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Entanglement of single-atom quantum bits at a distance


Quantum information science involves the storage, manipulation and communication of information encoded in quantum systems, where the phenomena of superposition and entanglement can provide enhancements over what is possible classically1,2. Large-scale quantum information processors require stable and addressable quantum memories, usually in the form of fixed quantum bits (qubits), and a means of transferring and entangling the quantum information between memories that may be separated by macroscopic or even geographic distances. Atomic systems are excellent quantum memories, because appropriate internal electronic states can coherently store qubits over very long timescales. Photons, on the other hand, are the natural platform for the distribution of quantum information between remote qubits, given their ability to traverse large distances with little perturbation. Recently, there has been considerable progress in coupling small samples of atomic gases through photonic channels2,3, including the entanglement between light and atoms4,5 and the observation of entanglement signatures between remotely located atomic ensembles6,7,8. In contrast to atomic ensembles, single-atom quantum memories allow the implementation of conditional quantum gates through photonic channels2,9, a key requirement for quantum computing. Along these lines, individual atoms have been coupled to photons in cavities2,10,11,12, and trapped atoms have been linked to emitted photons in free space13,14,15,16,17. Here we demonstrate the entanglement of two fixed single-atom quantum memories separated by one metre. Two remotely located trapped atomic ions each emit a single photon, and the interference and detection of these photons signals the entanglement of the atomic qubits. We characterize the entangled pair by directly measuring qubit correlations with near-perfect detection efficiency. Although this entanglement method is probabilistic, it is still in principle useful for subsequent quantum operations and scalable quantum information applications18,19,20.

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Figure 1: Experimental apparatus.
Figure 2: Experimental procedure.
Figure 3: Unrotated basis results.
Figure 4: Rotated bases results.


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

    MATH  Google Scholar 

  2. Zoller, P. et al. Quantum information processing and communication. Eur. Phys. J. D 36, 203–228 (2005)

    Article  ADS  CAS  Google Scholar 

  3. Duan, L.-M., Lukin, M. D., Cirac, J. I. & Zoller, P. Long-distance quantum communication with atomic ensembles and linear optics. Nature 414, 413–418 (2001)

    Article  ADS  CAS  Google Scholar 

  4. Sherson, J., Julsgaard, B. & Polzik, E. S. Deterministic atom-light quantum interface. Adv. At. Mol. Opt. Phys. 54, 82–130 (2006)

    ADS  Google Scholar 

  5. Jenkins, S. D. et al. Quantum telecommunication with atomic ensembles. J. Opt. Soc. Am. B 24, 316–323 (2007)

    Article  ADS  CAS  Google Scholar 

  6. Julsgaard, B., Kozhekin, A. & Polzik, E. S. Experimental long-lived entanglement of two macroscopic objects. Nature 413, 400–403 (2001)

    Article  ADS  CAS  Google Scholar 

  7. Chou, C. W. et al. Measurement-induced entanglement for excitation stored in remote atomic ensembles. Nature 438, 828–832 (2005)

    Article  ADS  CAS  Google Scholar 

  8. Matsukevich, D. N. et al. Entanglement of remote atomic qubits. Phys. Rev. Lett. 96, 030405 (2006)

    Article  ADS  CAS  Google Scholar 

  9. Duan, L.-M. et al. Probabilistic quantum gates between remote atoms through interference of optical frequency qubits. Phys. Rev. A. 76, 062324 (2006)

    Article  ADS  Google Scholar 

  10. Berman, P. (ed.) Cavity Quantum Electrodynamics (Academic Press, San Diego, California, 1994)

    Google Scholar 

  11. McKeever, J. et al. Deterministic generation of single photons from one atom trapped in a cavity. Science 303, 1992–1994 (2004)

    Article  ADS  CAS  Google Scholar 

  12. Wilk, T., Webster, S. C., Kuhn, A. & Rempe, G. Single-Atom Single-Photon Quantum Interface. Science 317, 488–490 (2007)

    Article  ADS  CAS  Google Scholar 

  13. Blinov, B. B., Moehring, D. L., Duan, L.-M. & Monroe, C. Observation of entanglement between a single trapped atom and a single photon. Nature 428, 153–157 (2004)

    Article  ADS  CAS  Google Scholar 

  14. Moehring, D. L., Madsen, M. J., Blinov, B. B. & Monroe, C. Experimental bell inequality violation with an atom and a photon. Phys. Rev. Lett. 93, 090410 (2004)

