the clockwise and the anticlockwise edge bands show reduced variations in the transmission compared with that in the bulk band. These spectra have been shifted along the frequency axis to superpose them, using an algorithm based on transmission and delay measurements, as detailed in ref. 23.

Extended Data Fig. 3 Transmission spectrum of a 2D device across three FSRs.

Measured transmission spectrum in the pump, signal and idler FSRs, corresponding to Fig. 2. ∆ν is the frequency relative to the longitudinal mode resonance, and Ω is the FSR. The shape of the transmission spectrum in these FSRs is almost identical. The small variation in the overall transmission across bands is mainly because of the frequency response of the grating couplers.

Extended Data Fig. 4 Joint spectral intensity.

a, The measured Γ(ωsωp) (see Fig. 2): that is, the intensity of generated signal photons at frequency ωs as a function of pump frequency, ωp. Each point on this plot represents a particular ωs and ωp. Using energy conservation, we can calculate the corresponding idler frequency at each point as ωi = 2ωpωs. Therefore, we can easily rescale the y axis of the plot and calculate the joint-spectral intensity (JSI; see refs 11,29) between the signal and idler frequencies, as shown in b. Note that this rescaling works only for a continuous-wave pump because for a pulsed pump source, the above energy conservation relation holds only up to the spectral bandwidth of the pump, signal and idler photons. Also, this measurement inherently assumes that the generated signal and idler photons are correlated. Using CAR and direct measurements of the signal and idler spectra (in Figs. 2, 3), we verified that the signal and idler photons are indeed correlated. The main advantage of such a spectral correlation measurement between the pump and the signal (or idler) photons is that it is fast and, for a continuous-wave pump, is equivalent to the JSI measurement.

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Mittal, S., Goldschmidt, E.A. & Hafezi, M. A topological source of quantum light. Nature 561, 502–506 (2018).

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  • Spontaneous Four-wave Mixing (SFWM)
  • Correlated Photon Pairs
  • Topological Edge States
  • Quantum Light Sources
  • Ring Resonance

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