When two indistinguishable single photons are fed into the two input ports of a beam splitter, the photons will coalesce and leave together from the same output port. This is a quantum interference effect, which occurs because two possible paths—in which the photons leave by different output ports—interfere destructively. This effect was first observed in parametric downconversion1 (in which a nonlinear crystal splits a single photon into two photons of lower energy), then from two separate downconversion crystals2, as well as with single photons produced one after the other by the same quantum emitter3,4,5,6. With the recent developments in quantum information research, much attention has been devoted to this interference effect as a resource for quantum data processing using linear optics techniques2,7,8,9,10,11. To ensure the scalability of schemes based on these ideas, it is crucial that indistinguishable photons are emitted by a collection of synchronized, but otherwise independent sources. Here we demonstrate the quantum interference of two single photons emitted by two independently trapped single atoms, bridging the gap towards the simultaneous emission of many indistinguishable single photons by different emitters. Our data analysis shows that the observed coalescence is mainly limited by wavefront matching of the light emitted by the two atoms, and to a lesser extent by the motion of each atom in its own trap.
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We acknowledge support from the European Union through the Integrated Project ‘SCALA’. J.D. was funded by Research Training Network ‘CONQUEST’. M.P.A.J. was supported by a Marie Curie fellowship.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
This file contains discussion about: the normalized height of the residual peak for non-interfering photons; alignment of the optical system and limits on spatial overlap; and the effect of the inhomogeneous broadening on the shape of the residual peak at zero delay, including one figure. (PDF 60 kb)
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Beugnon, J., Jones, M., Dingjan, J. et al. Quantum interference between two single photons emitted by independently trapped atoms. Nature 440, 779–782 (2006). https://doi.org/10.1038/nature04628
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