    Article  ADS  CAS  Google Scholar 

  15. Beugnon, J. et al. Quantum interference between two single photons emitted by independently trapped atoms. Nature 440, 779–782 (2006)

    Article  ADS  CAS  Google Scholar 

  16. Volz, J. et al. Observation of entanglement of a single photon with a trapped atom. Phys. Rev. Lett. 96, 030404 (2006)

    Article  ADS  Google Scholar 

  17. Moehring, D. L. et al. Quantum networking with photons and trapped atoms. J. Opt. Soc. Am. B 24, 300–315 (2007)

    Article  ADS  CAS  Google Scholar 

  18. Duan, L.-M., Blinov, B. B., Moehring, D. L. & Monroe, C. Scaling trapped ions for quantum computation with probabilistic ion-photon mapping. Quant. Inf. Comp. 4, 165–173 (2004)

    CAS  MATH  Google Scholar 

  19. Duan, L.-M. & Raussendorf, R. Efficient quantum computation with probabilistic quantum gates. Phys. Rev. Lett. 95, 080503 (2005)

    Article  ADS  MathSciNet  Google Scholar 

  20. Barrett, S. D. & Kok, P. Efficient high-fidelity quantum computation using matter qubits and linear optics. Phys. Rev. A 71, 060310(R) (2005)

    Article  ADS  Google Scholar 

  21. Olmschenk, S. et al. Manipulation and detection of a trapped Yb+ ion hyperfine qubit. Preprint at 〈〉 (2007)

  22. Berkeland, D. J. & Boshier, M. G. Destabilization of dark states and optical spectroscopy in Zeeman-degenerate atomic systems. Phys. Rev. A. 65, 033413 (2002)

    Article  ADS  Google Scholar 

  23. Maunz, P., Moehring, D. L., Olmschenk, S., Younge, K. C., Matsukevich, D. N. & Monroe, C. Quantum interference of photon pairs from two remote trapped atomic ions. Nature Phys. 3, 538–541 (2007)

    Article  ADS  CAS  Google Scholar 

  24. Dehmelt, H. Radiofrequency spectroscopy of stored ions. I: Storage. Adv. At. Mol. Phys. 3, 53–72 (1967)

    Article  ADS  CAS  Google Scholar 

  25. Hong, C. K., Ou, Z. Y. & Mandel, L. Measurement of subpicosecond time intervals between two photons by interference. Phys. Rev. Lett. 59, 2044–2046 (1987)

    Article  ADS  CAS  Google Scholar 

  26. Simon, C. & Irvine, W. T. M. Robust long-distance entanglement and a loophole-free Bell test with ions and photons. Phys. Rev. Lett. 91, 110405 (2003)

    Article  ADS  Google Scholar 

  27. Legero, T., Wilk, T., Kuhn, A. & Rempe, G. Characterization of single photons using two-photon interference. Adv. At. Mol. Opt. Phys. 53, 253–289 (2006)

    Article  ADS  CAS  Google Scholar 

  28. Bollinger, J. J., Heinzen, D. J., Itano, W. M., Gilbert, S. L. & Wineland, D. J. A 303 MHz Frequency Standard based on Trapped Be+ Ions. IEEE Trans. Inst. Meas. 40, 126–128 (1991)

    Article  CAS  Google Scholar 

  29. Roos, C. F. et al. Bell states of atoms with ultralong lifetimes and their tomographic state analysis. Phys. Rev. Lett. 92, 220402 (2004)

    Article  ADS  CAS  Google Scholar 

  30. Langer, C. et al. Long-lived qubit memory using atomic ions. Phys. Rev. Lett. 95, 060502 (2005)

    Article  ADS  CAS  Google Scholar 

  31. Madsen, M. J. et al. Ultrafast coherent excitation of a trapped ion qubit for fast gates and photon frequency qubits. Phys. Rev. Lett. 97, 040505 (2006)

    Article  ADS  CAS  Google Scholar 

  32. Bennett, 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  CAS  Google Scholar 

  33. Hill, S. & Wootters, W. K. Entanglement of a pair of quantum bits. Phys. Rev. Lett. 78, 5022–5025 (1997)

    Article  ADS  CAS  Google Scholar 

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This work is supported by the National Security Agency and the Disruptive Technology Office under Army Research Office contract, and the National Science Foundation Information Technology Research (ITR) and Physics at the Information Frontier (PIF) programmes.

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Correspondence to D. L. Moehring.

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Moehring, D., Maunz, P., Olmschenk, S. et al. Entanglement of single-atom quantum bits at a distance. Nature 449, 68–71 (2007).

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