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Experimental delayed-choice entanglement swapping

Nature Physics volume 8, pages 479484 (2012) | Download Citation

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Abstract

Motivated by the question of which kind of physical interactions and processes are needed for the production of quantum entanglement, Peres has put forward the radical idea of delayed-choice entanglement swapping. There, entanglement can be ‘produced a posteriori, after the entangled particles have been measured and may no longer exist’. Here, we report the realization of Peres’s gedanken experiment. Using four photons, we can actively delay the choice of measurement—implemented through a high-speed tunable bipartite-state analyser and a quantum random-number generator—on two of the photons into the time-like future of the registration of the other two photons. This effectively projects the two already registered photons onto one of two mutually exclusive quantum states in which the photons are either entangled (quantum correlations) or separable (classical correlations). This can also be viewed as ‘quantum steering into the past’.

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Change history

  • 26 April 2012

    In the version of this Article originally published online, the definition of the witness operator given in the paragraph after equation (4) was incorrect. This error has been corrected in all versions of the Article.

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Acknowledgements

We are grateful to N. Tetik and A. Qarry for help during the early stages of the experiment, and M. Aspelmeyer and P. Walther for fruitful discussions. We acknowledge support from the European Commission, Q-ESSENCE (No. 248095), ERC Advanced Senior Grant (QIT4QAD) and the John Templeton Foundation, as well as SFB-FOQUS and the Doctoral Program CoQuS of the Austrian Science Fund (FWF).

Author information

Author notes

    • Stefan Zotter
    • , Johannes Kofler
    •  & Thomas Jennewein

    Present addresses: Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Waehringer Guertel 18-20, A-1090 Vienna, Austria (S.Z.); Max Planck Institute of Quantum Optics, Hans-Kopfermann-Str. 1, 85748 Garching/Munich, Germany (J.K.); Institute for Quantum Computing and Department of Physics and Astronomy, University of Waterloo, 200 University Ave W., Waterloo, Ontario, Canada N2L3G1 (T.J.)

Affiliations

  1. Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, A-1090 Vienna, Austria

    • Xiao-song Ma
    • , Stefan Zotter
    • , Johannes Kofler
    • , Rupert Ursin
    • , Thomas Jennewein
    • , Časlav Brukner
    •  & Anton Zeilinger
  2. Vienna Center for Quantum Science and Technology (VCQ), Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria

    • Xiao-song Ma
    •  & Anton Zeilinger
  3. Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria

    • Časlav Brukner
    •  & Anton Zeilinger

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Contributions

X-s.M. designed and carried out the experiment and analysed data. S.Z. provided experimental assistance. J.K. provided the theoretical analysis and analysed data. R.U. provided experimental and conceptual assistance. T.J. conceived the research, planned and performed the experiment and analysed data. Č.B. provided theoretical suggestions and analysis. A.Z. conceived the research, designed the experiment and supervised the project. All authors wrote the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Xiao-song Ma or Anton Zeilinger.

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DOI

https://doi.org/10.1038/nphys2294

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