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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

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


  1. 1

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

  2. 2

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

  3. 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)

  4. 4

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

  5. 5

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

  6. 6

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

  7. 7

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

  8. 8

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

  9. 9

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

  10. 10

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

  11. 11

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

  12. 12

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

  13. 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)

  14. 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)

  15. 15

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

  16. 16

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

  17. 17

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

  18. 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)

  19. 19

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

  20. 20

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

  21. 21

    Olmschenk, S. et al. Manipulation and detection of a trapped Yb+ ion hyperfine qubit. Preprint at 〈http://arxiv.org/abs/0708.0657〉 (2007)

  22. 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)

  23. 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)

  24. 24

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

  25. 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)

  26. 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)

  27. 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)

  28. 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)

  29. 29

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

  30. 30

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

  31. 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)

  32. 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)

  33. 33

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

Download references


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.

Author information

Correspondence to D. L. Moehring.

Ethics declarations

Competing interests

Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Moehring, D., Maunz, P., Olmschenk, S. et al. Entanglement of single-atom quantum bits at a distance. Nature 449, 68–71 (2007). https://doi.org/10.1038/nature06118

Download citation

Further reading


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.