Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Letter
  • Published:

Experimental determination of entanglement with a single measurement


Nearly all protocols requiring shared quantum information1—such as quantum teleportation2 or key distribution3—rely on entanglement between distant parties. However, entanglement is difficult to characterize experimentally. All existing techniques for doing so, including entanglement witnesses4,11,12 or Bell inequalities5, disclose the entanglement of some quantum states but fail for other states; therefore, they cannot provide satisfactory results in general. Such methods are fundamentally different from entanglement measures that, by definition, quantify the amount of entanglement in any state. However, these measures suffer from the severe disadvantage that they typically are not directly accessible in laboratory experiments. Here we report a linear optics experiment in which we directly observe a pure-state entanglement measure, namely concurrence6. Our measurement set-up includes two copies of a quantum state: these ‘twin’ states are prepared in the polarization and momentum degrees of freedom of two photons, and concurrence is measured with a single, local measurement on just one of the photons.

This is a preview of subscription content, access via your institution

Access options

Buy this article

Prices may be subject to local taxes which are calculated during checkout

Figure 1: Experimental set-up for the measurement of entanglement using two copies of the quantum state.
Figure 2: Count rates.
Figure 3: Experimental values of concurrence.

Similar content being viewed by others


  1. Nielsen, M. & Chuang, I. Quantum Computation and Quantum Information (Cambridge Univ. Press, Cambridge, UK, 2000)

    MATH  Google Scholar 

  2. Bennett, C. H. et al. Teleporting an unknown quantum state via dual classical and Einstein-Podolsky-Rosen channels. Phys. Rev. Lett. 70, 1895–1899 (1993)

    Article  ADS  MathSciNet  CAS  Google Scholar 

  3. Ekert, A. K. Quantum cryptography based on Bells theorem. Phys. Rev. Lett. 67, 661–663 (1991)

    Article  ADS  MathSciNet  CAS  Google Scholar 

  4. Horodecki, M., Horodecki, P. & Horodecki, R. Separability of mixed states: necessary and sufficient conditions. Phys. Lett. A 223, 1–8 (1996)

    Article  ADS  MathSciNet  CAS  Google Scholar 

  5. Bell, J. S. On the Einstein Podolsky Rosen paradox. Physics 1, 195–200 (1964)

    Article  MathSciNet  Google Scholar 

  6. Bennet, C. H., DiVincenzo, D. P., Smolin, J. A. & Wootters, W. K. Mixed-state entanglement and quantum error correction. Phys. Rev. A 54, 3824–3851 (1996)

    Article  ADS  MathSciNet  Google Scholar 

  7. Wootters, W. K. Entanglement of formation of an arbitrary state of two qubits. Phys. Rev Lett. 80, 2245–2248 (1998)

    Article  ADS  CAS  Google Scholar 

  8. Vidal, G. & Werner, R. F. Computable measure of entanglement. Phys. Rev. A 65, 032314 (2002)

    Article  ADS  Google Scholar 

  9. Sancho, J. M. G. & Huelga, S. F. Measuring the entanglement of bipartite pure states. Phys. Rev. A 61, 042303 (2000)

    Article  ADS  MathSciNet  Google Scholar 

  10. White, A., James, D., Eberhard, P. & Kwiat, P. Nonmaximally entangled states: Production, characterization, and utilization. Phys. Rev. Lett. 83, 3103–3107 (1990)

    Article  ADS  Google Scholar 

  11. Häffner, H. et al. Scalable multiparticle entanglement of trapped ions. Nature 438, 643–646 (2005)

    Article  ADS  Google Scholar 

  12. Leibfried, D. et al. Creation of a six-atom ‘Schrödinger cat’ state. Nature 438, 639–642 (2005)

    Article  ADS  CAS  Google Scholar 

  13. Horodecki, P. Measuring quantum entanglement without prior state reconstruction. Phys. Rev. Lett. 90, 167901 (2003)

    Article  ADS  MathSciNet  Google Scholar 

  14. Mintert, F., Kuś, M. & Buchleitner, A. Concurrence of mixed multi-partite quantum states. Phys. Rev. Lett. 95, 260502 (2005)

    Article  ADS  MathSciNet  Google Scholar 

  15. Brun, T. A. Measuring polynomial functions of states. Quant. Inform. Comput. 4, 401–408 (2004)

    MathSciNet  MATH  Google Scholar 

  16. Kwiat, P. G. Hyper entangled states. J. Mod. Opt. 44, 2173–2184 (1997)

    Article  ADS  MathSciNet  Google Scholar 

  17. Barbieri, M., Cinelli, C., Mataloni, P. & Martini, F. D. Polarization-momentum hyperentangled states: Realization and characterization. Phys. Rev. A 72, 052110 (2005)

    Article  ADS  Google Scholar 

  18. Boschi, D., Branca, S., DeMartini, F., Hardy, L. & Popescu, S. Experimental realization of teleporting an unknown pure quantum state via dual classical and Einstein-Podolsky-Rosen channels. Phys. Rev. Lett. 80, 1121–1125 (1998)

    Article  ADS  MathSciNet  CAS  Google Scholar 

  19. Englert, B.-G., Kurtsiefer, C. & Weinfurter, H. Universal unitary gate for single-photon two-qubit states. Phys. Rev. A 63, 032303 (2003)

    Article  ADS  Google Scholar 

  20. Kwiat, P. G., Waks, E., White, A. G., Appelbaum, I. & Eberhard, P. H. Ultrabright source of polarization-entangled photons. Phys. Rev. A 60, R773–R776 (1999)

    Article  ADS  CAS  Google Scholar 

  21. Rarity, J. & Tapster, P. Experimental violation of Bell's inequality based on phase and momentum. Phys. Rev. Lett. 64, 2495–2498 (1990)

    Article  ADS  CAS  Google Scholar 

  22. Fiorentino, M. & Wong, F. N. C. Deterministic controlled-not gate for single-photon two-qubit quantum logic. Phys. Rev. Lett. 93, 070502 (2004)

    Article  ADS  Google Scholar 

Download references


We acknowledge support from the Brazilian agencies CNPq, CAPES, PRONEX, FUJB and FAPERJ. This work was performed as part of the Brazilian Millennium Institute for Quantum Information, and was supported by a fellowship within the postdoctoral programme of the German Academic Exchange Service (DAAD) as well as by a Feodor Lynen fellowship of the Alexander von Humboldt foundation. We are indebted to C. H. Monken for discussions.

Author information

Authors and Affiliations


Corresponding author

Correspondence to S. P. Walborn.

Ethics declarations

Competing interests

Reprints and permissions information is available at The authors declare no competing financial interests.

Rights and permissions

Reprints and permissions

About this article

Cite this article

Walborn, S., Souto Ribeiro, P., Davidovich, L. et al. Experimental determination of entanglement with a single measurement. Nature 440, 1022–1024 (2006).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing